Master's Thesis Master's Thesis

2010:056 CIV 2010:056 CIV MASTEMASTER’SR’S THESIS THESIS2010:056 CIV MASTER’S THESIS

The Seasonal Behaviour of Ice and 2010:056 CIV Features in Craters at the The SeasonalMASTE BehaviourR’S THESIS of Ice and NorthernFeatures Polar in Craters Region at of the Mars Northern Polar Region of Mars The Seasonal Behaviour of Ice and Features in Craters at the Northern Polar Region of Mars

Mitra Hajigholi Mitra Hajigholi MASTER OF SCIENCE PROGRAMME SpaceMitra Engineering Hajigholi MASTERLuleå OF University SCIENCE of Technology PROGRAMME Department of AppliedSpace Physics Engineering and Mechanical Engineering MASTER DivisionOF SCIENCE of Physics PROGRAMME Luleå UniversitySpace Engineering of Technology Universitetstryckeriet, Luleå Department#)6s)33. s)32.,45 %8 3% of Applied Physics and Mechanical Engineering Luleå University of Technology Division of Physics Department of Applied Physics and Mechanical Engineering Universitetstryckeriet, Luleå Division of Physics #)6s)33. s)32.,45 %8 3% Universitetstryckeriet, Luleå #)6s)33. s)32.,45 %8 3%

Preface

This report is the diploma thesis for a Master of Science in the Space Engineering Pro- gramme at Luleå University of Technology. It is the result of an internship at NASA Ames Research Center in California between October and January 2009/2010. It was possible due to a collaboration initiated by Luleå University of Technology. This thesis project was directed under Dr. Chris P. McKay (scientist at NASA Ames Research Center) and supervised by Dr. Adrian Brown (post-doc at SETI), who presented the idea upon which this project is based. I and my fellow student, Angelique Bertilsson, from Luleå University of Technology have worked side by side on this project. The resulting two thesis works are both about the geological characteristics of craters, on the Martian Northern Polar Region (NPR), related to seasonal ice coverage and features. I would like to begin by thanking Dr. Chris P. McKay for his time with us. He has supported the project with invaluable inputs, ideas and inspiration. He welcomed us with great hospitality to California and to NASA Ames Research Center. I would like to show my gratitude to Dr. Adrian Brown for his time, knowledge and encouraging enthusiasm for this project. He provided us with essential knowledge about craters and the NPR of Mars. Further, I would like to thank my examiner Prof. Sverker Fredriksson, for reading the report and giving constructive suggestions on it, and for giving me the opportunity to start with this thesis in collaboration with the NASA Ames Research Center. I would like to thank Angelique Bertilsson for helpful discussions and creative ideas, and for our unforgettable memories together in California. I also would like to thank Mi- kaela Appel, Robin Ramstad and everybody who made the stay at NASA Ames an unforgettable time. Last but not least I would like to send my huge gratitude to my family and beloved friends for supporting, inspiring and being there for me.

Mitra Hajigholi April 9, 2010

Abstract

This Master Thesis work in Space Engineering was conducted at NASA Ames Research Center. It is a pre-study to understand how the Martian weather conditions affect water and carbon dioxide ice annually in craters, located in the Northern Polar Region (NPR) of Mars. Different (known and unknown) features have been investigated when observing the 87 craters included in this work and located poleward of 60° in latitude. These craters are studied with images from CTX and HiRISE mainly, but also from CRISM. There are many images taken by both CTX and HiRISE of the craters over seasons, acquired from 2006 to 2008. Over 500 possible crater images were examined. To make a good scientific observation and a satisfactory description of the ice amount and features covering the crater, a sort out of some insignificant data was needed. Only images where the crater is visible and easily identifiable have been used i.e., not obscured by any clouds, dust storms or even glaring from the Sun to the camera. The 87 craters chosen to be investigated on the NPR have been carefully selected. Only craters larger than 10 km in were monitored, unless they have a given name. Since large craters are created by high energy impacts and cause a more complex crater, they are more interesting for this study. All craters were localized on a virtual spherical map over Mars and pinned to be saved in a folder, so that the observed place/surface/image is easily found again. To organize the entire collection of image data from varying solar longitude, of every 87 craters, a database called Information on Craters in the Martian Northern Polar Region was created by the writer and Angelique Bertilsson. The database is in addition to organize all the collected data, also designed to better, faster and easier use the information collected. For future scientific study of this work, Information on Craters in the Martian NPR will probably in the future be a public tool on a website easily accessible for scientist and students to use. Through this work both expected and unexpected seasonal variations have been observed. Theories of why the ice behaves, as it does, and how the features change seasonally are explained and discussed in this work.

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Nomenclature

DDS Dark Dune Spot GRS/NS Gamma Ray Spectrometer/Neutron Spectrometer

LS Solar longitude MEP Mars Exploration Program MGS-TES Mars Global Surveyor -Thermal Emission Spectrometer MRO Mars Reconnaissance Orbiter NPLD North Polar Layered Deposit NPR Northern Polar Region NPRC Northern Polar Residual Cap PLD Polar Layered Deposit SPC Southern Polar Cap

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List of Contents

1 Introduction ...... 1 1.1 NASA Ames Research Center ...... 1 1.2 SETI ...... 2

2 Mars...... 3 2.1 Mars orbit and seasons ...... 3 2.2 Crater formation ...... 5 2.3 Climate and atmosphere ...... 5 2.4 The polar regions ...... 7

3 Tools used to study the Martian craters ...... 13 3.1 Cameras on MRO ...... 13 3.2 IAS viewer ...... 16 3.3 Google Earth ...... 17 3.4 Database ...... 19 3.5 Criteria used for image selection ...... 19

4 The features that craters can contain in the Martian Northern Polar Region...... 21 4.1 Dunes ...... 21 4.2 Dust Devils ...... 22 4.3 Defrosting features ...... 24 4.4 Dark dune spots ...... 26 4.5 Polygons ...... 27

5 Crater characteristics, on Martian Northern Polar Region, related to seasonal ice coverage and other features ...... 31

6 Results ...... 67 6.1 Ice amount in all craters ...... 67

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6.2 Dunes ...... 75 6.3 Dust Devils ...... 76 6.4 Defrosting ...... 78 6.5 Dark dune spots ...... 79

7 Discussion ...... 81 7.1 Ice amount in all craters ...... 81 7.2 Dunes ...... 82 7.3 Dust Devils ...... 84 7.4 Defrosting ...... 85 7.5 Dark Dune Spots ...... 86

8 Future work ...... 89

References ...... 91

Appendix A ...... 95

Appendix B ...... 102

Appendix C ...... 103

Attachment ...... 104

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1 Introduction

As early as the eighteenth century the Martian polar caps were discovered. In the late nine- teenth and early twentieth century, telescopic drawings showed “canals” on the surface of Mars. The ice caps, canals and seasonal changes observed on Mars, were factors which suggested to scientists that life might exist on Mars. One essential factor for the existence of life is water. Some scientists suggest that lakes of water may exist several meters below Martian surface. Today we know that the surface of the red planet is probably uninhabited, but the question is whether life is underground or has been in the past. Different features have been investigated when observing the 87 craters located poleward of 60° in latitude, throughout this work. The reason for examining specifically 87 craters was due to time limits but also trying to include as many large craters as possible. These craters monitored, are studied with images mainly from CTX and HiRISE, but also from CRISM. To organize the entire collection of image data from varying solar longitude, of every 87 craters, a database called Information on Craters in the Martian Northern Polar Region (NPR) was created throughout this thesis work by the writer and Angelique Bertils- son. Life on Mars is a topic both NASA Ames Research Center and SETI are working with today, in preparation of future space exploration of Mars. This Master Thesis work in Space Engineering, conducted at NASA Ames Research Center, is a pre-study to better understand how the Martian weather conditions affect water and carbon dioxide ice annually in craters, located in the NPR of Mars.

1.1 NASA Ames Research Center

NASA is the foremost federal agency of the United States driving the advancement of space science and aeronautics. Founded in 1958, NASA now has over 16000 employees (FedScope, OPM, 2010) and runs on a budget of close to $20 billion dollars (NASA, 2010) per year. NASA aims to pioneer the future in space exploration, scientific discovery and aeronautics research (Wilson, 2010)

NASA Centers and facilities are all located in the United States. Some of the more commonly known are the Kennedy Space Center in Florida and the Johnson Space Center in Houston. Some of NASA’s current missions include Spirit and Opportunity, exploring the surface of Mars, and The Mars Reconnaissance Orbiter carrying the most powerful camera ever flown on a planetary exploration mission. (Jonas, 2008) Formerly a part of NACA, the Ames Research Center became a part of NASA upon its founding. Located at Moffet field in Mountain View, California, the Ames Research Center has over 2300 research personnel and a $600 million annual budget. Ames is a part of most NASA missions and thus has a wide area of expertise. Its research fields include Information technology, Nanotechnology, Biotechnology and Astrobiology. (Jonas, 2008)

1.2 SETI

Initially, when SETI was founded on February 1, 1985, the project’s purpose was the Search for Extra Terrestrial Intelligence (SETI). In part sponsored by the Ames Research Center, the SETI institute has now expanded to explore, understand and explain the origin, nature and prevalence of life in the universe. (SETI, 2010) The SETI institute is a private, non-profit organization with over 150 employed scientists, teachers and support staff. The institute is physically represented by three centres; Center for SETI Research, the Carl Sagan Center for the Study of Life in the Universe, and the Center for Education and Public Outreach. (SETI, 2010)

2 Mars

2.1 Mars orbit and seasons

Mars is a planet with many similarities to our own, but also with some important differences. To get an intuitive understanding of conditions on Mars, a comparison between the two planets can be made. One very important factor is its orbit. Mars orbits the Sun with an axial tilt of 25.19°, similar to that of the Earth (23.44°), giving rise to seasons during its orbit. The orbit is more elliptical than that of the Earth, with an eccentricity of 0.0935 compared Earth’s 0.0167. (Grayzeck, 2007) This gives rise to a different behaviour of seasons on Mars compared to the Earth.

Figure 1. Orbits of Mars and the Earth. Image source: NASA, Jet Propulsion Laboratory (2010)

A solar day on Mars is 24.66 hrs. At its perihelion Mars is at a distance of 2.0662∙108- km from the centre of the Sun, and at its aphelion at a distance of 2.4923∙108 km. (Grayzeck, 2007) In Figure 1 it is clearly visible that the south pole of Mars is closer to Earth at Mars perihelion, and is therefore until today better explored due to the fact that we can observe it with instruments from Earth. As mentioned before, seasons on Mars are different from those on Earth. As Earth’s orbit is less eccentric than Mars, each season will be of almost the same duration. Because

3 of Mars ellipticity and Kepler’s second law, the planet will move faster at perigee and slower at apogee. This of course generates a long summer and short winter. Spring and autumn of Mars are the same length of time. Because of Mars larger orbit the seasons will have a longer duration than those on Earth. (de Pater, 2001)

Figure 2. The seasonal change in degrees of solar longitude for the northern and southern polar region. The Martian months are defined as spanning 30° in solar longitude and its seasons 90°. The first month of the Martian year starts at spring at 0° in solar longitude. The Northern and Southern polar hemisphere summer is from 90° to 180° and from 270° to 360° in solar longitude, respectively. Image source: The Mars Climate Database (2007).

It is a common practice to use the parameter solar longitude, LS, when expressing the time of year on Mars. Solar longitude is defined as the number of degrees from its Mars- Sun line at vernal equinox to its current Mars-Sun line. When Mars is at its vernal equinox

LS = 0°. At the orbital aphelion, summer solstice, the solar longitude is 90°. At perihelion, winter solstice, it is 270°, see Figure 2. The seasons refer to those on the northern hemisphere of Mars. Mars northern and southern polar caps experience elevation change over Mars season, which is illustrated in Appendix C, in a graph showing how the Martian elevation (in meters) changes as a function of solar longitude, in degrees.

2.2 Crater formation

There are no planets or satellites in our solar system that have a more diverse impact re- cording than Mars, in terms of crater morphology and crater density. Mars has greater variations than observed on any other solar system. Its unique morphology has probably been influenced by subsurface volatiles, and also by the atmosphere reacting with the ejecta blanket emplacement. (Strom, 1992) The surface of Mars can be divided into two hemispherical regions based on the crater population, which is further described below. The southern hemisphere region, spanning from 30°N to 85°S, represents the period of late heavy bombardment, possibly from accretion remnants left over from the formation of the terrestrial planets. This region contains high crater density with a large range in crater diameters. (Strom, 1992) The northern region is dominated by a younger surface with a low density distribution of craters, which differs significantly from the southern, heavily cratered, terrain, especially in size. The craters have accumulated since the end of the heavy bombardment, from asteroids and comets, with asteroids dominating over comets. (Strom, 1992) Based on these two populations and their crater densities, the Martian surface units are assigned ages relative to the period of late heavy bombardment. Absolute ages may range from 4.2 Gyr for ancient cratered terrain to as young as 0.3 Gyr for Olympus Mons. The polar deposits are younger. (Strom, 1992)

2.3 Climate and atmosphere

The primary difference in climate between Mars and Earth may be a result of the smaller size of Mars. The largest difference is the absence of liquid water on Mars, which will be explained further in Section 2.4. Even though liquid water cannot exist on the Martian surface today, numerous channels on the planet are evidence of running water in the past. Some of these channels are tens of kilometres wide, several kilometres deep and hundreds of kilometres long. This, and the presence of tear drop-shaped islands in the outflow channels, suggests that vast flows of water must have been present, flooding the plains. 5

If water was present, then the Mars atmosphere must have been denser and warmer in the past. The channels are observed to be restricted to the old and heavily cratered terrain, and therefore the warm Martian climate did not extend beyond the end of the heavy bombardment era, about 3.8 billion years ago. (de Pater, 2001) Scientists’ estimations of Mars early atmosphere suggest a mean surface pressure of 1 bar and temperatures close to 300 K. The large source of CO2 and H2O at that time must have been supplied by widespread volcanism, impacts by planetesimals and tectonic activity. Impacts of large planetesimals may also have caused a loss of atmospheric gases through impact erosion. (de Pater, 2001) In a relatively small region called Tharsis, numerous volcanoes have been created, which appear to be from the same age. “The eruptions from these volcanoes must have enhanced the atmospheric pressure and, via the greenhouse effect, its temperature.” states de Pater (2001). The rarity of impact craters in this area implies though that the volcanic eruptions occurred well after the formation of the runoff channels on Mars highlands. (de Pater, 2001) Mars does not show any tectonic activity at present, which on Earth is one of the natural sources of carbon dioxide to the atmosphere. Consequently a large amount of CO2 is

Table 1, Basic atmospheric parameters for Mars compared with the Earth. Table source: de Pater (2001)

Parameter Mars Earth Mean heliocentric distance (AU) 1.524 1.00 Surface temperature (K) 215 288 Surface pressure (bar) 0.0056 1.013 Equilibrium temperature (K) 222 263

presently lost via adsorption onto regolith, condensation on the surface and carbonaceous weathering processes (the breaking down of rocks, soils and minerals through the direct contact with the planet's atmosphere). Without liquid water on the surface, weathering came to an end and the small fraction of carbon dioxide was retained. (de Pater, 2001) The present amount of H2O on Mars is mainly unknown. Most might have escaped, but recent studies show the existence of large amounts of subsurface water-ice on the NPRs of the planet.

The average surface pressure on Mars is 6 mbar, and the mean temperature 215 K. However, due to the planet’s low atmospheric pressure and therefore low thermal inertia, axial tilt and eccentric orbit (Milankovic cycles) (Byrne, 2009), the surface temperature shows large latitudinal, diurnal and seasonal variations (de Pater, 2001). These parameters may have caused large climate changes on Mars. On Earth, these parameters change every 104 years, and may be responsible for the series of ice ages and ice-free epochs during the past million years. For Mars these parameters have periods about ten times longer than for Earth. When Mars has a large obliquity the polar regions receive more sunlight, and large eccentricities increase the amount of sunlight falling on the summer hemisphere at perihelion. Periodic changes, taken place on Mars, can be observed in the layered deposits in Mars polar region. The Mars Pathfinder (in 2007) and the Viking space craft (in 1976), both measured significant variations of the atmospheric temperature, ranging from 0 to 70 km in altitude, on timescales of months to years. These variations are strongly correlated with the amount of dust carried along in the atmosphere. Noticeable pressure variations are caused by condensation of the important fraction of Mars carbon dioxide-dominated atmosphere onto the planet’s polar caps. Like Venus, Mars does not have a stratosphere, but a thermosphere at an altitude of 120 km, with a temperature nearly constant at about 140 K, compared with

250 K at the Earth. The low temperature is due to the cooling characteristics of CO2 in the lower atmosphere. (de Pater, 2001)

The H2O and CO2 clouds on Mars can modify the surface temperature by changing the radiative energy balance. Due to their high reflectivity they can decrease the amount of incoming sunlight, by cooling the surface. However, clouds can block the outgoing infrared radiation and increase the greenhouse effect, and play a major role in the formation of storms on the planet. (de Pater, 2001) At the equator of Mars, the surface temperature can drop to 200 K at night and peak up to 300 K during the day. The temperature in the polar region at winter is 148 K, while the summer pole temperature is 190 K.

2.4 The polar regions

Mars is an Earth-like planet with respect to its polar regions. They both have large kilome- tre-thick sheets of water ice that interacts with the planetary atmosphere and records cli-

7 matic variations in their stratigraphy (layers of accretion/deposit over time). (Byrne, 2009) The polar caps have the role of a summer time source and a winter time sink for water, with dynamic equilibrium determining the amount of water vapour in the atmosphere. (Jakosky, 1992) At the Martian poles water ice is permanently frozen. In the winter when the temperature drops below the freezing point of CO2, it will condensate to form a seasonal polar cap of dry ice. The ice sheet will then extend down to 50-55° of latitude. (de Pater, 2001) Under the ice layers dust is present, which becomes visible during the summer time when ice sublimes. Over time layering structures of dust and ice have been produced, and are clearly visible in images taken by satellites orbiting Mars, such Mars Reconnaissance Orbiter (MRO) with its high resolution camera HiRISE. (Byrne, 2009) Due to periodic variations in the orbital eccentricity, obliquity and season of perihelion of Mars, there are differences between the northern and the southern poles. In the Northern Polar Ice Cap (NPIC), the seasonal cap, i.e., the dry ice, sublimes completely away during the summer, leaving behind the 1000 km in diameter permanent ice cap of water. In the Southern Polar Cap (SPC), however, the carbon dioxide never completely sublimates away, leaving a permanent southern cap 350 km in diameter. This southern residual cap is not just a simple residue of CO2. Images obtained by MGS (Mars Global Surveyor) and MRO (The Mars Reconnaissance Orbiter) show geological features sugges- tive of depositional events unique to Mars south pole. Unique to the NPR is the huge field of dunes surrounding it, which is an indicative of the differences in dust storms between the north and the south poles. (de Pater, 2001)

Water Ice

Water is presently known to exist in the residual ice caps, at shallow depths in the regolith, on the surface and in the atmosphere. Major unresolved questions are the exchange of water between the north and the south polar reservoirs, i.e., what amounts and timescales are involved. (Titus, 2008)

Due to the low Martian atmospheric pressure, 5.6 mbar, water is present only as frost, ice or as vapour. As Figure 3 shows, pure liquid water may only exist at temperatures above 273.16 K (0.01°C) in correlation with a pressure above 6.12 mbar (611.73 Pa). According to Table 1, it is currently impossible on the surface of Mars.

Figure 3, Phase diagram for water, where the triple point temperature (T.Pt.) is the point where all the phases can occur. Stability fields for the solid, liquid and gas phases are indicated. At typical Martian temperatures and pressure, liquid water is not stable. Also shown, as the dashed line, is the vapour pressure of super cooled liquid water. Figure source: Chaplin (2009).

The north polar hood shows patterns from year to year that water-ice clouds form.

(Titus, 2008) It can be seen as far south as 48°N and obscure the residual cap as early as LS 167°. When the temperature at night gets low enough, clouds of water-ice can be created near an altitude of 10 km, in the equatorial regions. (de Pater, 2001) With consistent values over two Martian northern summers, the peak atmospheric water vapour was observed in the north at LS 120°, with MGS-TES (Mars Global Surveyor- Thermal Emission Spectrometer) (Calvin, 2008). In contrast, the water over the southern cap is observed to be highly variable. This implies that a water cap underlies the residual carbon dioxide ice in the south, with a history of highly variable exposure and sublimation. (Titus, 2008) The average water ice/frost precipitation in the north is 100 µm and 50 µm in the south. Models suggest that the amount of water sublimated from the northern residual cap

9 is insufficient to account for the peak amounts of water in the atmosphere, and that the regolith exchange must also contribute to the observed atmospheric reservoir in the northern summer (Titus, 2008). The Mars Odyssey Gamma Ray Spectrometer/Neutron Spectrometer (GRS/NS) shows large amounts of subsurface ice in both the north and the south reservoir (mid- to

Figure 4. Chart describing the principal events affecting the Martian water cycle over a year. The components of the water cycle are illustrated, including the migration of water ice along of the retreating seasonal caps. The cap at the NPR reaches 55°N in latitude, end of the northern fall and the southern spring. The cap at the SPR reaches 55°S in latitude, end of the southern fall and the northern spring. NPCS stands for North Polar Cap Sublimation; SCR stands for Seasonal Cap Recession. Figure source: Titus (2008).

high-latitude ice-permeated ground). Due to the fact that the southern hemisphere lacks a large water-vapour peak it means that the ground ice in the southern hemisphere is not in exchange with the atmosphere and may therefore be more deeply buried, as inferred from thermal inertia data. (Titus, 2008) As stated earlier the polar caps have the role of a summertime source and a wintertime sink for water. The seasonal variations of atmospheric water content may also depend on 10 the exchange with the regolith. (Jakosky, 1992) To better understand the water cycle on Mars, especially the role of clouds, general circulation models have been used, as the one in Figure 4. (Titus, 2008)

Carbon dioxide ice

The Martian atmosphere is composed mainly of carbon dioxide. Because of the shape of the Martian orbit, which is more elliptic than Earth, Mars will come closer to the Sun during its southern hemisphere summer and farther away during southern hemisphere winter. Hence the Martian seasons are more extreme compared to the seasons on Earth. As a result these extremes will cause seasonal change in the pressure and carbon dioxide content of the atmosphere. (Gardiner, 2010) As carbon dioxide needs five times the atmospheric pressure on Earth at sea level to become liquid, the carbon dioxide on Mars will go directly from solid ice to gas (sublimation). Carbon dioxide ice, also referred to as “dry ice”, is a non-polar molecule with a dipole moment of zero. It has a low thermal and electrical conductivity, where intermolecular Van der Waals forces act. In the northern Martian hemisphere the temperature will drop so much that the carbon dioxide gas, either condensates directly onto the surface or into the air on condensation nuclei, such as dust grains. These then fall down to the surface adding a coating of dry ice to the polar caps. In the meantime the southern hemisphere has the summer and the frozen carbon dioxide in the polar cap sublimes into carbon dioxide gas. As the southern summer ends and the northern summer begins the whole process reverses. (Titus, 2008) 25% of the atmosphere, of which 95% is carbon dioxide, will cycle seasonally between the northern and the southern polar caps annually. This is why the carbon dioxide cycle dominates the atmospheric circulation. The current Martian climate is driven by this process, where carbon dioxide freezes out of the atmosphere in autumn and the winter on the surface, and then sublimes back to the atmosphere during the spring. At the ambient Martian pressure of 6 mbar carbon dioxide and water ice will sublimate and condense at a temperature of 150 K and 200 K (Xie, 2008), respectively. Since carbon dioxide is more volatile than water, the surface will act as an efficient cold trap for water at low temperatures, when carbon dioxide frost is present.

Observing carbon dioxide and water ice in images, the reflectance of fresh carbon dioxide and water ice is similar to each other, which make them difficult to distinguish in monochrome or multiband reflectance imaging, unless coverage extends long-ward of about 1 µm. (Bennett, 2002) All frozen carbon dioxide will sublime during the northern summer, leaving a residual polar cap made of water ice mixed with Martian dust, which will last throughout the summer. In contrast, at the southern hemisphere the frozen carbon dioxide will remain frozen throughout the Martian year. Small amounts of water or dust will have a large effect on the reflectance, as pure carbon dioxide has a low absorption coefficient. (Titus, 2008) For example the reflectance will be 25% less, with 0.1% fine dust or 1% water, in a region of 1.5-2.5 µm bands. In visible wavelengths only dust can darken carbon dioxide. Looking with thermal IR even the grain size of carbon dioxide will have an important effect on emissivity. The variation of albedo can tell the size of the carbon dioxide grains. Seasonal frost with grain size less than 100 µm will be brighter than permanent ice, with grain size about 1 mm in midsummer. According to the work of James et al. (2003) pure carbon dioxide is bright with small variations in wavelength in the visible part of the spectrum. Visible albedo is then weakly dependent on the grain size of pure carbon dioxide. However, the emissivity of the surface carbon dioxide deposits and the albedo (which is wavelength dependent) control the process of deposition and sublimation in the Martian caps. The knowledge of the seasonal polar cap and the understanding of condensation and sublimation of carbon dioxide and water will allow us to understand the past, current and future Martian climate. Craters located at the seasonal polar cap regions provide a great opportunity to study condensation and sublimation of water and carbon dioxide, especially those with high albedo deposits of frost and/or ice. It is therefore important to understand how carbon dioxide ice changes and interacts with the Martian surface and atmosphere, exploring the craters located in the NPR.

3 Tools used to study the Martian craters

3.1 Cameras on MRO

Figure 5. Mars Reconnaissance Orbiter (MRO) monitors the present water cycle in the Mars atmosphere and the associated deposition and sublimation of water ice on the surface. The instruments involved are the shallow radar SHARAD, the CRISM spectrometer, the MARCI weather camera, the HiRISE high- resolution camera, the CTX context camera and the Mars Climate Sounder (MCS). Figure source: Watanabe (2005).

The Mars Reconnaissance Orbiter (MRO) is one of the satellites orbiting Mars as part of the NASA Mars Exploration Program (MEP). Some of the scientific objectives of MEP, advised to NASA by scientific communities, are: the search for evidence of past or present life; to understand the climate and volatile history of Mars; to understand geological processes and their role in shaping the surface and sub surface; and to assess the nature and inventory of resources on Mars in preparation for human exploration.

The MRO has mapped Mars since 2006, in its nearly polar, circular and low-altitude orbit (320 km above the surface). The spacecraft carries and operates six scientific instruments (shown in bold text in the Table below) for global mapping, regional surveying, and target observations and eight scientific investigation tools (Malin, 2007; Zurek, 2007) :

1. ACCEL, Upper Atmosphere Structure Investigation 2. CRISM, Compact Reconnaissance Imaging Spectrometer for Mars 3. CTX , Context imager 4. GRAVITY, Radio Science Investigation 5. HiRISE, High Resolution Imaging Science Experiment 6. MARCI, Mars Color Imager 7. MCS, Mars Climate Sounder 8. SHARAD, Shallow Subsurface RADAR

The mission has a much higher data return than any previous planetary mission, with 96 Tb (Terra bit) returned so far.

CTX

The Context Camera (CTX) is a camera providing black and white context images of the Martian surface, with high resolution imaging. These CTX images are used as a comple-

10 km

Figure 6. This is a CTX image (P15_006803 _2505_XN_ 70N257W) taken of the 39 km crater Louth, during Martian northern spring, at LS = 4.6°. North is up in this image. 14

-ment for the High spatial Resolution Imaging Science Experiment (HiRISE) camera. CTX has a spatial resolution of 6 m/pixel and a swath width of 30 km. (Malin, 2007)

HiRISE

The High Resolution Imaging Science Experiment, HiRISE, is a 0.5 m long reflecting telescope, which provides coloured (red, green, blue and IR) images with a detailed resolution of 0.25 to 1.3 m/pixel. (McEwen, 2007) HiRISE combines this capability with re- markably high signal-to-noise and can also acquire stereo images (though it requires target- ing the same site on different orbits). With HiRISE there are capabilities to provide incredi- ble detail and insight into Mars history, as represented by the surface morphology. (Zurek, 2007)

Figure 7. This is a HiRISE image (PSP_ 006869_2505) taken during Martian northern spring at

LS = 17.0°, of dark dune spots, in average 10 m in diameter, that emerged at the bottom of the Louth crater. In this image is north upwards.

CRISM

The Compact Reconnaissance Imaging Spectrometer for Mars, CRISM, can cover most of the planet at resolutions of 200 m/pixel in more than 70 bands covering wavelengths from 0.4 to 3.96 µm.

5 km

Figure 8.This is a CRISM image (FRT000092AB_ 07_IF187L_TRR2 ) taken during the northern spring LS = 12.13°, of the dune formation located at the bottom of the Louth crater. North is upward in this image.

It can achieve full spectral resolution over a swath 11 km wide by slowing down the apparent ground imaging speed with its articulating instrument, to a spatial resolution of 20 m/pixel. It can isolate the surface compositional signature by its ability to remove atmospheric features from the sunlight reflected by both the surface and the atmosphere. These measurements can provide key data about atmospheric thermal structure, dust loading and water vapour column abundance. (Zurek, 2007)

3.2 IAS viewer

Full resolution crater images were investigated from CTX, HiRISE and CRISM, by downloading them from the public internet web site provided by Arizona State University (2008). The images are stored in JPEG2000 (JP2) format, a relatively new format that provides potential to efficiently handle large images. One of the resources that help display the images is the free application IAS (Image Access Solutions) viewer. Some of the offers

IAS viewer gives to the user are features as zooming, panning, magnifications, chipping from display and band selection. (HiRISE, 2007)

Figure 9. IAS Viewer is a user friendly and a fast software programme to view jpg2000 images with.

3.3 Google Earth

Imagine a virtual global map that can browse Mars as it looks now. This is reality with the free software programme Google Earth. (Google, 2010) Google Earth has several global maps over the Martian surface, built up with images from different satellites orbiting Mars. Visible imagery, colourized terrain, day-time/night-time infrared imagery and Viking colour imagery, are some of the named maps covering Mars in 3D. With its zooming-function the Martian surface can easily be viewed to some extent in detail, and even more by downloading the full sized image available as a link from Google Earth to the image homepage. Interesting places can be marked with a pin and saved as a folder to share or store it. A ruler tool can be used to measure the scale length in km of an object found on the map. Above all, additional information about the observed place/surface/image can easily be found. The tools and features Google Earth provides were perfect to locate the craters chosen to be monitored for our project. The approximated crater diameter could be measured and the image data and source for the craters chosen could be directly found on the map.

Figure 10. With Google Earth, Mars can be studied in detail with the data provided from Mars Global Data Access. (University, 2008)

Figure 11. This is a zoomed in image with Google Earth at the crater called Louth. The amount of image data the crater is covered by and provided to any user is observed.

3.4 Database

To organize all the collected data of images acquired between 2006 and 2008 for the 75 craters monitored, a database called Information on Craters in the Martian Northern Polar Region was created by the author and fellow student Angelique Bertilsson. Every crater has a set of images with data, such as image ID; camera used; image location; acquisition date; solar longitude; and the website the image can be downloaded from. They are all recorded in the database. In addition to image observation, the amount of ice, features, crater diameter, location and a description of the area are recorded. Every crater has a unique crater ID based on its location on Mars. The ice coverage is denoted by none, less than 50%, more than 50% and full. The database was created and assembled with Microsoft Access and a search engine function, called Query, made the search for specific requests from the Information on Cra- ters in the Martian Northern Polar Region possible. The search engine was used to create statistical observations, to achieve a faster scientific result of craters. The database was made to organize all the data collected, to better, faster and easier use the information. Information on Craters in the Martian Northern Polar Region will probably be a public tool on a website, easily accessible for scientists and students to use.

3.5 Criteria used for image selection

There are many craters on the NPR, located polward of 60° in latitude. In this project, craters with larger diameter than 10 km have been selected to be monitored, unless they have a given name. Large craters are created by high energy impacts and cause a more complex crater, with interesting features. Over 500 possible crater images from all Martian season, 0-360° in solar longitude, have been examined. Firstly, the image must match with the right crater, selected to be monitored. Secondly, in all studied images, selected to the database, the monitored crater has to be visible and easily identifiable, i.e., no clouds or dust storms should obscure the crater. This is important, in order to better make a good description of how the ice amounts and the features in the craters vary with solar longitude.

4 The features that craters can contain in the Martian Northern Polar Region

4.1 Dunes

In geosystems a dune is defined as a hill of sand that has been created by aeolian processes. The dunes are formed by interactions with the wind, giving them different shapes and sizes. Based on the various types of dune formation they are categorized differently. By observing the changing pattern of the sand dunes, the interaction between the Martian surface and the atmosphere better can be understood. The observation of the dune can determine the activity of the Martian winds, but also how and with what rate the Martian winds move the sediment around. Sand grains are capable to move with the wind in two distinct ways, either by surface creep or by saltation, where saltation is the primary method. (Mangimeli, 2010) As the wind picks up the sand grains from the surface, the wind will give them a forward momentum. Depending on the weight of the grains, they will be carried away by the wind over different distances. Bigger grains will fall to the ground after a short distance. If the surface is composed by coarse sand grains, they will bounce up in the air and the wind will, again, provide the grain a forward momentum, while lighter grains will be moved longer distances by the wind.

Figure 12. The wind will continue to move the sand up to the top and create a pile of sand. When the pile will become too steep, it will collapse under its own weight. When the right steepness is reached the dune will be stable. Depending on the properties of the material, the angle of the steepness will be different. Figure source: Nature (2000)

When lighter grains strike the sandy surface they will more likely be buried selves, and the impact will eject a second grain into the air. (Mangimeli, 2010) As the wind picks up the grains, it will lose its force and velocity. Also, a small pile of sand can decrease the wind’s velocity and strength and cause even more sand to deposit and eventually create a large pile of sand, defined as a dune. However, since the gravitational force is three times weaker on Mars than on Earth, the sand grains will not be pushed downward by the gravity in the way that they are on Earth. They will therefore be able to stay in the air much longer before they strike the surface. Martian dunes were discovered for the first time in 1972 by Mariner 9 (Jet Propulsion Laboratory, 2010) and they are still actively studied. A major part of the observed craters on the Martian NPR contains dunes. The dunes are most likely located in the centre or in the middle part of the crater. Also, if the crater contains a central peak, some of them, contain dunes that are located on, or next to, the central peak. Some craters also have dunes located close to the crater wall, or have a large sea of dunes outside, around the crater.

4.2 Dust Devils

Figure 13. In contrast to tornados, dust devils are created through a different mechanism. When the Sun is heating up the dry surface, the air will start to produce convective rolls. Some of these rolls will get tilted upright with the wind, producing a dust devil. When dust and debris get caught inside the vortex, the dust devil will be visible. Weatherquestions (2003) 22

Suggested to be effective for raising dust in the low-density atmosphere, dust devils are leaving tracks of dark lines on the Martian surface. Dust devils are described as convective vortices made by dust and sand, emerging from high rotating wind speeds, significant elec- trostatic fields and reduced pressure. (Balme, 2006)

3.5 km

Figure 14. This image, B02_010507_2456_XN_65N231W, is taken with the CTX camera at LS = 146.09° (northern summer). At the bottom of this low latitude located crater (65N-128E), clear tracks of dust devils can be observed.

On both Mars and Earth, dust devils are common atmospheric phenomena. On Mars, dust devils have been observed by the Mars Pathfinder, by the MGS camera and by both the Viking orbiters and landers (Thomas, 1992). On Earth dust devils can be observed on terrestrial dry lands and desert landscapes. Dust devils are characterized by upward moving and spiralling flows that are caused by insulation that is heating up the near-surface air. When the ground is heated by the Sun, warm air will raise and interact with the surrounding wind. The air will move towards the centre of the updraft to spin, while attempting to conserve angular momentum. The friction of the surface will then reduce the angular momentum of the spinning air and disturb the balance between the centrifugal and pressure gradient forces (Thomas, 1992). When the centrifugal forces decrease, the warm, near-surface air will converge toward the centre of the vortex. In turn, the concentration of the ambient vorticity will increase by the radial inflow. If dust enters the rising vortex, a dust devil will appear. 23

By moving over nearby areas of hot air, the dust devils can be able to sustain them- selves for longer. When the dust devil enters a terrain where the surface temperature is lower, cooler air will be sucked in and disturb the balance and the dust devil will dissipate over seconds. According to the work of Murphy et al. (2000), the typical temperature and pressure within the dust devils varies between 4 and 8 K and from 2.5 to 4.5 hPa. Throughout this work, dust devils have been observed to be irregularly spread within the craters. Often the dust devils could be seen in the middle part of the crater, but also straight through the crater or behind and on the crater rim.

4.3 Defrosting features

As the Martian spring season begins, the atmospheric and surface temperature will gradually increase. Due to the fact that carbon dioxide ice has a lower sublimation point than water ice, so it will begin to sublimate back to the atmosphere and expose the water ice or regolith below. As the temperature increases it will become warm enough for the water ice to sublimate to the atmosphere. On average, the temperature in the NPR increases until the middle of the summer. This process is called defrosting. As defrosting occurs, interesting patterns and features can be observed, similar or dissimilar to what is observed on Earth. One way of observing defrosting features in CTX and HiRISE images of craters containing ice/frost, is by observing its albedo (in scales between 0 and 1, were 0 is the lowest and 1 is the highest) and monitoring its seasonal change during one Martian year. The regolith on the crater floor has a low albedo and ice has a high albedo. In particular carbon dioxide ice has a much higher albedo compared with water ice. An increase in albedo is most likely due to condensation of ice onto the crater floor/wall. A decrease in albedo could be due to defrosting of the area, but also due to the fact that ice can age, or as a result of usual dust storms occurring in the NPR periodically. Aged ice gives a lower albedo as a result of its larger ice/frost grain size. This is caused by accumulation of ice grains with time. An area of small-sized ice grains will have more surfaces to reflect the incoming light with, compared to the same area covered by larger sized ice grains. During the dust storm season, large amounts of dust from the large

24 fields of dunes in the NPR will hence cover the ice/frost deposit. When dust settles down on ice it will decrease its albedo significantly.

2 km

Figure 15. During the end of the Martian northern spring, at LS = 88.6°, this crater at 77N-89E experienced defrosting. Frost and ice have most likely experienced high and low temperatures and created patterns of waves and streaks next to the crater rim. These patterns can be seen to the left of this high resolution image (PSP_008926_2575), where a large amount of ice has sublimed. To the right a partly bare crater rim is visible. North is upward in this image.

Another way of observing defrosting features when observing crater images is by searching for the patterns that remind of defrosting patterns occurring on Earth. When ice/snow/frost melts it starts to move. Movement of ice can get the shape of stripes or waves that appear darker than the area surrounding it. The ice melts most likely only during the day when sunlight heats up the surface, and freezes during the night. The melting process is much slower on Mars than on Earth. So, as the ice melts and freezes during day and night, patterns of ice layers can be observed. Generally, the defrosting patterns start to appear on the rim (the location that gains most sunlight during the day and has the highest incline will also show most defrosting) and then on the crater wall and last on the floor (which is usually deep and covered by the crater wall’s shadow).

4.4 Dark dune spots

Dark Dune Spots (DDSs) (Kereszturi, 2009) seem to form only in craters containing dunes, and when defrosting patterns occur. In rare cases DDSs can be observed even when defrosting is not present. These dark albedo features can be located upon and next to thin ice/frost covered dune formation on the flat ground. DDSs seem to be formed under these ice sheets, unexposed to the atmosphere. The first signs when DDSs start to form are similar to spots, being a few meters in diameter. Under these sheets of ice, the spots develop by increasing radially in size until they become exposed to the atmosphere.

Figure 16. At a latitude of 74°N, during the middle of the summer (LS = 42.60°), DDSs have emerged on and next to the partly ice covered dunes located at the bottom of this unnamed crater (74N-13E). The spots are between 0.5 m and 1.25 m in diameter. This image (PSP_007584 _2550) was taken by HiRISE, and north is directed upwards in this image.

Compared to the surrounding area the features appear to be dark/black. With time and increasing thermal heat they increase in number and grow in size. Depending on where the spots appear they will develop differently. If they appear on top of a dune peak they will in time stream down similar to how liquid stream downhill, looking like streaks featuring the same albedo as the spots.

4.5 Polygons

In some craters, a structure of polygonal features can be observed on the crater floor, on the crater wall and on and beside a central peak within the crater. These land formations are similar to those patterned grounds that are common in periglacial environments on Earth (at the northern and southern polar regions). At extreme cold temperatures these regions surface soil and sediment freeze to a depth of up to 1500 m. This layer is called permafrost. (Pidwirny, 2009)

6 km

Figure. 17 CTX image (P15_007013_2437_XN_63N228W) of polygonal nets at the bottom of the crater, 64N-132E.

Periglacial environments are defined as landforms created by processes associated with intense freeze-thaw actions that drastically modify the ground surface. A number of types of modification observed on Earth are migration of ground water, and the formation of unique landforms. Patterned ground features on Earth and on Mars arise on horizontal surfaces, but also on slopes. Their shapes range in size from a few cm up to 100 m in diameter, and look similar to mud beds. The polygonal cracking surface is usually the result of desiccation (the state of extreme dryness) or thermal contraction. (Pidwirny, 2009)

470 m

Figure 18. HiRISE image (PSP_007571_2490) of polygons at the bottom of the crater, 68N-13E.

There are a variety of patterned ground formations observed on Earth that includes frost-crack polygons, ice-wedge polygons, sorted and non-sorted circles, and stone or soil stripes. From thermal contractions in rock or frozen ground with ice content, steep fractures are formed, and called frost-cracks. (Humlun, 2004)

Figure 19. This polygonal network, formed in dry and cold areas, is located in Svalbard but resembles polygons found in Antarctica and in Martian craters in the NPR. Figure source: Portal (2010).

Ice wedge polygons are similarly formed as frost-cracking polygonals, but the cracks are gradually filled with ice. Sorted circle formations are often delineated with a border of stones surrounding a central area of finer material, varying in size from a few centimetres to over 3 m and can extend to depths of about 1 m. The stones are largest at the surface and decrease with depth. Unsorted rings usually 0.5 to 3 m in diameter, have a lack of definite ring of stones and can be found singly or in groups. Soil stripes are linear patterns of soil or vegetation on slopes without related lines of stones. (Price, 1972) On Mars there are large-scale polygonal nets, identified commonly in craters and on level terrain. The small ridges or furrows from polygonals are sometimes observed to be covered by ice during late and early spring, which makes the structure more distinct from the surrounding crater floor.

5 Crater characteristics, on Martian Northern Polar Re- gion, related to seasonal ice coverage and other features

In this Section follows short descriptions of 87 craters. A summary is included about the crater ice amount and features that are monitored and interpreted. This is done by observing images of the craters from all Martian seasons. Some craters have names. The nameless craters are numbered by degrees in centre latitude (North) and longitude (East).

The short description of every crater includes:  The crater location  The number and the type of images covering the crater during all Martian

seasons referred in degrees of LS (solar longitude)  The diameter in km  The formation (complex or simple)  The amount and location of the ice  The observed features in the crater, described in the previous Section (as dunes, dust devils, defrosting features, dark dune spots (DDS) and polygons).

The craters listed below are in order of rising latitude. By looking at the map over the NPR where also the NPRC is visible, in Appendix B, one can see were the craters are located. The seasons of Mars in solar longitude are illustrated in a graph in Appendix C.

Crater: Kunowsky Location: 56.50°N, 350.58°E At Vastitas Borealis region, the 67 km crater is located. Kunowsky is covered by eight images taken between LS = 16.7° and 351.92° (i.e., at early spring, beginning and end of the summer and the winter season). Next to the circular group of peaks in the middle of the complex crater, dunes are observed. There is not much snow or ice visible in this crater,

31 only thin layers are visible on the peaks, on the rough crater floor between the dunes and partly on the crater rim. The situation does not change for the ice amount during early summer, but there are tracks of dust devils visible over dune formations. Images from the winter season show a fully ice covered crater, with a thick ice layer on the rim and crater floor and a thin layer of ice in hollowed out areas and on the rough floor.

Crater: 60N-222E Location: 60.12°N, 221.9°E The 16 km crater is localized on the Vastitas Borealis area. Three out of four images covering the crater are taken by CTX and one by HiRISE. The images are all taken during Mar- tian spring season, LS = 20.29-60.56°, and all show a cloudy area partly obscuring the dunes in the centre of the crater. The eastern crater wall and rim have a thin ice layer that decreases fairly rapidly with time. Tracks of dust devils are visible at LS = 60.56°.

Crater: 60N-281E Location: 60.15°N, 280.83°E In the Vastitas region is the 37 km crater located. The three CTX and two HiRISE images cover the crater over LS = 51-141.7°. The crater has several gullies formed on its rim, small dune formations at the bottom of the crater next to its large peak, and a mini crater. This crater experiences a thin ice layer covering the whole crater during the middle of the spring. The dunes become darker with solar longitude, and dark dust is spread north by the wind. Also some of the gullies, northwest of the crater, are observed to become darker in albedo.

Crater: 60N-251E Location: 60.25°N, 251.08°E The 22 km sized crater is a complex one. It has only one CTX image covering the crater, showing a flat peak and a crater bottom with polygonal nets. This image covers the whole crater during the middle of the northern spring (at LS = 58.27°). No signature of an ice cover can be seen but tracks of dust devils on the eastern side of the crater floor are seen.

Crater: 60N-313E Location: 60.32°N, 313.47°E On the region called Vastitas Borealis is this 20 km crater located. Three CTX images cover the crater during LS = 44.11-100.63°. A thin layer of ice is observed on the crater wall in the earliest image. The regolith is observed to have polygonal patterns over wave- formed hills and a peak in the middle of the crater. A couple of gullies are observed on the north-eastern crater wall. At the end of the spring defrosting patterns start to show on the northern crater wall. Dust devil tracks are visible in the southern part of the crater close to the dunes. They increase in amount during the beginning of the summer. With increasing solar longitude the dunes become darker and almost all ice sublimes.

Crater: 60N-129E Location: 60.36°N, 129.37°E All of the images are taken by the CTX camera, starting from the middle of the northern spring and reaching the middle of the northern summer, from 44.81° to 140.42° in solar longitude. There are five images covering the 24 km crater. In these images, almost no change can be discerned. The crater is almost uncovered except for some ice on the eastern crater wall and on the crater rim. Structures of polygonal nets are visible in the middle region of the crater. Black traces of dust devils are visible behind and across the crater.

Crater: 60N-101E Location: 60.40°N, 101.25°E The 21 km sized crater is located close to the area called Alba Fossae. This complex crater, with a peak in the middle, is covered by three images, two taken by CTX and one by

HiRISE, during the northern spring and the summer, at LS = 58.27°, 68.6° and 128.7°. Images show that the crater has a polygonal shaped ground. Defrosting patterns are visible during late northern spring, and thinner layers of ice are observed in the whole crater. How- ever, earlier in spring the crater has an ice/frost layer covering almost the whole crater.

Crater: 60N-148E Location: 60.43°N, 147.72°E Three images are taken over the 22 km wide polar crater, during the middle of the northern spring, reaching from 41.57° to 58.85° in solar longitude. During this time the crater has a lot of ice, although, the middle part is almost ice free. Some ice is still left in small hollowed out areas within the polygonal structure, which cover the middle region of the crater. In the latest image, the ice is still covering larger parts of the eastern and the southern regions of the crater, the crater walls and the crater rim. In the middle, which is uncovered, dust devils can be seen.

Crater: 60N-90E Location: 60.59°N, 89.66°E There are ten images taken of the 21 km wide crater, located in the southern part of the northern hemisphere. The images are taken from the beginning of the northern spring and reach to the middle of the northern summer, 28.59° to 142.01° in solar longitude. At the western crater rim gully formations are visible. Some vague structures of polygonal nets are visible in the middle of the crater. Also some dunes are located in the north-western part of the crater. In the end of the spring and in the beginning of the summer, dust devils have appeared around the dunes. Some small amount of ice is visible on the eastern crater rim in the beginning of the spring. Except for that, the crater is more or less empty in all images taken during the spring and the summer.

Crater: 60N-88E Location: 60.65°N, 87.74°E The CTX camera has taken five images of the 61 km crater, located in the southern part of the NPR. The images are taken during the northern spring and up to the middle of the northern summer, 36.09° to 147.19° in solar longitude. The floor in the middle of the crater is composed of raised ridges and some vague structure of polygonal nets is visible as well. The crater has more or less no ice during this period. From the middle of the spring, to the latest image, in summer, dust devils are visible around the dunes in the middle of the crater.

Crater: 61N-312E Location: 61.19°N, 311.61°E The 30 km crater, covered by three images, is located at Vastitas Borealis. Two CTX images and one HiRISE image cover the crater between LS = 41.4° and 63.59°. All images show barely any ice, except in hollowed out areas at the peak region and some on the rim. Some high peaks of the large dune formation at the bottom of the crater show some ice deposit. Data observations from CRISM during this time period does not show any water or carbon dioxide ice deposit.

Crater: 61N-22E Location: 61.31°N, 21.51°E The 13 km wide crater is located in the southern region of the northern hemisphere. Two images are taken by the CTX camera during the northern spring, 47.98° and 69.79° in solar longitude. On the earlier image, only small amounts of ice can be seen. Some of this ice is located on the inner side of the north-eastern crater rim, which becomes even smaller in later image. Some ice is located in small hollowed out areas on the crater floor, which is composed of a polygonal structure.

Crater: 61N-308E Location: 61.35°N, 307.7°E The 22 km crater is located in the Vastitas Borealis. It has two CTX and two HiRISE images covering the crater in total between LS = 20.6° and 43.7°, which is the beginning and the middle of the spring. The crater is observed to have several gullies around the entire crater rim, which are partly covered in thin ice. A large field of dark dunes is visible at the bottom of the crater. Early in the spring, the southeast of the crater has a small part that has a thin ice layer.

Crater: 61N-229E Location: 61.46°N, 229.45°E One CTX image covers this 22 km crater, located northwest on the Vastitas Borealis region, at LS = 68.63°. The rough terrain on the crater bottom shows spots of ice layering in

35 hollowed out areas. A thicker ice layer is observed at the north-eastern rim with darker spots visible.

Crater: 62N-6E Location: 61.7°N, 6.36°E The crater is located in an area called Acidalia Planitia. There are three images taken by CTX during the northern hemisphere spring, reaching from solar longitude 41.29° to 68.46°. In the middle of the 24 km crater, dunes are seen, but also a structure of polygonal nets can be seen on the crater floor. The crater is almost completely empty of ice during this period. Some ice is still left from winter and can be seen on the edge of the crater rim, in the east of the crater image.

Crater: 62N-222E Location: 62.41°N, 221.76°E The crater is located, in an area called Scandia Colles. The 16 km crater is covered by two images, both during the middle of the northern spring. The crater shows a thin ice layer on its western rim and wall. However, in the north of the crater, a thick ice layer is visible on top of the unusual hills created on the crater rim. Other features visible, are tracks of dust devils in the middle of the crater.

Crater: 63N-187E Location: 62.53°N, 186.81°E On the area called Vastitas Borealis, the 35 km crater is located. Nine images cover the crater from the middle of the spring to the middle of the summer. In the beginning of the Martian spring the crater is observed to have a thin ice layer on part of its rim and on its peak, which is surrounded by dunes. With time, more ice accumulates on the peak, and consequently in the end of the spring the peak has a thick ice layer covering it. The ice cover increases in area, reaching and surrounding the dunes next to the peak as well. However, the ice on its rim decreases with time. At Martian summer the dunes in the crater become dark, and tracks of dust devils are visible during the middle and late summer. The rugged crater floor shows no sign of ice,

36 but small valleys in the centre of the crater do, and the crater peak has a thin ice layer in the end of the summer.

Crater: 63N-239E Location: 62.8°N, 238.52°E This 34 km crater has three CTX images in total cover it during the middle of the Martian Spring. 63N-239E has a rough and bubbly patterned crater floor. Dunes are visible next to a peak. The dunes become darker with time and the ice layer thinner as ice sublimates.

Crater: 63N-12E Location: 63.4°N, 11.92°E This crater is located in the southern part of the northern polar region. There are three images taken over the 41 km wide crater, where two are taken by the CTX camera and one by the HiRISE camera. All images are taken during the northern spring, reaching from 29.16° to 64° solar longitude. Some kind of plateau formation can be seen in the middle of the crater. Also a structure of polygonal nets can be seen around this plateau on the crater floor. A couple of dunes are located north-east of the crater. The crater is almost empty of ice during this period. However, the dunes are still covered, and some DDSs have appeared on their peaks. Some of the ice can also be seen in small hollowed out areas on the crater floor. Aside from this, the only ice seen is located around the crater, on the crater rim.

Crater: 63N-292E Location: 63.46°N, 292.482°E The 17 km crater is covered by seven images (four CTX and three HiRISE images). At

LS = 37.2° the crater barely shows any sign of ice, as most is on the rim. Its many gully formations, that may be a proof of water flowing down the crater, are partly ice covered by a thin layer. The large dunes in the bottom of this crater are also partly covered by thin ice at this time and both are so until middle of the spring. During the beginning of the summer, the dunes have become much darker as all the ice has sublimated. Tracks of dust devils are observed to cross the dunes and the crater bottom. Still, there can be seen a thin layer of ice on the eastern crater rim.

Crater: 63N-296E Location: 63.49°N, 295.87°E This 21 km crater is located in the region called Vastitas Borealis. Four images in total cover the crater during LS = 22.11-62.26°. The CTX image taken during early spring shows that some dunes are ice covered, even though the crater is partly obscured by clouds. Dur- ing the middle of the spring the crater has a frost, or thin ice, layer covering its walls and hollowed out areas at the bottom of the crater. The dunes seem to no longer have any ice coverage and have therefore become dark in colour. Odd frost features are observed around the crater as stripes going straight from the rim and out. Weak dust devil tracks are visible crossing the crater, and become stronger in pattern during the end of the spring.

Crater: 64N-132E Location: 63.53°N, 131.82°E The 25 km crater is located in the southern part of the northern hemisphere. Three images are taken, with one in high resolution. All of the images are taken during the northern spring, from 22.33° to 45.26° in solar longitude. In the earliest image, ice can be seen in depths on the crater floor and wall. Also, structures of polygonal nets are visible in the middle of the crater floor. In the latest image, almost all the ice has sublimated. The only ice left, is as small stripes on the crater rim.

Crater: 64N-234E Location: 64.26°N, 233.73°E On the area Vastitas Borealis, the 14 km crater is located. Only one image, taken by CTX, covers the crater during the northern spring at LS = 41.92°. At this time the crater is covered by a thin ice/frost layer on partial areas on the crater floor, but also on the rim. Weak tracks of dust devils are visible in this simple crater, which does not show any peak.

Crater: 65N-210E Location: 64.58°N, 209.64°E This crater is located in the Vastitas Borealis region. The 24 km crater is covered by two CTX images in total during the Martian spring. During the early spring the crater has ice coverage on the rim and on the field of dunes, with DDSs emerging on them. The ice

38 amount in the rest of the crater is hard to discern due to clouds covering it, but defrosting patterns and polygonal nets are visible. Later during the spring the dunes are dark, and almost no ice is visible in the crater, except on its rim and a thin ice layer at the north- eastern crater wall.

Crater: Lomonosov Location: 64.95°N, 350.80°E Lomonosov is located in an area named Vastitas Borealis. With 13 images in total, the 112 km crater has close to full image coverage of its total area. Four HiRISE and nine CTX images show how the seasons in Lomonosov change during one Martian year, reaching from LS = 22.04-352.44°. Lomonosov is a complex crater with a flat floor and dunes next to a concentric ring of peaks. Smaller craters are visible on the flat crater floor, where the largest crater has a of 3 km. Five out of seven images from the middle of the Martian spring show that hollowed out areas have hills of ice surrounding them. These hills become smaller with time, ice layers become thinner and dark dunes become more visible as ice sublimates. In the last images during the spring, only the peak and some mini craters next to it, have a thick ice cover left. Not much ice exists in the crater during the summer, but at LS = 160.43° ice is observed in valleys between dunes. Lomonosov has one image during the winter, 352.44° in solar longitude, when the crater is fully covered by a thick water ice layer and a thinner carbon dioxide ice layer above it. In all images covering Lomonosov during the spring, defrosting patterns are visible. At the same time a strange behaviour of frost is seen on the dunes and on the central peak, compared to the area around it. Four images have DDSs visible on dunes at the crater floor. Two of them are from the middle of the spring (37.6° and 44.98° in solar longitude) and the other two during late summer (151.5° and 160.43° in solar longitude).

Crater: 64N-31E Location: 64°N, 31°E The almost 21 km wide, low-latitude crater has only one image covering it. It is taken by the CTX camera during the middle of the northern spring, at 48.58° in solar longitude. In this image a structure of polygonal nets is visible on the crater floor. Only a small amount

39 of ice is visible as small stripes on the crater rim. Except for that, the crater is completely empty of ice.

Crater: 65N-178E Location: 65.12°N, 177.98°E There are nine images covering the 51 km crater, with three images taken by the HiRISE camera. The images are taken from the middle of the northern summer in Martian year 28 and reach to the middle of the summer the year after, 159.6° to 124.4° in solar longitude. However, only one image is taken during the northern winter, 351.64° in solar longitude. Also, two of the images taken by the HiRISE camera have corresponding images taken by the CTX camera at that the same time, at 159.6° and 131.8° in solar longitude. During the summer in Martian year 28 dust devils are visible in the middle of the crater, around the small peak. Also, some small dunes are located behind the peak, to the east. The crater is more or less free of ice, but still, some small areas show traces of ice. When the winter then comes, a thin layer of ice seems to be covering the whole crater. Larger stone formations are still visible through the ice on the crater floor. At this time,

LS = 159.6°, in Martian year 28, there can be seen DDSs upon the dunes. As the spring turns to summer the following year, the ice sublimates to be visible only in small hollowed out areas on the crater floor.

Crater: 65N-284E Location: 65.31°N, 283.85°E Above an area called Tantalus Fossae, the 22 km crater is located. Three CTX images, during Martian spring, and one HiRISE image, during Martian summer, cover the crater in total between LS = 15.83° and 128.7°. In the earliest image by CTX, the crater is partly obscured by water-ice clouds. Yet it can be observed that the dunes are all completely ice covered, on the large peak and next to a large field of strange features (looking like cracks filled with ice). DDSs are also clearly visible on the dunes. However, the rest of the crater has a thin ice layer that shows defrosting patterns. The images during the middle of the spring show that the dunes have emerged from the ice cover, and the rest of the peak has an even thicker ice layer than before. Spots of ice are also visible on the rim and inside hol- lows on the eastern crater wall.

Crater: 65N-330E Location: 65.37°N, 329.58°E The 18 km crater is located on the area Vastitas Borealis. Four CTX and two HiRISE images cover the crater over LS = 20.14-176.1°. In the earliest image the crater has a thin ice layer covering almost the whole crater. Polygonal nets have valleys filled with ice on the crater floor, and the dunes clearly visible on the bottom of the crater are covered by a thin ice layer. However, defrosting patterns are visible and DDSs have emerged and prospered over all the dune formations. Thicker ice layers are observed on the eastern side of the crater and at the top of the rim. During the middle of the spring most of the ice has sublimated and the dunes are dark and uncovered by ice. Still the eastern crater wall, next to the dunes, is covered by a thicker ice layer compared with the rest of the crater, and is observed to be so during the summer as well. Streaks of ice can be seen all around the crater, from the rim and down to the bottom of the crater. When the summer season comes, the crater gets hit by a dust storm. All over the crater, inside and outside the crater, tracks of dust devils can be observed in images taken during the middle of the Martian summer.

Crater: 65N-339E Location: 65.36°N, 338.77°E The 21 km crater is located in the Vastitas Borealis region. 65N-339E is covered by three

CTX images in total, covering its seasonal behaviour between LS = 33.9 ° and 144.75°. In the earliest image, the crater is obscured by clouds, although by observing the albedo difference, the southern and western parts of the crater seem to have a thicker ice layer compared with the surrounding area. Partly ice covered dunes are also visible. Later, during the middle of the spring, the crater is observed to have a thin ice layer covering the southern rim. The regolith seems to have a cracking pattern at the bottom of the crater, next to the dunes. A small crater is observed northwest of the bottom of the crater. During the middle of the spring the crater shows no sign of water. The dune formation has become dark, and tracks from dust devils are visible. Another mini crater is visible in the last image, located northeast of the crater wall.

Crater: 65N-128E Location: 65.43°N, 128.33°E There are six images taken by the CTX camera, over the 29 km wide crater. The images are taken during the northern spring and reach to the middle of the northern summer, from 37.43° to 146.09° in solar longitude. During the beginning of the spring, a thin layer of ice covers the crater. As the spring goes to summer, the ice layer is sublimated away and only a small amount of ice is left on the crater rim. In the latest image, in the middle of the summer, no ice is visible. In the middle of the crater, there is a structure of polygonal nets, and circular formations can be seen around its centre. During the end of the spring dust devils have started to appear, and are still visible in the latest image, during the middle of the summer.

Crater: 66N-163E Location: 66.35°N, 163.44°E The approximately 24 km wide crater is located in the southern part of the northern hemisphere. Five images are taken by the CTX camera and one is taken by the HiRISE camera during the northern summer, reaching from 22.77° to 82.63° in solar longitude. In the earliest image, ice is visible all over the crater. In the middle of the crater a small peak can be seen, and around this a structure of polygonal nets. Also, the crater floor has some kind of cliffs around the middle region. Within these a structure of polygonal nets is visible as well. In the middle of the spring, at 61° in solar longitude, there is a larger amount of ice covering the crater, compared to the image before. Almost no structure of the crater floor can be seen. The polygonal nets that were visible earlier in the spring, can hardly be seen at this time. After this, the ice sublimates again and in the latest image almost no traces of ice are visible. Some stripes on the crater rim are visible in the east. Also some dust devils have appeared in the middle region, north of the peak.

Crater: 66N-144E Location: 66.38°N, 144.02°E The CTX and HiRISE camera have together taken four images of the 32 km wide, low- latitude crater. The images are taken from the beginning of the northern spring and reach to the middle of the spring the same year, from 13.07° to 61.1° in solar longitude. The almost circular crater has a small peak in the middle, with a couple of dunes upon it. In the beginning of the spring, a thin layer of ice covering of the larger parts of the crater is visible. The dunes in the middle have DDSs randomly spread out upon them. In the middle of the spring, in the latest images, the DDSs are gone, as is the ice. Only small stripes of ice are visible, on the crater rim.

Crater: 67N-252E Location: 66.95°N, 252.05°E The 9 km wide, simple crater has two images (one taken by CTX and one by HiRISE) covering the whole crater during the northern spring, at LS = 43.7 ° and 60.97°. The crater floor is covered by polygonal nets that are larger in size than those close to the bottom of the crater, and smaller in size compared to those on the walls. The first image shows a partly thick layer of ice, located on the northern crater wall, which thins out closer to the bottom of the crater. The polygonal cracks are observed to be filled with ice or frost. In the latest image, more ice has sublimated, and a thin layer as a streak is observed on the northern crater rim.

Crater: 66N-40E Location: 66°N, 39.5°E The 37 km wide crater has five images taken by the CTX camera during the northern spring. The images are taken from the beginning of the spring and reach to the middle of the northern spring, from 26.75° to 59.43° in solar longitude. The peak in the middle has a structure of polygonal nets. South-west of the peak a couple of ice covered dunes are located. In the beginning of the spring, ice covers the crater and the dunes, except for some spots. As the spring goes on, the ice sublimates. In the latest image, in the middle of the spring, the ice is still visible as a thin layer within the crater, and as small stripes on the crater rim.

Crater: 67N-114E Location: 67.09°N, 113.56°E With six images in total, CTX and HiRISE have taken images of the 22 km crater from the beginning of the northern spring to the beginning of the northern summer, 28.55° to 110.10° in solar longitude. In the western region of the crater, the crater floor has low circular formations. The crater has almost no ice covering during this time. Some traces of ice are visible on the eastern crater rim, but as the summer begins, the ice has sublimated away.

Crater: 67N-250E Location: 67.12°N, 249.76°E

The 25 km crater is covered by three CTX images from LS = 28.37 to 63.22°. The crater is observed to have a polygonal patterned crater floor. Early in the spring the crater has a thin ice layer with defrosting patterns covering the whole crater. A thicker ice layer is located on the crater rim. An odd regolith feature is observed, and it looks like old frozen lava flow. During the middle of the spring most of the ice has sublimed and only a thin ice layer is left on its rim.

Crater: 67N-98E Location: 67.13°N, 97.54°E There are three images taken by the CTX camera of the 20 km wide crater. All of the images are taken in the beginning of the northern spring, reaching from 29.52° to 39.78° in solar longitude. The crater floor is bubbly, surrounded with darker lines and a structure of polygonal nets. A thin layer of ice covers the whole crater at this time, although the structure of the crater floor is clearly visible. The latest image has, comparable to the earliest, less ice cover, which points at defrosting.

Crater: 67N-223E Location: 67.44°N, 222.89°E On Scandia Colles, the 14 km crater is located. This crater has three CTX images in total that cover the spring period, LS = 35.92-68.19°. In all images the crater shows small amounts of ice covering it. Most of the ice/frost is located as a thin layer on the rim or the

44 southern and eastern part of the crater. The ice undoubtedly sublimates with time, so that almost no ice is visible during the end of the spring. Features observed in the crater are polygonal nets that cover the crater floor, and dust devil tracks are observed in the latest image during the spring over a dark region, which probably indicates the location of dunes or dust.

Crater: 68N-93E Location: 67.91°N, 92.84°E Seven images in total have been taken of the 29 km crater. The images are taken mostly during the northern spring, but also during the summer, reaching from 21.42° to 123.56° in solar longitude. In the beginning of the spring, larger parts of the crater are covered by ice. DDSs are visible on the dunes that are located between two stone formations/hills in the middle of the crater. Around the dunes, a structure of polygonal nets is visible on the crater floor. As the spring goes to summer, the ice sublimates to eventually be almost gone at summer. Small stripes of ice are still left on the crater rim in the latest image.

Crater: Heimdal Location: 68.20°N, 235.72°E The 10 km crater, Heimdal, is located northwest of the NPR in the region called Scandia Colles. It is covered by 14 (nine CTX and five HiRISE) images in total during the spring and the summer, between LS = 37.29° and 154.30°. During the middle of the spring the crater has a thin ice layer on the north-eastern and southern crater wall, which increases in amount radially upward to the top of the rim. The crater floor is rough and has a bubbly pattern. High resolution images show odd DDSs appearing as spots on the crater floor. They are similar to shadows, and unlike those DDSs commonly observed on dunes. With solar longitude, the ice sublimates and leaves an almost naked crater floor behind, with some spots of ice on the southern and north-eastern wall. Later during the spring, polygonal nets are visible.

Crater: 68N-190E Location: 68.25°N, 189.6°E The 12 km crater is located in the Vastitas Borealis region. The crater has five images in total covering it, four CTX and one HiRISE images, taken between LS = 35.49° and 135.8°. The earliest taken image has a thin cloud obscuring the bottom of the crater. However, there are polygonal nets that are filled with ice in the hollowed out cracks. The polygonal nets on the crater floor are gone during the end of the Martian spring and the early summer. On the western crater wall a large cliff is visible. The ice thickness covering this cliff changes with time, from being thick to thin, and then nothing. At the middle of the spring a lot of tracks of dust devils on the crater floor are visible, and no ice layer is visible in the high resolution image.

Crater: 68N-13E Location: 68.31°N, 12.55°E There are five images taken over the 15 km crater, with one taken in high resolution. The images are taken during the middle of the northern spring, reaching to the middle of the summer, 42.02° to 140.08° in solar longitude. There is a thin layer of ice, covering almost the whole crater in the earliest image. In later images the ice has sublimated away to only be seen on a small area, to the north-east, at the inner edge of the crater rim. With a structure of polygonal nets, the crater has an overall smoother crater floor, compared to other craters.

Crater: 69N-26E Location: 68.56°N, 26.46°E Four images are taken from the beginning to the end of the northern spring, reaching between 28.19° to 82.8° in solar longitude. There are three images taken by the CTX camera and one in high resolution by HiRISE, taken at the same time as one of the CTX images, at 82.8° degrees in solar longitude. The circular crater is 13 km wide and shows a structure of polygonal patterns in the middle. The crater has almost no ice covering, although a small amount can be seen in small hollowed out areas on the crater floor and on the inner edge of the south-western crater rim. In the latest image, as the season goes toward summer, the ice

46 amount eventually becomes only a small stripe along the crater rim. Also, in the same image, dust devils have appeared in the south-western part of the crater.

Crater: 69N-41E Location: 69°N, 41°E There are two images taken of the 20 km crater. The images are taken during the northern spring, 29.59° and 69.31° in solar longitude. The images taken by the CTX camera show, on the first image, a thin layer of ice/frost covering the crater and its small dunes located in the north-eastern region of the crater. Behind the dunes to the west a larger spot of ice is visible. DDSs are visible upon the dunes and on the ice spot. The crater floor is composed of a structure of polygonal nets. In the latest image, the DDSs on the dunes are gone, as is the ice spot behind them. The only ice left, are the small stripes located on the crater rim.

Crater: 70N-285E Location: 69.50°N, 285.42°E Southwest of Chasma Boreale, the 20 km crater is located. Two CTX images and one

HiRISE image cover the crater in total, during the northern spring between LS = 41.85 ° and 70.07°. In one image during the middle of the spring the crater is obscured by water-ice clouds, so the amount of ice covering the crater is hard to discern. However, by observing its southern rim and just outside of the crater and comparing it with the other images, a thicker ice layer was present during this season. In the later two images the crater has almost no sign of ice, apart from the outer edge on the rim. This complex crater has a large area of dunes on the bottom and south-western crater wall. Towards the end of the spring the high resolution image shows that the large dunes start to become darker in colour.

Crater: 70N-352E Location: 70.03°N, 352.07°E This crater is located in a region named Acidalia Planitia next to Vastitas Borealis. With 13 images in total, the 40 km crater has full image coverage of its total area. Five HiRISE and eight CTX images show how the seasons in 70N-352E change during one Martian year, reaching from solar longitude 24.91° to 349.8°. It is a complex crater with a flat floor,

47 having polygonal nets and dunes north of a concentric ring of peaks. Smaller craters are visible on the crater floor. The earliest image, is from the end of the winter season showing a totally ice covered crater, partly obscured by thick clouds. During early spring a thick ice layer covers the gullies, observed on the eastside of the crater, but also the rest of the crater. Defrosting patterns start to form as ice starts to sublimate during the middle of the spring, and with it dark features start to form on the dunes. The ice layer, on the crater wall and on top of the peaks in the bottom of the crater, gets thinner at this time. Until the beginning of the summer the dunes are ice covered, which is clearly visible as the dunes become darker when they become exposed to the atmosphere. Images from the end of the spring show that most of the ice has sublimated from the crater wall, the rim and from the bottom of the crater. There is only some ice left on the dunes. During the beginning of the summer, dust devils have left tracks on and close to the dunes. These are not clearly visible during the middle of the summer.

Crater: Louth Location: 70.16°N, 103.26°E The 36 km crater, Louth, is located three degrees lower in latitude than crater Korolev. There are 27 images in total, taken by both the CTX camera and in high resolution with the HiRISE camera. The images are taken from the middle of the northern summer in Martian year 28 and reach to the middle of the summer in Martian year 29, 133.7° to 149.2° in solar longitude. Throughout the Martian year, the crater has a slightly elliptical mound of water ice, located almost in the middle of the crater. Except for this water mound, a small amount of ice is visible on the south-eastern crater wall and rim. In the eastern end of the water mound, some dune formations are visible. During the summer in Martian year 28 the crater is almost uncovered by ice, except for a smooth cap of water mound in the middle and some ice on the south-eastern crater wall and rim. Also, in the western region of the water mound, it looks like some sort of dune formation is getting through the ice. Outside the mound, the crater floor is composed by a structure of polygonal nets. During the spring in Martian year 29, everything in the crater is covered by a thin layer of ice/frost. DDSs are located on and behind the dunes, on the water mound. Defrost- ing patterns can be seen on the western part of the water mound, and along the crater rim. 48

In the middle of the spring, at 50.31° in solar longitude, larger parts of the dunes are free of both ice and DDSs. Still DDSs can be seen behind the dunes, in the middle region of the water mound. Defrosting still takes place, and larger parts of the crater are now uncovered except for the water mound in the middle and some ice in the south-eastern part of the crater. Also, defrosting patterns are visible on the water mound, especially from the middle towards the dunes in the north-east. Lines of defrosting are visible in the western region of the water mound, as well. In the end of the spring the dunes are uncovered and no DDSs are visible. The water mound is now comparable to a smooth cap of residual water ice. Also, some ice is still left on the south-eastern crater rim, comparable to the year before.

Crater: 70N-267E Location: 70.16°N, 266.55°E Southwest of Chasma Boreale, the 23 km crater is located. Eight images in total cover the crater, four CTX and HiRISE images, from LS = 27.4° to 184.9°. This complex crater, having a peak in the middle, shows defrosting patterns early in the spring, at LS = 27.4°. The thick ice layer becomes thinner and the ice movement can be observed. However, a large area in the southern part of the crater is still covered by a thick ice layer. High- resolution images show thick ice layers on dunes with dark features around them. Also, odd features are visible, similar to polygonal nets, having white hills and darker valleys, which seem to be green/blue coloured. Images during the middle of the spring show a partly ice covered crater, most located on the crater wall with some on the bottom of the crater. Al- most no ice is observable in the crater during the summer. Only fragments of it exist on the western side of the crater, on the wall.

Crater: 71N-194E Location: 70.55°N, 193.57°E The 36 km wide, complex crater is located on the area called Olympia Planum. Six images cover the whole crater during the northern spring season, with five taken by CTX and one by HiRISE. The high resolution image is taken during early spring, but the small area visible shows a thin ice layer. This is strange. Some five solar degrees later the crater is observed to have a thick ice layer covering almost the whole crater. However, after five solar degrees again, the crater shows a thin ice layer. Defrosting patterns are visible in all images,

49 more in the middle of the spring and less in the beginning and end of the spring, together with features as polygonal nets.

Crater: 72N-345E Location: 71.53°N, 344.83°E Located at called Vastitas Borealis, the crater is 22 km wide. It is covered by two CTX and two HiRISE images during LS = 42.2 to 129.6°. During the middle of the spring the crater has an overall thin ice layer except on the dunes and on the western crater wall, which have a thick ice cover. Gullies are visible on the western side of the crater rim. The gullies have dried out channels, of which only the lower parts are ice covered at this time of season. During late spring the gullies have absorbed ice and have now a thicker ice layer covering them. A thin cloud is obscuring the crater bottom, but the white dunes and the DDSs appearing on and around them are still visible. It is clearly noticeable that a large amount of ice has sublimed from the crater rim and partly on the crater wall, but especially outside the crater. The high resolution image taken during the beginning of the summer, shows that the western crater wall and half of the dunes are visible. At this season thin spots of ice are visible only in hollowed out areas and on the rough floor of the crater wall.

Crater: 72N-146E Location: 72.07°N, 145.91°E The crater has two images taken during the northern summer by the CTX camera, at 42.02° and 61.55° in solar longitude, and one image taken in the northern summer in high resolution, at 134.3° in solar longitude. The 22 km crater has a ragged crater floor with areas with a polygonal structure. Around the middle region, the crater has a cliff-like appearance and stripes towards the centre. During the spring, the crater is covered by a layer of ice, although when defrosting takes place the crater floor can be seen. In the summer, the ice has sublimated and larger areas within the crater are uncovered.

Crater: 72N-144E Location: 72.45°N, 144.20°E The CTX camera has taken five images of the 21 km wide crater. These are taken from the beginning of the northern spring and reach to the beginning of the northern summer, 28.98°

50 to 103.15° in solar longitude. Up to 63.8° in solar longitude, DDSs can be seen on the top of the dunes, in the north-east of the crater. Large areas within the crater are covered by ice. A larger amount is seen in the eastern part of the crater, from north to south. After

LS = 63.8°, the ice sublimates and the dunes no longer have any DDSs. The dunes and the crater are now almost uncovered by ice. In the later images, in the beginning of the summer, only stripes of ice are visible on the crater rim. Also, some dust devils have started to appear in the middle, around the dunes.

Crater: Lonar Location: 72.59°N, 38.27°E Six images cover the 7 km crater, located at mid-latitude of the northern hemisphere. Two of the images, one by CTX, the other in high resolution, are taken during the same time in the middle of the northern spring, 47.2° in solar longitude. The other four images are taken during the middle of the northern summer, reaching from 116.42° to 157.1° in solar longitude. Also, two of these images are taken at the same time, at 157.1° in solar longitude, one by CTX and one by HiRISE, in Martian year 28. In the images taken during the summer of Martian year 28, only a small amount of ice can be seen in the east and in the south of the crater. During the northern spring, the year after, more ice is seen around and on the small peak in the middle of the crater. Also, a structure of polygonal nets can be seen on the peak. When spring goes to summer the same year, in the two later images, only a small amount of ice can be seen in the south, similar to the images taken in the year before.

Crater: Korolev Location: 73.0°N, 164.5°E With a diameter of 84 km, the Korolev crater is the biggest crater on the northern hemisphere. There are 18 images taken by both the CTX camera and with the high resolution, HiRISE camera. One of the images is taken during the northern winter, 350.6° in solar longitude. The other images are taken during the northern spring and in the northern summer, reaching from the middle of the summer in Martian year 28, to almost the same time in Martian year 29, 141.39° to 142.2° in solar longitude. Throughout the Martian year, Korolev is seen with a large amount of ice on the crater floor, probably residual ice. The earliest images show defrosting patterns all over the crater. In the west of the crater some- 51 thing seems to be flowing under a dune-like structure. The inner edge of the northern crater rim is uncovered, in contrast to the southern crater rim, which is covered by a thin layer of ice. During the beginning of the spring in Martian year 29, the crater is completely covered by ice and earlier defrosting patterns are covered by a new layer of ice. DDSs are seen on the top of what could be dunes, in the southern part of the crater at 20.36° in solar longitude. In the end of the spring, at 83.07° in solar longitude, sharp black lines have appeared in the cliff between the ice and the rim, located at the east of the crater. As the summer starts, these lines are getting fainter, and the defrosting patterns are even more apparent. Even in the latest images there can be seen, as in the first images, something comparable to water flowing under a dune-like structure, located in the north-western part of the crater.

Crater: 73N-22E Location: 73.12°N, 21.53°E There are two images taken of the 12 km crater. One is taken by CTX and one in high resolution, by HiRISE. Both of the images are taken during the same time in the middle of the northern spring, 43.1° in solar longitude. The crater is more or less empty at this time. Small amounts of ice can be seen on and behind the crater rim. The crater floor is rugged with polygonal nets in the southern part.

Crater: 73N-2E Location: 73.35°N, 2.21°E The 17 km crater is covered by three images. All of the images are taken during the northern spring, reaching between 15.70° and 66.21° in solar longitude, with two images taken almost at the same time, at 15.7° in solar longitude. One of these is taken by the CTX camera, the other one is taken in high resolution by HiRISE. The crater is almost completely covered by ice. In the earliest images DDSs have started to appear on the southern edge of the dunes, in the middle of the crater. Also, defrosting patterns are clearly visible on the dunes, as a cracked covering structure. In the latest image, larger areas in the crater are uncovered by ice, as well as some of the dunes. The DDSs on the dunes, seen on earlier images, are gone, and a structure of polygonal nets can be seen on the inner edge of the crater rim, together with small amounts of ice.

Crater: 73N-178E Location: 73.4°N, 178.22°E Four images are taken over the 22 km wide crater, with one taken in high resolution with HiRISE. The images are taken during the northern spring, between 39.7° and 61.96° in solar longitude. At LS = 39°, the crater is covered with one image taken by the CTX camera and one image in high resolution, by the HiRISE camera. In the images, a small peak in the middle is visible, covered with polygonal nets and a small amount of ice. In the north- western, behind the peak, an area with dunes is located. At this time, defrosting patterns are visible like a cracked covering surface on the dunes. Above and behind the dunes, DDSs have appeared as well. The crater has over all a thin layer of ice covering it. In the direction of the north-western crater rim, the ice amount is less, compared to the rest of the crater.

Crater: 74N-187E Location: 73.52°N, 186.81°E On the area called Olympia Planum is the 26 km crater is located. Four CTX images in total cover the crater during the middle and late Martian spring season, LS = 35.49-70.03°. The crater has a peak in the middle, as complex carters do, with dunes and polygonal nets covering it, strongly visible during the middle of the spring. During early spring the crater has a thin ice layer covering the whole crater, including the dunes, on which DDSs appear but also around them. In the middle of the spring, two images show how the crater has clear defrosting patterns, as wavy layering of ice caused by ice movement. At some areas on the crater rim and wall, the ice has sublimed completely, and bare regolith emerges. At LS = 70.03° the crater is almost empty of ice. The dunes are uncovered by ice and have become dark. On the southern and western rim of the crater, a thin ice layer and spots can be seen.

Crater: 74N-320E Location: 74.16°N, 319.49°E This crater is 17 km and located below the region Chasma Boreale. Its four CTX images, taken during LS = 14.31° and 72.1°, show the effect of ice sublimation on the large field of dunes covering almost the entire crater floor. At early spring the whole crater wall have a thin ice layer covering it. The dunes are totally ice covered, but the cover is thin enough to

53 see the DDSs on the dunes, below the ice. During the middle of the spring the thin ice layer on the crater wall has become thick, due to condensation of ice, and the dunes show an increased amount of DDSs. During late spring defrosting patterns are visible and a large amount of ice has sublimated and made the ice cover on the northern crater wall disappear. The dunes have a thin ice layer, if at all, at some areas. The DDSs have increased. In the last image at late or end of the spring, almost no ice is visible on the dunes, which have become dark (as how sand becomes when wet). Ice is visible only on the top of the rim, which decreases in amount and thickness downwards to the bottom of the crater.

Crater: 74N-347E Location: 74.38°N, 346.84°E The 22 km crater is located at Vastitas Borealis. There are three images in total covering the crater between LS = 27.29° and 99.2°. The earliest image shows thick ice coverage over the whole crater, including the large dune formation in the middle of the crater. Dark features are visible at this time north of the dunes. In the middle of the spring defrosting patterns are clearly visible. The ice is thinner and ice layering has been created on the crater walls. However, the dunes are still ice covered, but more DDSs have appeared around them.

Crater: 74N-13E Location: 74.43°N, 13.24°E Three images are taken over the 12 km crater, with one taken in high resolution. The images are taken during the middle of the northern spring, 44.9° to 62.16° in solar longitude. A thin layer of ice covers the whole crater in all the taken images. A larger amount can be seen south-west of the crater. As time goes by, the ice sublimates and in the latest taken image larger areas of the crater floor are seen. Also, a structure of polygonal nets can be seen in the middle, together with a small crater impact in the south-west.

Crater: 74N-14E Location: 74.43°N, 13.54°E There are three images covering the crater. One is taken by the CTX camera during the northern winter in Martian year 28. The other two are taken almost at the same time, during

54 the northern spring in Martian year 29, by the HiRISE camera. The crater is 12 km. The crater is more or less covered by ice in the earliest image. DDSs can be seen on and behind the dunes, which are located north-west in the crater. The later images show DDSs, together with defrosting patterns and less snow, compared to the first image. Small amounts of ice can be seen in small hollowed out areas within the crater. Also, a larger amount of ice is still left on the inner edge of the north-eastern crater rim. A hint of polygonal nets can be seen on the rugged crater floor.

Crater: 75N-15E Location: 74.52°N, 14.59°E The crater is located in the Vastitas Borealis region, also referred to as the northern lowlands. The images are taken during the middle of the northern spring up to the beginning of the northern summer, 47.68° to 94.13° in solar longitude. There are four images taken of the crater, with one in high resolution taken at the same time as an image taken by CTX, at 94.1° in solar longitude. The crater has a 12 km diameter and is in the earliest image covered by a thin layer of ice. On and behind the dunes located in the middle of the crater, DDSs can be seen. As the spring goes to summer, the ice sublimates away and a structure of polygonal nets can be seen on the crater floor, around the dunes. In the latest images the dunes are uncovered, and only a small amount of ice is left. Some of the DDSs are still left, located behind the dunes.

Crater: 75N-340E Location: 75.3°N, 340.49°E Between the areas called Vastitas Borealis and Gemina Lingula, this 16 km crater is located. Two images from CTX cover the crater in the beginning (LS = 21.57°) and in the end

(LS = 81.3°) of the Martian spring. In the first image the crater is almost completely ice covered. The eastern crater wall and rim has a thicker ice layer compared with the opposite side. The large dune formations located on the bottom of the crater and western wall, are ice covered with dark features emerging on and around them. At the end of the spring the dunes are dark, having some ice cover on the outer edges of the high dune peaks. Overall, the crater shows almost no sign of ice, except on the highest point on the rim.

Crater: 75N-158E Location: 75.31N, 158.28°E The 14 km crater is located in the Vastitas Borealis region. It is covered by five images, between LS = 40.62 ° and 138.34°. During early spring the crater is covered by a thick ice cover, which is observed to be a mixture of carbon dioxide and water, using the CTX camera at this time. However, defrosting patterns are visible during the whole Martian spring and the summer. The crater has a polygonal net as a crater floor. During the end of the spring and the middle of the summer, ice has sublimed away from the northern crater wall, and left a partially thin ice layer.

Crater: 76N-27E Location: 75.59°N, 27.11°E There are two images taken of the 17 km wide crater. The images are taken by the CTX camera during the middle of the northern spring, 48.89° and 68.43° in solar longitude. On the earliest image, a thin cover of ice can be seen. This cover is almost gone in the latest image. Here the ice is seen only in small depressions in the polygonal structure of the crater floor. Around the small dune formations in the northern part of the crater, DDSs are seen in the earlier image, but due to bad image quality in the latest image, the DDSs are not confirmed to be present.

Crater: 76N-333E Location: 76.18°N, 332.8°E Just above the area Vastitas Borealis is this 12 km crater located. The crater has three images in total, all taken with CTX during the spring, at LS = 33.9°, 46.37° and 61.31°. In all images the crater has ice on its rim, more early during the spring and less during late spring. The large field of dunes at the bottom of the crater is not covered by ice in any images, but shows an increase of DDSs with time, together with defrosting patterns. However, at LS = 46.37° the crater adsorbs a thick layer compared with the previous image, and causing the DDSs to decrease in size and amount.

Crater: 76N-159E Location: 76.00°N, 159.00°E There are three images covering the 14 km wide crater. All of the images are taken by the CTX camera in the middle of the spring and in the middle of the summer, reaching from 40.62° to 138.34° in solar longitude. During the northern spring, a thin layer is visible over the rugged crater floor. As the spring goes to summer, the ice sublimates to be visible only in the south-western part of the crater in the middle of the summer.

Crater: Escorial Location: 77.03°N, 305.17 °E In the Chasma Boreal region is this 17 km Escorial crater located. It is covered by three CTX images during the Martian spring. At early spring the crater has a thick ice cover on its wall and rim, but the ice layer gets thinner to the bottom of the crater. The Escorial crater has a small peak at the bottom, and the rough crater floor has the patterns of long streaks, which sometimes form from the rim to the bottom of the crater. DDSs are visible on the western crater wall even though no dunes are visible. In images from the late spring these spots are gone, but leave a dark area behind, as dunes are observed to do in other craters. At this late spring time, defrosting patterns are also visible, showing almost no sign of ice on the crater walls, except on the rim.

Crater: Dokka Location: 77.10°N, 214.46°E The 45 km Dokka crater is located at the Olympia Undae region. It is covered by five CTX images between LS = 12.48° and 353.16°. In the earliest taken image, the whole crater is covered by a thick ice layer. In the middle of the spring there are visible defrosting patterns. Ice has sublimed more on the southern rim of Dokka, and made the ice layer thinner, van- ishing on some spots. During the Martian winter season a large amount of ice has accumulated on the whole crater surface and rim, making the ice layer thick.

Crater: 77N-196E Location: 77.2°N, 195.79°E Located in the region called Olympia Planum, this 22 km crater is covered by four CTX images between LS = 28.91° and 125.86°. A thick ice layer is visible, covering the crater and the dunes at the bottom of the crater. The dunes have visible DDSs emerging on and around them, which increase in amount and size with increasing solar longitude during the spring. During the middle of the spring, this expansion of DDSs has reached the crater wall together with visible defrosting patterns. At the end of the spring defrosting patterns are emphasised, and at the north-eastern crater wall almost no ice is left. DDSs are rarely found at this time, and only on the dune peaks. During the middle of the summer all ice has sublimed from the northern part of the crater, including the dunes. Only a thin layer of ice, starting from next to the dunes and going south, is left, and has patterns of defrosting visible.

Crater: 77N-89E Location: 77°N, 89°E The 34 km wide crater is covered by images taken by both CTX and HiRISE. Most of the images are taken during the spring and the summer, reaching from the middle of the northern summer in Martian year 28 to the beginning of the northern summer in Martian year 29, 113.5° to 145.92° in solar longitude. In Martian year 29, two of the images are taken during the northern winter, 349.11° and 351.76° in solar longitude. During the northern summer in Martian year 28, the crater is more or less completely covered by ice. Only a small part between the ice within the crater and the crater wall is visible. During the winter the whole crater is covered. Only a vague outline of the crater is visible. In the spring of Martian year 29, the crater is completely full of ice. Defrosting patterns are seen to take place in the south-western region of the crater. In the middle of the spring the crater rim is more visible in south-west. Behind the crater, a lot of dunes are located. At this time of the year, DDSs have appeared. Compared to the images taken during the winter, the ice amount has decreased a lot within the crater. During the end of the spring, defrosting patterns are clearly visible. Between 59° and 69° in solar longitude, the crater seems to have been covered by frost. The visible part of the south-western crater rim that was uncovered in LS = 59°, is covered again at LS = 69°. 58

After this, the ice is again seen sublimating, and the crater rim becomes visible. In the end of the spring and in the beginning of the summer the whole crater rim is visible and some small regions of the crater floor are faintly made out.

Crater: 77N-46E Location: 77.00°N, 46.00°E The 11 km wide crater is located within a region of dune fields, Siton Undae. The images are taken by the CTX camera during the winter of Martian year 28, and reach to the beginning of the northern summer in Martian year 29, 350.34 to 116.68 in solar longitude. Dur- ing the Martian winter the whole crater is covered by ice. Ice covered dunes are located in the north. Behind the dunes, to the south, DDSs can be seen. In the middle of the spring, the DDSs have started to appear upon the dunes as well. Strong defrosting patterns are visible in the whole crater. In the latest image, at the beginning of the summer, the dunes are uncovered. On a line from the south-western crater rim towards the dunes, the ice is gone as well.

Crater: 78N-52E Location: 78.01°N, 52.35°E There are four images taken of the 16 km wide crater. All of the images are taken by the CTX camera during the northern spring and the summer. Also one image is taken during the northern winter in Martian year 28. The images cover the crater from 28.62° to 114.78° in solar longitude. During the summer of Martin year 28, everything except for the dunes in the west of the crater is covered by ice. Some traces of dust devils are visible towards the west. In the image, taken during the winter, the whole crater is covered by ice. Early in the spring, the following year, the crater is still covered, but all over the dune’s DDSs have appeared. When the summer comes, the DDSs are still visible. Defrosting takes place and the crater floor has started to become visible in the northern part of the crater. Except for this region, the crater is still covered by ice in the latest image, in the beginning of the summer.

Crater: 78N-347E Location: 78.38°N, 346.93°E The crater is located just south of the area called Gemina Lingula. Three CTX images in total cover the crater, during the middle of the Martian spring and the summer, reaching from LS = 35.28° to 138.88°. The 9 km sized crater is observed to have a large amount of ice left in the western part of the crater during the middle of the summer, which could be residual water ice, i.e., ice that stays in the crater around the year. The frost/ice outside the crater is observed to behave strangely. On the outer west side of the crater, the area is bare, and the east side is ice covered in the shape of a half-circle embracing the crater. This behaviour could be due to wind direction, blowing the ice in the same direction, or due to a warmer spot being present to the west of the crater and not letting frost deposit there. Dur- ing the middle of the spring, the crater is observably full with ice.

Crater: Inuvik Location: 78.6 °N, 331.4 °E Just below the Chasma Boreale, the 20 km Inuvik crater is located. Nine (seven CTX and two HiRISE) images in total cover the crater between LS = 21.09° and 145.05°. During early Martian spring, Inuvik has a thick ice layer covering the whole crater, including its large field of dunes located on its western crater wall and bottom. The Dunes have at this time DDSs on and around them. Later, closer to the middle of the spring, DDSs are observed to have emerged on the crater walls as well, which is uncommon. Defrosting patterns are seen close to the dune formations, as darker wave or stripe traces from ice movement. With time, still during the middle of the spring, defrosting patterns start to appear on the western crater wall, as the ice layer becomes thinner. Dune peaks are also darker now than the surrounding area. When the end of the spring is close, the dunes are showing large amounts of DDSs that slide downhill from the dune peaks, when the dunes are still thickly ice covered. As the temperature rises, and ice is subliming with increasing solar longitude, the ice layer gets thinner and thinner on the crater walls. However, the thick ice cover stays on the crater rim and bottom covering the dunes, which are still bleeding dark features. As the Martian summer comes, the thick ice cover vanishes from the large dune formations, and a thin ice layer surrounds it, having patterns of defrosting. A thin ice layer still covers the crater wall, but the thick ice cover is now gone from the rim, which now only 60 has spots of ice covering it. In a high resolution image, a polygonal patterned crater floor is visible. During the middle of the summer, the crater walls have no ice cover and the dunes are dark without any DDSs. However, spots of ice or snow are visible on the dunes and a larger amount of ice is visible north, west and especially south of the dunes. As the ice sublimates Inuvik’s small crater peak in the centre has become visible.

Crater: 79N-240E Location: 78.7°N, 240.46°E

Six images in total cover this 16 km crater during LS = 43.28-350.48°. During the middle of the spring the crater has a thin ice layer covering its walls. At the bottom of the crater, a large dune formation is observed that is partly ice covered, with many dark features on and around the crater. High resolution images show polygonal patterns covering the crater floor next to the dunes. Later during the spring, it is observed that ice sublimes firstly from the northern crater wall and from the rim. However, it is observed that ice has accumulated on the western part of the dune formation. During Martian winter the crater is covered by a thick layer of ice.

Crater: 78N-41E Location: 78°N, 41°E Five images taken by both CTX and HiRISE cover this mid-latitude crater. The images are taken during the summer and the winter, in Martian year 28 and during the spring in Mar- tian year 29, at 146.51°, 352.36° and between 42.6° to 62.57° in solar longitude, respectively. The crater is 14 km and has dunes located in the western region. During the summer, in Martian year 28, the crater is uncovered by ice until later into the winter, when it becomes totally covered. When the spring comes the following year, defrosting features are visible on the surface of the dunes, similar to a cracked surface. Upon this, DDS are randomly spread on the dunes. Also, a structure of polygonal nets is visible on the crater floor around the dunes. In the latest image, in the middle of the spring, the DDSs are still visible together with a larger amount of ice around within the crater. However, some of the ice is gone in the northern part of the crater.

Crater: Sevel Location: 79.12°N, 323.85°E South of the region called Chasma Boreale the 6 km Sevel crater is located. It is covered by two CTX images in total from LS = 53.2° and 109.9°. During early Martian spring, Sevel is covered by a thick ice layer with DDSs visible to have emerged all over the crater, especially north of the crater. A strange behaviour of the ice is observed around the outside of the crater. It has a border of thicker ice layers embracing the crater in an oval shape. The area inside this oval (which is more visible during the summer), has a much thinner ice layer or none at all during the summer. This is an odd behaviour, which could be due to wind directions or an occurrence of higher thermal heat in some regions. During the early summer Sevel has a large and thick ice cover left in the bottom of the crater, which probably is annual residual ice. Defrosting patterns are visible in both images.

Crater: 79N-62E Location: 79.00°N, 62.00°E The crater is located in an area with dune fields, called Siton Undae. There are eleven images taken of the 18 km wide crater. These are taken from the middle of the northern summer in Martian year 28 to the same time the following year, reaching from 145.65° to 145.96° in solar longitude. Three of the images are taken during the winter at the Martian turn of the year, 28/29. The crater is completely full of ice during the winter. The crater rim and the dune formations, located in the middle region of the crater, can hardly be distin- guished. As early as three degrees in solar longitude, DDSs have started to appear on the dunes. In the middle of the spring, a cracked surface can be seen on the dunes together with DDSs. In the middle of the northern summer, defrosting takes place in the whole crater and larger parts of the dunes are visible. Still some of the DDSs are visible. Also, the north- eastern crater wall has become uncovered. In the latest images, during the middle of the northern summer, half of the northern region of the crater is uncovered by ice, like the dunes.

Crater: Boola Location: 81.15°N, 254.84°E Close to the NPRC is the 15 km Boola crater located. It is covered by two CTX images from the middle of the Martian spring and the summer. During the middle of the spring Boola seems to have a large amount of ice at its bottom and a thick ice layer on the eastern crater wall. The rest of the crater has a thinner ice cover. During the middle of the summer defrosting patterns are visible and a large amount of ice has sublimed away. Spots of ice are visible on the rim, and a large area of thick ice is left at the bottom of the crater, which could possibly be visible residual ice.

Crater: Jojutla Location: 81.33°N, 190.64°E In the Olympia Undae region, close to the NPRC, is Jojutla with its 22 km crater. It is covered by nine (seven CTX and two HiRISE) images in total, during the middle of the spring and middle of the summer. In the earliest images defrosting patterns are visible and ice sublimation is clearly visible on the north-western rim. The large field of dunes on the bottom of the crater has a thick, fully covering, ice layer, were DDSs have emerged and slid downhill from high dune peaks. During the end of the spring, defrosting patterns are visible and most of the ice has sublimed to a thin layer in almost the whole crater, but not at the south-western crater wall, where a thick ice layer is visible. Strong winds have probably blown in the same direction, because hills, southern dune peaks and rough regolith, all have a thick ice layer on their north-eastern side. In the beginning of the summer, Jojutla shows no ice layering in the northern part of the crater. Some spots of ice are left on the dunes, but no DDSs are active. Only darker dunes are observed. The southern crater wall still has thick ice coverage, which gets thinner during the middle of the summer. Defrosting patterns are clearly visible during the whole Martian summer on the southern crater wall.

Crater: 81N-117E Location: 81.36°N, 117.28°E There are two images taken of this high latitude crater. These are taken by the CTX camera during the spring and the summer, in two different years, at 116.11° in Martian year 28 and at 62.74° in Martian year 29. The crater is 12 km wide and completely covered by ice. Defrosting patterns are visible in the northern part of the crater, during the spring. A couple of DDSs are visible on the ice in the middle of the crater, but also outside, behind the crater, to the west.

Crater: Udzha Location: 81.98°N, 77.2°E The high-latitude crater is located on the edge of the NRPC. There are five images taken by the CTX camera, covering the 45 km wide crater. One of the images is taken during the northern summer in Martian year 28, 112.84 ° in solar longitude. The other images are taken during the middle of the spring and in the beginning of the northern summer in Mar- tian year 29, reaching from 41.92° to 90.86° in solar longitude. The crater is almost completely full of ice during this period. Only small parts of the rim are visible and confirm the crater’s existence. As the spring goes to summer, starting in solar longitude 46.68 °, some hints of defrosting patterns can be seen in the southern part of the crater. In the beginning of the summer, defrosting is more apparent in the whole crater, but still the crater is completely covered by ice.

Crater: Crotone Location: 82.18°N, 298.59°E Crotone, the 17 km crater, is located in the Chasma Boreale region. It is covered by two images (CTX) during Martian spring and two (HiRISE and CTX) during the summer. At

LS = 51.41° and 53.68° the crater has a thick ice layer covering the whole crater. Defrosting patterns are visible at the north-western crater wall, were ice movement can be observed from the shape of horizontal stripes or waves, which appear darker than the area surrounding it. During the middle of the summer the crater rim has a thin ice layer covering it with spots of thicker layering. The darker stripes surrounding the inner crater are strange. It

64 could be the crater floor that has this shape or it could be an aged thick layer ice left annually, and partially being frost covered by a thin layer during the summer.

Crater: Puyo Location: 83.79°N, 138.69°E There is one image taken by the CTX camera of this high-latitude crater. The crater is 10 km. The image is taken during the middle of the northern summer. At this time the crater is covered by a layer of ice. Dunes, located in the southern part of the crater, are still covered, and DDSs can be seen on the crater floor together with some parts of the crater floor that are seen through the ice cover.

6 Results

Monitoring the 87 craters have resulted in a better understand of the seasonal behaviour of the crater content. As mentioned in Section 4, the craters in the NPR are observed to contain features such as dunes, periodic occurring dust devils, defrosting patterns and DDSs, but also seasonal and residual ice/frost. With the help of the information in the database, Information on craters in the Martian NPR, relations was determined and plotted in graphs of the ice amount and the features existing in craters, with parameters as latitude, longitude, LS, crater diameter and Martian year. In the subsections below, the graphs are analysed, in order to obtain clarified results of the seasonally/annually behaviour of the crater content.

6.1 Ice amount in all craters

Figure 20 illustrates the amount of ice covering all craters and their respective images taken during two Martian years, 28 to 29, which represent Earth years 2006 to 2008. Every dot represents an image of a crater, its colour representing the amount of ice covering the crater at a specific time in solar longitude LS. The ice amount in the craters are categorized in four different intervals depending on how much ice covers each crater; full, more than 50%, less than 50% and empty. In this work the ice amount represents only how much the ice is covering the surface of the crater and not how deep this layer is. If the crater has a thin layer covering the whole surface of the crater, i.e., if the structure of the crater floor is visible, the crater is set to have a full ice cover. If the crater is almost covered by ice, i.e., has small regions uncovered, the ice amount is set to be more than 50%. If the crater has large regions uncovered by ice, the crater is set to be covered by less than 50% ice. According to Figure 20, images of craters with no sign of visible ice/frost within them are located in the range between 60° and 70° in latitude, during a seasonal period that ranges from the beginning of the northern spring to the middle of the northern summer, 28° to 160° in solar longitude.

Images of craters with less than 50% ice are observed to be located in the range between 54° and 84° in latitude within a seasonal period between 15° to 158° in solar longitude. Over the latitudinal range of 65-75°, during the same seasonal period, the craters containing less than 50% ice/frost are distributed more densely.

Figure 20. The ice coverage of the craters on the Martian NPR over different latitude as a function of solar longitude. The white, yellow, red and black markings indicate when the crater has no ice, less than 50%, more than 50% or when its full of ice, respectively. The green crosses represent observed tracks from dust devils in the craters.

The amount of ice follows a pattern illustrated with an ellipse with their colours representing the ice amount. The crater images showing no signs of ice are basically observed on the lower part of the NPR during the end of the northern spring. The yellow ellipse represents crater images observed to have less than 50% of ice coverage. It covers almost the whole NPR, between 55° up to 84° in latitude with a steep positive angle. The red and the black ellipses follow the outline that with rising amount of ice in craters the ellipse gets smaller in size, and its central point increases in latitude and decreases in solar longitude. The results acquired in Figure 20 are reasonable, as colder temperature follows higher latitude. The closer the crater is to the NPRC, located poleward of 80° in latitude, the more ice it contains, and is possibly preserved more or less the hole Martian year.

Detailed Analysis

From the overview analysis of the ice amount, a detailed analyse of specific unusual cases are compared with other craters, described below.

 Craters fully covered by ice can be observed between 71° and 83° in latitude, starting

during late winter season at LS = 351° and lasting until middle of the spring, LS = 63°. One of these craters is Korolev, located 73° in latitude. One observed image of Korolev, taken during the Martian year 28, shows a fully ice covered crater in the

middle of the northern summer, LS = 141°. Still the year after, in the middle of the

northern spring at LS = 43°, Korolev is full of ice. The ice amount in the crater is

thereafter observed to decrease with increasing solar longitude until LS = 141°.

 Crater 79N-62E shows, an ice coverage in late summer, in Martian year 28 that is more than 50%. The crater is full of ice between 349° and 3° in solar longitude, which is not surprising due to its high latitudinal location in the NPR.

 Sevel crater is another crater observed with a large amount of ice. The crater is located at 79° in latitude and is observed to be full of ice in the middle of the spring and the beginning of the northern summer at, 53° and 110°, in solar longitude. Craters at high latitudinal location usually contain a large amount of ice, but it is unusual for them to be fully covered during the summer.

 At lower latitudes, craters are observed to contain less ice than those at higher latitudes. It is therefore more common to find craters at higher latitudes having residual ice annually, than those at lower latitudes. This pattern is clearly observed in Figure 20.

 Two craters located on the eastern and the western side of the NPR, at the same latitude and during the same Martian season, show a contrast in the observed amount of ice covering it. One crater contains no ice and the other crater contains more than 50% ice. 69

Summer frost and residual ice deposits

Figure 21. Craters on the Martian northern polar region with residual ice deposit, over different latitude as a function of diameter. The yellow, red and black markings indicate when the crater has less than 50%, more than 50% or when it is full of ice, respectively. The yellow and red lines represent the linearity of craters containing less than 50% ice and more than 50%, respectively.

Out of 87 craters observed in the NPR, poleward of 60°, there are 26 craters (listed below) showing ice deposits (at least less than 50% of ice) in images taken at LS = 75-180° (late spring and throughout the summer). The majority of the 87 craters monitored have a seasonal ice deposits that sublimate away during late spring and early summer. By plotting the crater latitude vs. the crater diameter, a weak linearity (due to lack of data) between them can be observed in Figure 21. Most of the craters on the NPR are between 10 and 30 km. In this diameter range, the majority of the craters over all latitudinal range contain ice deposit during the summer. However, Figure 21 shows that smaller craters contain less ice, and larger more than 50% or full of ice. This is, however, only true for latitudes up to 74°, with an exception of one crater being fully covered by ice deposit, with a small diameter size of 20 km. Above 74° in

70 latitude, the craters have between less than 50% and more than 50% of ice deposit covering it, with diameter sizes of 6-45 km. The following 26 craters, in Figure 21, contain ice deposit during the late northern spring and throughout the summer, during two Martian years (28 and 29):

Lomonosov, Sevel, Boola, Jojutla, Udzha, Louth, Heimdal, Puyo, Escorial, Korolev, 63N- 187E, 67N-267E, 69N-26E, 72N-345E, 72N-144E, 73N-38E, 70N-267E, 75N-5E, 75N- 340E, 76N-159E, 77N-89E, 68N-93E, 78N-52E, 61N-90E, 79N-62E, 82N-299E.

Residual Ice

Similar to the polar region’s residual water ice cap beneath the seasonal ice cap, some craters have a large area of ice deposit on the crater floor that never sublimates. Eight possible craters, identified with image observation and albedo analysis, are candidates for having residual ice deposit in them. They are located poleward of 70° in latitude and can be observed in Figure 22-29. One of them is Korolev (Figure 29) that is famous for its coverage by residual water ice. Following craters contain large amount of ice during Martian years 28 and 29:

9 km

Figure 22. 79N-62E, LS = 145.65°, B02_010496_2592_XN_79N299W.

10 km

Figure 23. Udzha, LS = 112.84°, T01_000803_2620_XN_82N280W.

5 km

Figure 24. Puyo, LS = 134.33°, B01_010203_2601_XN_80N221W.

5 km

Figure 25. Inuvik, Ls = 142.17°, B02_010407_2587_XN_78N028W.

3 km

Figure 26. Sevel, LS = 109.9°, P22_009537_2612_XI_81N039W.

12 km

Figure 27. 77N-89E, LS = 89.03°, P20_008939_2572_XN_77N270W.

13 km

Figure 28. Louth, LS = 133.68°, P01_001370_2503_XI_70N257W.

21 km

Figure 29. Korolev, LS =133.68°, P01_001370_2503_XI_70N257W. 74

6.2 Dunes

In Figure 30 all the observed craters are plotted with their respective latitude location as a function of their diameter size. All craters that contain dunes are marked with a cross. When comparing the craters that contain dunes with the crater diameter, a relation between them can be observed as a pattern. In the range between 56° and 84° in latitude, craters are observed to contain dunes. Of all the observed craters within this work, none of them with diameter less than 11 km contain any dunes. In Figure 31 the locations of all observed craters, with and without dunes, are plotted with latitude and longitude. Those craters containing dunes are marked with a cross. All the craters are observed to indicate some sort of grouping. Craters containing dunes seem to be located sparse and fewer between 135° and 270° in longitude. However, between 0° and 90° and 270° and 360° in longitude, the craters containing dunes seem to be located much denser.

Figure 30. All observed craters together with those containing dunes (marked with a cross). Both are plotted with latitude vs. the crater diameter.

Figure 31. The location of all observed craters is plotted with latitude and longitude. All craters with observed dunes within them are marked with a cross.

6.3 Dust Devils

In Figure 32 all observed craters are plotted with their locations in latitude and longitude. Craters that contain dunes and dust devils are coloured with black and orange, respectively. Upon these markings, craters containing both dunes and dust devils are indicated with a red cross, while craters containing no dunes but showing tracks of dust devils are indicated with a blue cross. Analyzing Figure 32, the dust devils seem to appear between 37° and 160° in solar longitude, during the northern spring and until end of the summer, and over latitudes between 60° and 78°. No dust devils are observed above 68° in latitude in craters without dunes. Below 68° dust devils appears in craters both with dunes and without, randomly spread. Craters not containing dunes, but with dust devil tracks can be seen only between 60° and 68° in latitude and between 128° and 25° in longitude. However, craters with dunes and

76 dust devil tracks visible can be observed between latitudes 60° to 78° over a span of 52° to 352° in longitude. Also, looking at the craters with dust devils but without any dune formations, they seem to appear during the middle of the spring and disappear at the end of the northern summer. Since dust devils are a result of temperature differences, this might indicate that dust devils depend of temperature. Early in the spring and the late summer or beginning of the autumn it might get too cold for the dust devils to form. Many of the craters that contain dunes also show traces of dust devils, but they are also observed in craters that do not contain dunes. Those specific craters have been plotted in Figure 33 with latitude and solar longitude, from early spring to early autumn. As can be seen in Figure 33, most of the dust devils appear during the middle of the northern spring. A few can be observed during the middle of the northern summer at higher latitudes.

Figure 32. All observed craters, at their respective location in latitude and longitude, and their content of dunes and dust devils.

Figure 33. Craters that do not contain dunes, but have traces of dust devils in latitude and solar longitude. Most of them appear in the middle of the northern spring.

6.4 Defrosting

Figure 34 illustrates when in solar longitude and where in latitude, defrosting patterns are observed in all craters, during two Martian years, 28 to 29. Every green cross represents an image having visible patterns of defrosting. Defrosting patterns are observed to appear in almost every crater, between 56° and 82° in latitude and between 10° and 160° in solar longitude. The relation of defrosting patterns with latitude and solar longitude seems to follow the shape of a thick arc, with its ends starting from LS = 0° and 56° latitude, having its arc peak value at latitude of 78° and

LS = 30°, and ending at latitude of 77° and LS = 145°. A dense and thick region of green crosses, dominate the central region of the arc, as can be seen in Figure 34. Many craters at high latitude, 75° poleward, are observed to have defrosting patterns early in spring at 0-40° in solar longitude. These patterns were expected later in the spring, because of the crater’s location close, or on, the northern seasonal polar cap. Two of the largest craters on the NPR, Korolev and Lomonosov have defrosting patterns late in winter, located at latitude of 73° and 65°, respectively. This is not unusual with respect to their location. Since defrosting patterns usually are observed during late winter and during the spring.

Figure 34. Green crosses represent observed defrosting patterns in all crater images investigated in this work, which were taken throughout two Martian years, 28 and 29.

6.5 Dark dune spots

Figure 35. DDSs on and next to dunes seem to emerge during the northern spring and the summer.

All craters in this work, with observable DDSs emerging on and next to the dunes, are illustrated in Figure 35 as with latitude and solar longitude. The black filled circles represent observable DDSs on the dunes in the crater, and the red crosses represent DDSs next to the dunes. DDSs seem to emerge on and next to dunes between 0-150° in solar longitude, i.e., during early northern spring until late summer, and located between 60° and 82° in latitude. However, these patterns on, and next to, dunes are more common during the middle and late spring season, LS = 0-70°, and are more densely located in 60° and 82° in latitude. In the craters 79N-62E, 77N-46E and 65N-182W, DDSs can be seen during the middle of the northern winter, at solar longitude 350°. When DDSs are observed to appear on the dunes during the spring and the summer, they are most likely observed next to dunes as well.

7 Discussion

For future Mars missions an understanding of the Martian environment is essential. This can be achieved by mapping and observing the craters on the Mars NPR. In return, knowledge of why the Mars environment looks like it does today will be gained, and a glimpse of the Martian future might be seen. By observing, for example the dust devils, scientific questions about the Martian climate will be answered, i.e., the cycles of erosion and sedi- mentation and the surface-atmosphere interaction. A better understanding of the Martian climate will benefit future Martian robotic missions, but as well possible human missions. Furthermore, understanding the water and carbon dioxide cycle will give knowledge of what might have happened with the Martian atmosphere.

7.1 Ice amount in all craters

At lower latitudes, less ice is observable in the craters comparable to higher latitudes. At higher latitudes it is more common to find craters having a large amount of ice during the entire Martian year. The ice amount in the observed craters follows a pattern, as on Earth. But in contrast to the Earth, ices on Mars sublime instead of melting. As the northern spring goes to summer, both water and carbon dioxide ice sublimes into the Martian atmosphere. As expected, almost all craters that are full of ice are located at high latitude, above 71°N in latitude. During the northern spring these craters are full of ice, but as the time goes, the ice sublimates. This is also true for craters that have more than 50% ice and less than 50% ice. However, these craters are more wide-spread, and depending on their size and depth, the ice amount within will be retained. For craters to be able to retain ice they need to be located higher in latitude or have a larger diameter size. Small, low-latitude craters are more likely empty during all Martian year. If they contain ice, this will be only during a short period of time during the winter, and when the temperature rises during the spring, the ice will sublimate. Those low-latitude craters with summer ice are, on average, larger in diameter compared to the average size of

81 the observed craters, e.g., Korolev and Lomonosov, where Lomonosov is the largest located at lower latitude. Impacts are often associated with planet formation and evolution. By analyzing the impacts, information of the subsurface can be reached. For example, the fluidized ejecta, which can be observed around some craters on Mars, suggest the presence of volatiles, such as water in the subsurface. By monitoring and analyzing the images of craters in the Martian NPR, especially ice covered craters like Korolev, a deeper understanding of how water behaves on the Martian

NPRC can be reached. But also what kind of influence it has on the CO2 cycle between the northern and southern hemisphere. To understand what might cause, for example, the change of the surface albedo in some craters, more images have to be taken in high resolution, with HiRISE, over different seasons. With these images the change of surface albedo can be monitored over time, and what relation it might have to temperature, could better be understood. Also, by understanding all features that are observed within some of the craters, knowledge of the prevalence of life might be better understood. In mapping the craters on the NPR, more aspects of the Martian history will be achieved, which in turn will be important for understanding the Martian future.

7.2 Dunes

Craters containing dunes seem to be sparsely located between 135 ° and 270° in longitude, and densely located between 0° to 90° and 270° to 360° in solar longitude. The craters studied in this work, located between 56° and 84° in latitude, are observed to contain dunes. There are extensive dune deposits surrounding the NPR. Between 75° and 80° in latitude the polar region is surrounded by crescentic dune formations. Linear and finely rippled dune formations are adjacent to the cleft of Chasma Boreale and to the Polar Lay- ered Deposit (PLD) in Olympia Planitia. There are not many craters located in the region between 75° and 80° in latitude. According to Figure 31, there are 16 craters located in this region, with the main body containing dunes. What kind of dunes they are has not been analyzed within this work, but their formations are most crescentic or linear. The topography around Martian craters may be a factor in whether or not the crater contains dune features. Some regions of the Martian surface have different composition

82 depending on the latitude. Due to the residual ice and the seasonal ice cap, the terrains are different compared to lower latitudes, with less or no ice deposits. This might influence if dunes can be created or not. Dunes were thought to be largely frozen in place on Mars over time, but sometimes they move like dunes usually do on Earth. If dune formations are due to dust storms, craters with dunes should be more densely located where there could be more dust storms in these specific regions. This is the case in Figure 32, where craters containing dunes and dust storms are located in the longitude range between 290° and 360° over the span of 60-73° in latitude. Craters containing dunes seem to be randomly and sparsely located over the Martian NPR over latitude. Analyzing Figure 30 there seems to be no dunes within craters with a diameter of less than 11 km. Depending on how deep these craters are, they may not have a high enough crater wall and rim to be able to hold on to the sand grains within them, from being blown away by the wind. Compared to deeper craters, a higher crater wall and rim will enclose the dust grains, hindering them from being blown away. The size of the crater could also have an influence. A larger crater with a large area will, most likely, be deeper and consequently have a higher crater wall and rim to enclose the sand. According to Figure 30, only one of the bigger craters has any dunes, Korolev. This crater could theoretically have dunes, but since the crater is more or less covered by ice the Martian year around, this cannot be confirmed. A crater with large diameter and a central peak points out to a high energy impact. Most likely a large diameter results in a deeper crater and that could be why smaller craters cannot trap dust into the crater. This could be one reason why we do not observe any dunes within these craters. Also, the formation of the crater impact might have an influence as well. Depending on with what velocity the impact strikes the surface, the crater floor will be composed differently and most likely, affect how and if dunes are formed. Last but not least, one more factor affecting dune formations are the winds and their direction on Mars, which creates and shapes the dunes. Craters that seldom experience winds probably do not contain any, or only have a very few, dunes in total.

7.3 Dust Devils

Throughout this work, dust devils have randomly been seen within the craters, often during late spring and the summer. The reason why we observed most of the dust devils to occur at this time of Martian year, on lower latitude, could be because of the higher surface temperature. The temperature difference between the surface and the atmosphere is large enough at this location to cause a rapid rise of hot air. As a result, this will give it a spin effect due to the conservation of angular momentum. As the regolith and the dunes, in the crater, are no longer ice covered during the spring and the summer, the tracks of the dust devils are more easily located, due to the fact that the winds more easily can move the dust particles and loose gravels. If the dust devils appear in craters with dunes, their tracks will become more apparent as loose grains of the dunes will be easily embedded in the vortices. Why craters containing dunes at higher latitude do not have any dust devils could be because of their higher latitude location. Those craters are covered by residual ice and have a lower surface and atmosphere temperature compared to craters at lower latitudes. Even if the conditions were right for dust devils to be created, they would likely not be seen because of the ice. Since the craters at higher latitude are covered by ice most of the year, there is no sand or gravel for the dust devils to leave any tracks on. When analyzing Figure 33 the dust devils that are observed in craters without dunes seem to appear in the middle of the northern spring and disappear in the end of the summer,

LS = 43-146°. This observation might support our theory about the temperature dependence of the existence of the dust devils. At early spring, late summer and early autumn it might be too cold for the dust devils to form. If the dust devils do not depend on the temperature, they should be seen in (high resolution) images taken at this time of year. Dunes could be a source for dust devils, leaving traces on nearby craters with no dunes. Unfortunately no such craters have been observed in this work. It is essential to find out more about the dust devils, i.e., how they appear, how they are affected and affect their surrounding environment, but also what kind of traces they leave on the Martian surface, more craters must be explored to be able to certify this theory. Still some questions are unanswered, for example, why do craters without dunes contain dust devils only at certain latitudes and longitudes, 60° to 68°, and 128° to 251°? What kind of influence do dunes have on the appearance of dust devils? By observing the 84 dust devils, a better knowledge can be achieved about the Martian surface, how the temperature varies, but also how the Martian winds blow. Our work has only scratched on the surface of what all the craters on the Mars NPR could contain. One of all features the crater can contain, explained in the subsections above, is the unexplored dust devils that leave black traces on the Martian surface.

7.4 Defrosting

Defrosting patterns are observed to appear during LS = 0-160°, as expected compared to similar seasons on Earth. Even though the resulting spread of crosses in Figure 34, over season was expected, the latitudinal spread as a function of LS was not. Too many craters at higher latitudes show an early signature of defrosting during early spring. These patterns were expected later in the spring, because of the crater location close, or at, the NPRC. The necessary thermal radiation received from the Sun needed to sublimate carbon dioxide ice and later water ice is expected to appear during the late spring or the beginning of the summer, compared with regions at lower latitude, if the Martian tilt of 25° is stable. Anomalous seasonal behaviour of ice in craters could also be explained by the type of ice covering it. Latitudinal variations in insulation drive the atmospheric circulation of water, carbon dioxide and airborne dust (Jakosky, 1992). These can affect the constituents of the ice, resulting in a mixture of water and carbon dioxide ice, perhaps with airborne dust, which settles down on the ice after seasonal dust storms. These factors in turn determine the stability characteristics of the ice (Snyder, 2005). When ice becomes old (as its grains grow in size) (Brown, 2009) or when dust settles down on the ice, it will become dark and then absorb more sunlight, thereby warming relatively quickly and defrosting more rapidly (Calvin, 2008). The likely factors controlling ice accumulation, sublimation and defrosting are the surface properties, the length of day and night, the distance to the Sun, the solar angle, latitude, altitude, clouds, seasonal dust storms and crater depth. These factors are described in more detail below. The crater diameter is possibly a factor that can control the crater floor temperature. The temperature decreases as the diameter, to, depth ratio increases, causing shadowed areas. (Ingersoll, 1992) Shadowed areas are also a result of the solar angle, the crater location and formation of the rim, i.e., if high or low.

Seasonal dust storms or local dust devils have airborne dust grains that absorb sunlight. These will heat the atmosphere locally, or the ice they have settled down on. The effect of clouds on surface temperature is the net effect of three things; sunlight reflecting from their top side, the greenhouse effect of absorbing and radiating back the thermal radiation of Mars surface and the reflection of the thermal radiation from the surface and back down to Mars. A large difference between the day and night side of the planet will cause the air to flow from the hot day side to the cool night side. These triggered winds, heated by the solar radiation, are called thermal tidal winds. Not only will ice rapidly sublime and accumulate by this effect, but also the heated winds will be a probable source of dust storm and dust devil formations. The length of the day and night will also influence the surface temperature. When Mars is near perihelion, when north has the winter, the days are much shorter at the NPR, and as a result colder. This is reversed during aphelion, when it is summer in the northern hemisphere.

7.5 Dark Dune Spots

Most of the DDSs (Kereszturi, 2009) seem to appear in the spring and the summer, and are most likely related to defrosting, as they appear during the spring and early summer. The first signs of emerging DDSs are observed during the early spring. In some craters, DDSs are observed to first of all appear next to dunes and then as solar longitude proceeds. The number of spots increases and they start to appear on the dunes as well. However, other craters show a different behaviour. DDSs are first observed on the dunes and then they either vanish or grow with proceeding solar longitude. Then they emerge on top of the dune peaks and slide down-hill as the spots increase. During the end of Martian spring until the end of the summer, hidden DDSs under the ice start to disappear and finish developing, as they emerge to the surface and start to react with the Mar- tian atmosphere. At this time, when no new dark features can be produced due to ice sublimation, the dunes are observed to become dark in colour, as how wet sand becomes on Earth. DDSs observed next to dunes appear intensely during the middle and end of the spring, over the span of LS = 20-70°. Only two craters have DDSs appearing during the

86 summer, which are 72N-215W and 65N-182W, located at latitude of 72° and 65°, respectively. Why do so few craters have DDSs appearing during the summer, and why do DDSs next to dunes disappear before those on the dunes? Can they be explained by the surface and low atmospheric temperatures, on the NPR? Why and how these formations occur is still not clear. One hypothesis suggested by Appel et al. (2010) suggests that these dark features are all based on a solid-state greenhouse effect. Their different appearances are explained as due to different terrains in which they occur. Depending on the composition of the dune, they consist of dark soil and sometimes finer grained soil that is bright when dry, but dark when mixed with water. During the beginning of the northern Martian spring the dunes have a layer of water ice with an additional layer of carbon dioxide ice. The surface is usually covered by a thin layer of bright, fine grained sand, which has been blown on top. (Appel, 2010)

Figure 36. An illustration, made by Appel (2009), of the step by step process of how a DDS emerges.

With spring the temperature rises and the dune receives direct sunlight. The carbon dioxide absorbs nearly none of the solar energy through direct absorption (Appel, 2010). However, water ice does absorb parts of the IR spectrum, and consequently heats up. The solar radiation will reach the darker soil, underneath the ice as well, which absorbs most of the solar energy and heats up faster than the surroundings. When the warmer soil heats the 87 water ice just above it, a liquid interfacial layer will form between the water ice and the soil and mix them together to a degree. (Appel, 2010) As illustrated in Figure 36Error! Reference source not found., the ice eventually melts up to the surface, after creating a hole where the CO2 ice layer is weakest. When the soil-water mixture seeps out through the hole, it will carry some of the dark dune materials with it, and create the dark dune spot. Water in the soil-water mixture will eventually evaporate and the deposited soil will remain. (Appel, 2010) If the disappearance of DDSs next to dunes is due to high temperatures during the Martian summer, then we should expect them to sustain in craters with dunes at high latitudes, above 70° in latitude. This is however, not the fact since Figure 34, shows only one crater, 72N-215W, following this trend. However, at a quite low latitude (65°N), the 65N-

182W crater has DDSs visible next to dunes during the middle of the spring, at LS = 132°. This is a quite strange behaviour, considering the theories stated above. The crater floor and the lower altitude atmosphere should experience a much higher temperature than those located on higher latitude. It should be too warm for these spots to survive, so why are they there? Maybe the crater experienced a shock of cold air, freezing the crater temporarily Martian year 29.

8 Future work

To understand and to observe change in ice deposits, and other features that have been observed within the craters, this work needs to be followed up in the future. Only over time, will we be able to answer all the questions that we have today. But also to see or to recog- nize different patterns around the northern hemisphere more craters need to be mapped. Only then, can we be more certain of how the ice changes. This work has analyzed almost 90 craters, which is only a small part of all the impacts on the northern hemisphere. Still there are craters with no images covering them, which then need to be mapped and analyzed. The crater impacts on the northern hemisphere are much fewer compared to the southern hemisphere. But in contrast to the southern hemisphere, the northern hemisphere is harder to observe due to the Martian tilt. For future science of the Martian northern hemisphere, information on all images of the craters has been stored, together with a small description in a database. The database is not yet official, but as the work continues to map all craters on the northern hemisphere, the database will eventually be accessible for all scientists. To make it easier for future work, the author will be working to put the database online on the NASA Ames homepage. It will then be accessible also on Dr. Adrian Brown’s homepage: http://abrown.seti.org/.

References

Appel, M. et al. (2010) 'Potential model for dark albedo features in the Martian polar region observed at 81°N 156°E', Lunar and Planetary Conference XXXXI, Houston, 1562.

Arizona State University (2008) Mars Space Flight Facility, Available: http://global- data.mars.asu.edu/ (29 October 2009).

Balme, M. et al. (2006) 'Dust devils on Earth and Mars', Geophysical Reasearch, 44, 1.

Bennett, J. et al. (2002) Life in the Universe. United Kingdom: Benjamin Cummings, ISBN: 978-0805385779.

Brown, A.J. et al. (2009) 'Louth Crater: Evolution of a layered water ice mound', Icarus, 196, 433.

Byrne, S. (2009) 'The polar deposit of Mars', Annual Review of Earth and Planetary Sciences, 37, 535.

Calvin, W.M. et al. (2008) 'Summer season variability of the north residual cap of Mars as observed by the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES)', Planetary and Space Science, 56, 212.

Chaplin, M. (2009) Water Properties (including heavy water data), Available: http://www1.lsbu.ac.uk/water/data.html (5 March 2010). de Pater, I. et al. (2001) Planetary Sciences, Cambridge: Cambridge University Press. ISBN 0-521-48219-4.

Delamere, A. et al. (2003) 'MRO High Resolution Imaging Science experiment (HIRISE): Instrument', Available: http://marsoweb.nas.nasa.gov/HiRISE/instrument. html#components (08 February 2010).

FedScope, OPM (2010) National Aeronautics and Space Administration, , Available: http://data.bestplacestowork.org/bptw/detail/NN00 (5 January 2010).

Gardiner, L. (2010) Windows to the Universe, Available: http://www.windows.ucar. edu/tour/link=/physical_science/chemistry/carbon_dioxide.html&cdp=/windows3&edu =high (10 March 2010). 91

Google (2010) Google Earth, , Available: http://earth.google.com/ (10 March 2010).

Grayzeck, E. (2007) Mars Fact Sheet, Available: http://nssdc.gsfc.nasa.gov/planetary /factsheet/marsfact.html (5 March 2010).

HiRISE (2007) Department of Planetary Science, Lunar and Planetary Laboratory, HiRISE & JPEG2000 Images, Available: http://hirise.lpl.arizona.edu/jp2.php (27 October 2009).

Humlun, O. et al. (2004) Geoindicators, Available: http://www.lgt.lt/geoin/ doc.php? did=cl_frozen (1 March 2010).

Ingersoll, A.P. et al. (1992) 'Stability of polar frosts in spherical bowl-shaped craters on the Moon, Mercury, and Mars', Icarus, 100, 40.

Jakosky, B.M. et al. (1992) 'The seasonal behaviour of water on Mars', in Kieffer, H.H., Jakosky, B.M., Snyder, C.W., Matthews, M.S. (eds.) Mars, The University of Arizona Press. ISBN 0-8165-1257-4.

Jakosky, B.M. (1998) The search for life on other planets, Cambridge: Cambridge University Press. ISBN 0-521-59165-1.

James, P. et al. (2003) 'The carbon dioxide cycle', Advances in Space Research, 35, 14.

Jet Propulsion Laboratory (2010) Mars Odyssey: Multimedia, Available: http://mars.jpl.nasa.gov/gallery/sanddunes/ (20 February 2010).

Jonas, D. (2008) Overview, 11 August, Available: http://www.nasa.gov/centers/ames/ about/overview.html (5 February 2010).

Kereszturi A. et al. (2009), 'Recent rheologic processes on dark polar dunes of Mars: Driven by interfacial water?', Icarus, 201, 492.

Malin, M.C. et al. (2007) 'Context camera investigation on board the Mars Reconnaissance Orbiter', Journal of Geophysical Research, 112, 1.

Mangimeli, J. (2010) Geology of sand dunes, Available: http://www.nps.gov /archive/ whsa/Sand%20Dune%20Geology.htm (12 February 2010).

Mars Global Color Image Mosaic (2003) U.S. Department of the interior, U.S. Geo- logical Survey.

McEwen, A.S. et al. (2007) 'Mars Reconnaissance Orbiter's high resolution imaging science experiment (HiRISE)', Journal of Geophysical Research, 112, 1.

Murphy, J. et al. (2000) 'Martian and terrestrial dust devils: Test of a scaling theory using Pathfinder', Journal of Geophysical Research, 105, 1859.

NASA (2010) Fiscal Year 2011, Budget estimates, Available: http://www.nasa.gov /pdf/420990main_FY_201_%20Budget_Overview_1_Feb_2010.pdf (5 February 2010).

NASA, Jet Propulsion Laboratory (2010) All About Mars, Available: http://mars program .jpl.nasa.gov/allaboutmars/extreme/martianyear/ (11 March 2010).

National Park Service (2000) Dune, Available: http://www.nature.nps.gov/geology/ usgsnps/dune/ dunefmtn1.gif (15 February 2010).

Pidwirny, M. (2009) Physical Geography, Chapter 10: Introduction to the Lithosphere, Available: http://www.physicalgeography.net/fundamentals/10ag.html (5 March 2010).

Portal, Arctic (2010) The International Permafrost Association, Available: http://ipa.arcticportal.org/index.php/component/option,com_morfeoshow/gallery,1/task , (10 March 2010).

Price, L.W. (1972) 'The Periglacial Environment, Permafrost, and Man', Association of American geographers, 14, 1, Available: http://eric.eds.gov/ERICDocs/data/ericdocs2 sql/content_storage_01/0000019b/80/37/b2/6f.pdf .

SETI (2010) Our Mission, Available: http://www.seti.org/ourmission (5 February 2010).

Snyder, H. et al. (2005) 'Monitoring the perennial Martian northern polar cap with MGS MOC', Icarus, 174, 502.

Strom, R.G. et al. (1992) 'The Martian impact cratering record', in Mars, Kieffer, H.H, Jakosky, B.M., Snyder, C.W., Matthews, M.S. (eds.), The University of Arizona Press. ISBN 0-8165-1257-4.

The Mars Climate Database (2007) Martian Seasons and Solar Longitude, Martian

Solar Longitude LS, Available: http://www-mars.lmd.jussieu.fr/mars/time/ solar_longitude.html (5 March 2010).

Thomas, P. et al. (1992) 'The Polar Deposits of Mars', in Mars, Kieffer, H.H, Jakosky, B.M., Snyder, C.W., Matthews, M.S. (eds.), 767, Univ. of Arizona Press, Tucson.

Titus, T.N. et al. (2008) 'Martian polar processes', in The Martian Surface: Composition, Mineralogy, and Physical Properties, Bell, J.F. (eds.), United Kingdom: Cambridge University Press. ISBN-13 978-0-511-41372-8.

Watanabe, S. (2005) NASA, MRO 'Follow the Water' Theme, Available: http://www. nasa.gov/mission_pages/MRO/multimedia/multispectral.html (10 March 2010).

Weatherquestions (2003) Dust Devils, Available: http://www.weatherquestions.com/ dust_devil.jpg (15 February 2010).

Wilson, J. (2010) What does NASA do, NASA, Available: http://www.nasa .gov/about/highlights/what_does_nasa_do.html (5 February 2010).

Xie, H. et al. (2008) 'A conceptual model for explanation of Albedo changes in Martian craters', Planetary and Space Science, 56, 887.

Zuber, M.T. et al. (1998) 'Observations of the north polar region of Mars from the Mars Orbiter laser altimeter', Science, 282, 2053.

Zurek, R. et al. (2007) 'An overview of the Mars Reconnaissance Orbiter (MRO) science', Journal of Geophysical Research, 112, 1.

Appendix A

All crater images examined are listed below in order of increasing latitude. The nameless craters are name given after its location on Mars. For example, crater 61N-312E is located at 61° in latitude and 312° in longitude. P17_007547_2429_XN_62N049W is the image ID, and one of the crater images listed below the crater name. Images taken by the CTX camera have a specific image ID, starting with a P and then a number, and can be found on the following home page: http://hirise.lpl.arizona.edu/anazitisi.php. Images taken by the HiRISE camera have a specific image ID starting with PSP, and can be found on the following home page: http://viewer.mars.asu.edu/planetview/inst/ctx/#start.

57N-351E, Kunowsky PSP_009721_2370 60N-313E P03_002179_2371_XI_57N009W P17_007626_2418_XN_61N047W P10_005080_2370_XN_57N010W P18_008127_2407_XN_60N046W P11_005225_2371_XN_57N008W P21_009274_2407_XN_60N046W P13_006214_2368_XN_56N010W P15_006860_2371_XN_57N009W 60N-129E PSP_006860_2370 P20_008727_2406_XN_60N230W PSP_002179_2375 P18_008081_2406_XN_60N230W P17_007646_2406_XN_60N230W 60N-222E P20_008793_2406_XN_60N230W P15_006957_2405_XN_60N138W B02_010362_2406_XN_60N230W PSP_008104_2405 P17_007603_2405_XN_60N138W 60N-101E P18_008104_2424_XN_62N138W P19_008340_2409_XN_60N101W PSP_010054_2410 60N-281E P16_007193_2409_XN_60N102W P17_007825_2405_XN_60N079W PSP_010396_2405 60N-148E P19_008326_2405_XN_60N079W P17_007553_2406_XN_60N213W PSP_008181_2405 PSP_008054_2405 P18_008181_2405_XN_60N079W P18_008054_2407_XN_60N212W

60N-251E 60N-90E P18_008037_2405_XN_60N109W P17_007832_2408_XI_60N270W P21_009274_2407_XN_60N046W PSP_007832_2410 P18_008127_2407_XN_60N046W P16_007186_2400_XN_60N270W P17_007626_2418_XN_61N047W P21_009269_2409_XN_60N270W 95

P21_009335_2409_XN_60N272W PSP_001908_2430 P18_008122_2400_XN_60N270W PSP_010228_2430 P20_008768_2408_XN_60N270W P18_008013_2431_XN_63N173W P22_009836_2408_XN_60N270W P18_008158_2431_XN_63N173W B02_010403_2408_XN_60N270W 63N-239E 60N-88E P16_007431_2423_XN_62N122W P21_009335_2409_XN_60N272W P17_007576_2424_XN_62N121W P18_008122_2400_XN_60N270W P18_008222_2424_XN_62N121W P20_008768_2408_XN_60N270W P22_009836_2408_XN_60N270W 63N-12E B02_010403_2408_XN_60N270W PSP_008204_2440 P19_008333_2409_XN_60N272W P16_007202_2440_XN_64N348W P17_007542_2409_XN_60N272W P17_007558_2439_XN_63N349W P16_007397_2409_XN_60N272W B02_010535_2409_XN_60N272W 63N-292E PSP_007429_2440 61N-312E P16_007429_2440_XN_64N067W P17_007547_2429_XN_62N049W PSP_007574_2440 P18_008193_2416_XN_61N048W PSP_007640_2440 PSP_007547_2415 T01_000875_2440_XI_64N067W P18_008141_2440_XN_64N067W 61N-22E PSP_008141_2440 P17_007729_2417_XN_61N338W P19_008375_2418_XN_61N338W 64N-296E P15_007007_2440_XN_64N064W 61N-308E P17_007508_2440_XN_64N064W P15_006967_2418_XN_61N051W PSP_007508_2440 PSP_007613_2420 P18_008154_2440_XN_64N064W PSP_006967_2420 P17_007613_2424_XN_62N052W 64N-132E P17_007659_2438_XN_63N228W 61N-229E PSP_007659_2440 P19_008341_2420_XN_62N131W P15_007013_2437_XN_63N228W

62N-6E 64N-234E P17_007545_2422_XN_62N354W P17_007563_2446_XN_64N126W P19_008336_2422_XN_62N353W P17_007690_2422_XN_62N353W 65N-210E P17_007788_2450_XN_65N150W 62N-222E P16_007142_2449_XN_64N152W P18_008104_2424_XN_62N138W P18_008249_2429_XN_62N138W 65N-351E, Lomonosov P17_007585_2450_XN_65N007W 63N-187E P16_007440_2450_XN_65N008W PSP_010439_2430 P13_006227_2449_XN_64N007W PSP_009806_2430 B01_009866_2451_XN_65N009W P17_007657_2432_XN_63N172W P17_007730_2454_XN_65N006W P17_007512_2432_XN_63N173W P15_007005_2452_XN_65N010W P02_001908_2431_XI_63N173W PSP_002047_2450 96

P03_002047_2451_XI_65N008W P20_008752_2466_XN_66N196W P17_007651_2452_XN_65N009W P18_008238_2465_XN_66N196W PSP_010644_2455 PSP_008172_2465 PSP_010011_2460 P15_007025_2465_XN_66N196W P19_008297_2451_XN_65N009W P18_008119_2420_XN_62N189W PSP_007440_2455 66N-144E 64N-31E PSP_008120_2465 P17_007755_2451_XN_65N329W P18_008120_2477_XN_67N216W P15_006973_2465_XN_66N216W 65N-178E P15_006762_2466_XN_66N215W B01_010136_2453_XN_65N182W PSP_010136_2455 67N-252E P18_008066_2453_XN_65N182W PSP_007615_2470 PSP_010413_2455 P18_008116_2474_XN_67N108W PSP_002027_2455 P17_007710_2455_XN_65N180W 66N-40E P03_002027_2453_XI_65N182W P16_007135_2465_XN_66N320W P13_006207_2454_XN_65N181W P17_007491_2465_XN_66N321W P17_007565_2453_XN_65N182W P17_007636_2465_XN_66N320W P17_007781_2465_XN_66N320W 65N-284E P18_008071_2465_XN_66N320W P15_006836_2457_XN_65N076W P17_007693_2455_XN_65N076W 67N-114E P17_007627_2469_XN_66N076W P22_009545_2472_XN_67N246W PSP_010053_2455 P20_008912_2473_XN_67N246W PSP_008912_2470 65N-339E P16_007185_2473_XN_67N246W P16_007335_2448_XN_64N020W P17_007541_2478_XN_67N247W B02_010473_2458_XN_65N021W P18_008253_2473_XN_67N246W P17_007691_2446_XN_64N021W 67N-250E P18_008182_2474_XN_67N110W 65N-330E P18_008037_2474_XN_67N111W P03_002417_2456_XI_65N030W P16_007180_2474_XN_67N111W P15_006953_2462_XN_66N030W P18_008100_2462_XN_66N030W 67N-98E PSP_010236_2460 P17_007502_2472_XN_67N261W P15_006953_2462_XN_66N030W P16_007212_2472_XN_67N263W B02_010236_2459_XN_65N030W P16_007357_2472_XN_67N262W PSP_008456_2460 67N-223E 65N-128E P19_008328_2480_XN_68N137W P17_007580_2473_XN_67N231W P16_007392_2479_XN_67N138W P18_007936_2456_XN_65N232W P17_007682_2479_XN_67N137W P18_008147_2456_XN_65N231W P20_008661_2456_XN_65N231W 68N-93E B02_010507_2456_XN_65N231W P17_007786_2919_XN_68N267W P16_007435_2452_XN_65N231W B01_009915_2481_XN_68N267W P19_008346_2481_XN_68N267W 66N-163E P15_006988_2480_XN_68N267W 97

P17_007489_2480_XN_68N267W P19_008310_2502_XN_70N008W PSP_009915_2480 P16_007229_2530_XN_73N009W P21_009348_2481_XN_68N267W P15_007084_2501_XN_70N009W P13_006161_2502_XI_70N007W 68N-236E, Heimdal P17_007519_2502_XN_70N007W P18_008077_2482_XI_68N124W P22_009457_2502_XN_70N008W PSP_010299_2915 P18_008165_2502_XN_70N008W PSP_010358_2485 PSP_009457_2505 PSP_009778_2485 P16_007374_2502_XN_70N007W P22_009646_2484_XI_68N125W PSP_009734_2505 P22_009580_2485_XI_68N124W PSP_010512_2505 PSP_009580_2485 PSP_010868_2505 P21_009435_2484_XN_68N125W PSP_008165_2505 P21_009092_2484_XI_68N124W P21_009079_2485_XI_68N124W 70N-103E, Louth P19_008433_2484_XI_68N124W PSP_006869_2505 P16_007431_2482_XI_68N124W PSP_010587_2500 PSP_010714_2485 PSP_009242_2505 P17_007787_2481_XI_68N125W PSP_008886_2505 P20_008965_2504_XN_70N255W 68N-190E PSP_008952_2500 PSP_010241_2485 PSP_007515_2500 P18_008171_2486_XN_68N170W P01_001370_2503_XI_70N257W P18_008026_2486_XN_68N170W PSP_009453_2500 P17_007525_2486_XN_68N170W PSP_001370_2505 P16_007380_2486_XN_68N170W PSP_001700_2505 P02_001700_2506_XI_70N256W 68N-13E PSP_006737_2505 P17_007571_2487_XN_68N347W P15_006737_2504_XI_70N256W B02_010353_2487_XN_68N347W PSP_007449_2505 P20_008718_2486_XN_68N347W PSP_007159_2505 P18_008072_2487_XN_68N347W PSP_008741_2505 PSP_007571_2490 PSP_007739_2505 PSP_007805_2505 69N-26E P17_007805_2504_XN_70N256W P18_008177_2491_XN_69N333W PSP_007950_2505 P16_007175_2492_XN_69N332W PSP_008095_2500 PSP_008757_2490 PSP_008161_2505 P20_008757_2491_XN_69N333W PSP_008240_2500 PSP_008530_2505 70N-285E P15_006803_2505_XN_70N257W PSP_008352_2500) PSP_007153_2505 P17_007561_2500_XN_70N075W P16_007153_2505_XN_70N093W P19_008273_2500_XN_70N075W P04_002617_2505_XI_70N092W PSP_009935_2505 69N-41E PSP_009031_2505 P19_008361_2496_XN_69N318W P16_007214_2496_XN_69N318W 70N-267E PSP_007153_2505 70N-352E P16_007153_2505_XN_70N093W 98

P04_002617_2505_XI_70N092W P22_009754_2529_XN_72N195W PSP_009935_2505 P16_007473_2538_XN_73N193W PSP_009031_2505 PSP_007961_2530 P16_007364_2505_XN_70N095W P18_007961_2529_XN_72N197W PSP_009803_2505 P20_008765_2530_XN_73N193W P17_007799_2513_XN_71N093W P20_008831_2529_XN_72N195W PSP_007799_2505 P20_008963_2529_XI_72N195W P21_009042_2528_XI_72N197W 71N-194E P03_002291_2530_XN_73N193W P16_007182_2513_XN_71N165W P22_009477_2530_XN_73N193W P15_007037_2509_XN_70N166W P15_007038_2530_XN_73N194W P16_007393_2508_XN_70N167W PSP_010387_2530 P17_007683_2513_XN_71N166W B02_010387_2529_XN_72N195W P18_008184_2510_XN_71N166W P21_009332_2529_XN_72N194W PSP_006892_2510 PSP_001592_2530 P17_007816_2528_XN_72N198W 72N-345E P01_001592_2530_XI_73N195W PSP_010077_2520 P18_008073_2520_XN_72N015W 73N-22E P17_007572_2520_XN_72N015W PSP_007597_2530 PSP_007572_2520 P17_007597_2532_XN_73N338W

72N-146E 73N-2E P17_007566_2523_XN_72N214W P15_006833_2537_XN_73N357W PSP_010203_2525 PSP_006833_2540 P18_008133_2522_XN_72N214W P19_008270_2537_XN_73N357W

72N-144E 73N-178E P16_007197_2524_XN_72N215W PSP_007499_2535 P17_007698_2526_XN_72N216W P17_007499_2535_XN_73N181W P18_008199_2526_XN_72N215W P18_008145_2536_XN_73N181W P20_008845_2526_XN_72N215W P18_008079_2536_XN_73N181W P21_009346_2526_XN_72N215W 74N-187E 73N-38E, Lonar P16_007380_2540_XN_74N173W PSP_007715_2530 P17_007736_2540_XN_74N174W PSP_001966_2530 P17_007670_2541_XN_74N172W P17_007755_2531_XN_73N333W P19_008382_2540_XN_74N173W P02_001966_2531_XI_73N321W P17_007715_2532_XN_73N321W 74N-320E P22_009719_2531_XI_73N321W P18_008153_2544_XN_74N040W P22_009798_2530_XI_73N321W P17_007507_2543_XN_74N040W P15_006795_2544_XN_74N040W 73N-27E P19_008443_2544_XN_74N040W P17_007755_2531_XN_73N333W P16_007151_2552_XN_75N041W P19_008335_2532_XN_73N332W 74N-347E 73N-165E, Korolev P16_007150_2543_XN_74N013W P15_006959_2528_XN_72N196W P18_008086_2544_XN_74N012W P13_006181_2527_XN_72N195W P18_007941_2543_XN_74N012W 99

P17_007748_2563_XN_76N144W

74N-13E 77N-196E P17_007584_2549_XN_74N346W B01_009977_2580_XI_78N166W P13_006292_2569_XN_76N349W P17_007696_2568_XN_76N165W PSP_007584_2550 P16_007195_2577_XN_77N165W P18_008178_2829_XN_77N164W 70N-14E P17_007650_2503_XN_70N346W 77N-89E PSP_007650_2505 P22_009638_2573_XN_77N272W P18_008151_2502_XN_70N346W P20_008715_2567_XN_76N271W P16_007133_2566_XI_76N270W 75N-15E P16_007259_2832_XN_76N271W P18_008164_2553_XN_75N345W P17_007568_2567_XI_76N270W P17_007729_2557_XN_75N344W PSP_007779_2570 PSP_009087_2550 P18_007924_2551_XN_75N269W P21_009087_2550_XN_75N345W P18_008069_2569_XN_76N270W P19_008359_2567_XI_76N270W 75N-340E P15_006922_2569_XN_76N270W P15_006992_2551_XN_75N020W P19_008570_2567_XI_76N270W P20_008713_2848_XN_75N019W P13_006172_2582_XN_78N314W P20_008860_2566_XI_76N270W 75N-158E PSP_008926_2575 P17_007526_2556_XN_75N201W P20_008939_2572_XN_77N270W P20_008667_2844_XN_75N201W P13_006238_2573_XN_77N315W B02_010308_2568_XI_76N202W P17_007596_2568_XN_76N313W P17_007592_2564_XN_76N202W P18_008031_2567_XN_76N313W P22_009726_2833_XN_76N313W 76N-333E P19_008551_2832_XI_76N270W P16_007335_2564_XN_76N026W P15_006711_2568_XI_76N270W P17_007691_2564_XN_76N027W P14_006566_2568_XN_76N270W P18_008126_2564_XN_76N027W P13_006210_2576_XN_77N271W P13_006144_2577_XN_77N269W 76N-159E P02_001819_2570_XN_77N270W P17_007526_2556_XN_75N201W P02_001687_2575_XN_77N269W P17_007592_2564_XN_76N202W P15_006777_2569_XI_76N270W B02_010308_2568_XI_76N202W 77N-46E 77N-305, Escorial P13_006172_2582_XN_78N314W P13_006202_2559_XN_75N051W P20_008860_2566_XI_76N270W P13_006189_2558_XN_75N056W PSP_008926_2575 P20_008773_2570_XN_77N054W P20_008939_2572_XN_77N270W P16_007191_2561_XN_76N054W P13_006238_2573_XN_77N315W P15_006980_2590_XN_79N056W P17_007596_2568_XN_76N313W P18_008031_2567_XN_76N313W P22_009726_2833_XN_76N313W 77N-215E, Dokka P13_006245_2587_XN_78N148W P15_006746_2572_XN_77N145W 78N-52E P13_006179_2582_XN_78N145W P13_006264_2574_XN_77N305W P17_007682_2573_XN_77N143W P16_007187_2581_XN_78N307W 100

P18_008110_2581_XN_78N307W 81N-255E, Boola T01_000857_2581_XI_78N307W P01_001496_2603_XN_80N103W P17_007667_2614_XN_81N105W 78N-347E P18_008097_2787_XN_81N105W P01_001506_2600_XN_80N015W P17_007664_2585_XN_78N012W 81N-191E, Jojutla P16_007374_2595_XN_79N015W P01_001427_2787_XI_81N170W TRA_000865_2615 79N-331E, Inuvik P18_008131_2616_XN_81N169W P02_001665_2596_XN_79N031W PSP_008131_2615 TRA_000860_2585 P18_008065_2616_XN_81N170W P15_006979_2587_XN_78N028W P02_001723_2610_XN_81N176W P17_007625_2599_XN_79N030W P21_009299_2784_XN_81N169W P18_008028_2813_XN_78N028W P20_008877_2784_XN_81N169W B02_010407_2587_XN_78N028W P18_007941_2785_XN_81N167W P21_009062_2597_XN_79N029W P20_008693_2587_XN_78N028W 81N-117E PSP_008416_2585 T01_000894_2622_XN_82N243W P18_008168_2786_XN_81N242W 79N-240E HRL0000AB3B_07_IF185L_TRR2 P13_006178_2578_XN_77N118W .LBL P18_008103_2582_XN_78N120W PSP_008103_2580 82N-77E, Udzha P17_007602_2582_XN_78N120W P17_007700_2602_XN_80N279W P17_007563_2779_XN_82N283W 78N-41E P20_008992_2618_XI_81N282W P13_006225_2581_XN_78N321W T01_000803_2620_XN_82N280W P18_008163_2581_XN_78N320W P02_001702_2572_XN_77N318W 82N-299E Crotone P17_007728_2581_XN_78N320W P18_007903_2611_XN_81N066W PSP_007583_2580 PSP_009986_2625 P17_007837_2611_XI_81N064W 79N-324E, Sevel P02_001653_2623_XN_82N070W P22_009537_2612_XI_81N039W P21_009115_2606_XN_80N036W 84N-139E, Puyo P18_007889_2603_XN_80N039W B01_010203_2601_XN_80N221

79N-62E P13_006290_2583_XN_78N296W P13_006145_2572_XN_77N296W P13_006211_2582_XN_78N299W P14_006501_2583_XN_78N297W P15_006857_2592_XN_79N299W P18_008183_2808_XN_79N299W P20_008795_2592_XN_79N299W P02_001688_2617_XI_81N303W P22_009507_2586_XN_78N298W B02_010496_2592_XN_79N299W PSP_008426_2595

101

Appendix B

Map over the Northern Polar Region of Mars

Figure A1. Polar stereographic projection of the NPR of Mars. Planetographic latitude and west longitude coordinate system is shown in red. Planetocentric latitude and east longitude coordinate system is shown in black. (Mars Global Color Image Mosaic, 2003)

102

Appendix C

Elevation change as a function of solar longitude

Figure C1. The Martian elevation changes, in meters, vs. solar longitude, in degrees (Zuber, 1998).

103

Attachment

On the following pages are two abstracts for the Lunar Planetary Science Conference 2010. The first one describes expected and unexpected seasonal variations of the amount of ice in seven Martian craters. The next one describes the seasonal change of ice and other features, in Korolev crater, during the summer months.

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MONITORING SEASONAL BEHAVIOR OF ICES IN THE CRATERS IN THE MARTIAN NORTHERN POLAR REGION WITH CTX AND HIRISE. M. Hajigholi1, S. A. M. Bertilsson2, A. J. Brown3, C. P. McKay4 and S. Fredriksson5 1Department of Physics, Luleå University of Technology, 971 87 Luleå, Sweden, mi- [email protected], 2Department of Physics, Luleå University of Technology. 3SETI Institute, 4NASA Ames Research Center, 5 Department of Physics, Luleå University of Technology.

Introduction: To better understand the behavior of age observation, the crater rim status (clear, diffuse or ice in craters on Mars, seven Martian craters in the none), the amount of ice (None, less than 50 %, more Northern Polar Region (NPR) have been monitored than 50 % and full), unusual features, crater diameter, over a range of latitudes (60-90°). Each crater has location and a description of the area are recorded. been examined during varying solar longitude, during Criteria used for crater selection: Our first analy- which the amount of ice has shown both expected and sis using the database included seven craters chosen unexpected seasonal variations. throughout the Martian NPR, creating better data cov- CTX and HiRISE: The Context Camera (CTX) on erage. We have chosen to focus on craters with diame- the Mars Reconnaissance Orbiter (MRO) (orbiting ters ≥ 10 km. The crater should be covered by images Mars since 2006) is a camera providing black and from at least two different seasons. In all studied im- white context images of the Martian surface. These ages, the crater should be clearly visible and easily CTX images are used as a complement for the High identifiable i.e. no clouds or dust storms should ob- spatial Resolution Imaging Science Experiment (Hi- scure the crater area. RISE) camera. CTX has a spatial resolution of 6 m/pixel [1] and HiRISE 0.25 to 1.3 m/pixel [2]. Analysis: A way of determining ice coverage in The crater images we investigated from CTX and craters is by comparing images from different seasons. HiRISE were downloaded from the public internet web In the case of a fully covered crater it is quite easy to site provided by Arizona State University [3]. see, as the inner crater wall is white with frost. As ice Crater mapping: We are creating a database coverage disperses we see called Information on Craters in the Martian Northern darker spots emerge and the Polar Region. 75 Craters are monitored over, using underlying Martian surface images acquired between 2006 and 2008. The craters becomes more prominent. monitored are located poleward of 60° in latitude, with When dunes are present in the the help of CTX and HiRISE images and Google Earth crater, they become visible (a tool we used for locating the craters on Mars and to when ice sublimes from their measure the crater diameter). immediate vicinity. In order to Every crater has a set of images with data, such as categorize the craters we have image ID, image location, acquisition date, solar longi- divided the coverage into four tude and website the image can be downloaded from levels; full, >50%, <50% and which are recorded in the database. In addition to im- empty.

Figure 2. The 34 km Crater C, in solar longitude sequence. A:

=76.42, B: =81.37 and C: =86.32. We can see the behavior in the amount of ice coverage during the spring season going from (A) more than Figure 1. The ice coverage of the craters on the Martian northern polar region over different latitude as a function of 50 % to (B) full to (C) less solar longitude. The red markings indicate when defrosting patterns are visible for Korolev and Dokka, which are than 50 %. completely covered by the seasonal ice cap during a Marsian Year. the ice coverage over different latitude. An expected result is that the ice coverage in crater increases with latitude from solar longitude 135° to 360° and decreases from longitude 0° to 135°. This is true in figure 1 for crater F (Kunowsky), A, E and D located in figure 3. Seasonal ice coverage was variable for crater C, (Figure 2). During the late spring of Mars Year 29 [5] the ice in this crater accumulates, from containing more than 50 % ice to being fully covered and then the ice disperses to less than 50 %. Conclusions: Anomalous seasonal behavior of ice in craters could be explained by the type of ice covering it. Latitudinal variations in insulation (which drives the atmospheric circulation of water, carbon dioxide and airborne dust [6]) affect the constituents of the ice, resulting in a mixture of water and carbon dioxide ice, perhaps with airborne dust which has settled on the ice after seasonal dust storms. When ice becomes dark due to dust it will absorb more sunlight, thereby warming relatively quickly and defrosting more rapidly [8].These factors in turn determines the stability characteristics of the ice [7]. Our observations suggest the ice in crater C experiences two periods of accumulation during the spring and winter season, in Mars Year 29 [5]. This could be due to unusual events but also may be a periodic behavior as the Martian season turns into summer and winter. At this stage we cannot verify either hypothesis however we will be looking at other craters at similar latitudes for similar accumulation periods. The likely factors controlling ice accumulation are the length of day and night, distance to the sun, the solar angle, latitude, altitude, clouds and seasonal dust storms. Craters on lower latitude than Korolev crater and on higher latitude than Dokka crater do not show ob-

Figure 3. (Top) The Martian northern polar region (map from Google vious evidence of ice accumulation. One conclusion Earth) with the 7 monitored craters pointed out, with the locations as we can draw from this is that over and under roughly follows: A: 186.81°E- 62.53°N; B: Korolev, 164.5°E-73.0°N; C: 73° and 77° latitude, the ice coverage in craters is 89°E-76.5°N; D: 62°E-79°N; E: 266.55°E-70.16°N; F: Kunowsky, 350.58°N-56.50°E and G: Dokka (not pictured), 214.46°E-77.10°N . more stable. The craters on lower latitude contain less Five CTX images below the map represents the craters A ( = ice than those on higher latitude during a Martian year. 40.13 °) (P17_007512_2432_XN_63N173W), B ( = 55.67 °) (P18_ We intend to look at craters similar to crater C in order 007961_2529_XN_72N197W), F ( = 16.72 °), (P15_006860_2371_XN_ to investigate whether ice coverage fluctuates during 57N009W), D ( = 84.1 ° ) (P20_008795_2592_XN_79N299W) and E ( = 27.4 ° ) (P16_007153_2505_XN_70N093W) from the same sea- spring time as we are observing in crater C. References: [1] Malin, M. C. et al. 2007. JGR 112 son, the Martian northern spring i.e. 0° ≥ ≤ 90°. doi:10.1029/2006JE002808. [2] McEwen, A. S. et al. During winter, five of our seven craters are com- 2007. JGR 112 doi:10.1029/2005JE002605. [3] pletely covered by the seasonal ice cap. The ~84 km http://global-data.mars.asu.edu/. [4] Bertilsson S. A. in diameter Korolev crater on ~73° latitude is perma- M. et al. 2009. LPSC XXXXI, this meeting. [5] nently covered by residual water ice [4]. This is also http://www.mars.lmd.jussieu.fr/mars/time/martian_tim true for the ~45 km in diameter crater Dokka e.html. [6] Jakosky B. M. and Haberle R. M. 1992. at ~77.16° latitude. Only one of the craters monitored SAO/NASA Astrophysics Data System, 969-1016, had an image which could be classified as having abso- doi:1992mars.book..969J. [7] Snyder Hale A. et al. lutely no ice in the whole crater, which was during the 2005. Icarus 174 502-512, Martian summer and at a quite low latitude (~62.5°). doi:10.1016/j.icarus.2004.10.033. [8] Calvin W. M et All the craters have many images from the spring al. 2008. PSS 56 212-226. season, showing expected and unexpected results of 41st Lunar and Planetary Science Conference (2010) 1569.pdf

MONITORING THE KOROLEV CRATER ON SPRING AND SUMMER IMAGES IN THE MARTIAN NORTHERN POLAR REGION WITH CTX AND HIRISE. S. A. M. Bertilsson1, M. Hajigholi1, A. J. Brown3, 4 1 1 C. P. McKay , S. Fredriksson . Department of Physics, Luleå University of Technology 971 87 Luleå, Sweden, 3SETI Institute, Mountain View, CA 94043, 4NASA Ames, Moffett Field, CA, 94035. Email: [email protected]

Introduction: Change in albedo during spring and spring and summer, ordered by Ls. The crater should summer of the water ice covered crater Korolev have be clearly visible in all studied images and easily iden- been previously reported. According to Armstrong et tified, i.e. no clouds or dust storms obscuring the cra- al. [1] Korolev exhibits an increase in summer time ter. For information about Korolev crater all year albedo, which they linked to water ice condensing around the reader is referred to M. Hajigholi et al. [5]. during the summer months. Analyzing images of Ko- rolev crater from different solar longitude, Ls, during spring and summer can help us understand how water behaves in the Martian Northern Polar Region, NPR. This work will analyze the images from CTX and

HiRISE to map water ice and seasonal change in Koro-

lev crater during northern spring and summer.

Location: Korolev crater is one of the northern

Martian lowlands largest craters, roughly circular with

an ~80 km diameter, containing significant ice rich

material [2]. Figure 1 shows the location of Korolev

crater relative to the residual cap at 73°N 165°E.

Figure 2. Korolev crater at different solar longitudes, Ls. Showing (left to right) how the albedo of the surface changes with time.

P01_001592_2530_XI_73N195W, Martian Year 28,

P20_008831_2529_XN_72N195W, Martian Year 29, P20_008963_2529_XI_72N195W, Martian Year 29,

P21_009332_2529_XN_72N194W, Martian Year 29, P22_009754_2529_XN_72N195W, Martian Year 29, Analysis: In Figure 2 images of Korolev crater are B02_010387_2529_XN_72N195W, Martian Year 29 ordered in solar longitude from L =142.2°, northern Figure 1. Topography map of Martian northern polar cap showing s summer in Martian Year (MY) 28 to L =141.4°, north- the location of Korolev crater at 73°N 165°E. s Source: Viking Color Image. ern summer, MY 29. The images are taken by CTX

and show how a darker color from the left bottom and HiRISE and CTX: HiRISE, High Resolution Im- middle right corners change with time over the crater. aging Science Experiment, is a high resolution camera Figure 3 shows four zoomed in images at the same riding on MRO. It commenced operations in 2006 and area in the middle of the crater, from Figure 2, is a 0.5 m reflecting telescope which give a colored L =142.2° in MY 28, L =85.33° in MY 29, L = 89.86° (red, green and IR) and detailed resolution of 0.25 s s s in MY 29 and L =141.4° in MY 29. We have observed meter per pixel [3]. CTX is a context camera designed s dark lineated features covering the icy surface in to obtain grayscale images with a resolution of 6 meter spring which later disappear in the summer. This beha- per pixel and a swath width of 30 km [4]. vior may be related to the albedo changes observed by Korolev Data: With help of Google Mars, we Armstrong et. al. [6]. have identified eight CTX images and three HiRISE Figures 4 – 8 show images from CTX and HiRISE images covering Korolev from northern spring to (from top to bottom) located at 73°N 165°E, taken at summer. We have created a database (Information on the same solar longitude, L =142.2° in MY 28. Figure craters in the Martian Northern Polar Region) [5], to s 5, a HiRISE image, shows how the surface in the mid- store information about craters from the Martian north- dle of the crater has smooth rippled terrain with darker ern polar region. When selecting the images of Koro- lines of roughened terrain [7]. This could be due to a lev crater the following criteria have been utilized. The surface with a mixture of different material and layers images of the crater should be over different seasons, with an icy regolith underneath. According to 41st Lunar and Planetary Science Conference (2010) 1569.pdf

Armstrong [6] a thermal pulse traveling through the misphere can be reached. To understand what might regolith is capable of releasing water vapor from the cause the change of the surface albedo in Korolev, icy regolith. This water vapor can then supply as a more images has to be taken in high resolution with local reservoir that can condense in mid summer. HiRISE over different seasons to monitor the change over time and what relation the change might have due to temperature when comparing high resolution images with Thermal Emission Spectrum, TES, observations and thermal models. Figure 4: Right Top image, P01_ 001592_2530_XI_73N195W, is an overview CTX image located at 73°N 165° E taken at solar longitude 142.2°, MY 28

Figure 5: Left Top image is an en- larged version of the high resolution Figure 7 image.

Figure 6: Left Bottom image is zoomed in image of the Right Top (red circle) image

Figure 3. Images zoomed in to the same region in the middle of the Figure 7: Bottom Center image is a crater ordered by L . Left to right top images: s HiRISE image, PSP_001592_2530 P01_001592_2530_XI_73N195W, MY 28, taken at the same time and solar longi- P20_008831_2529_XN_72N195W, MY 29, Left to Right bottom tude as above. images: P20_008963_2529_XI_72N195W, MY 29,

B02_010387_2529_XN_72N195W, MY 29 Figure 8: Right Bottom image is zoomed in image of Figure 7 (red The higher albedo of ice allows the light to pene- circle). trate ice relatively easily. Depending on what might be underneath the high albedo surface, the sub-surface will absorb the sunlight and cause the covered material to heat up. This “solid state greenhouse” effect will eventually cause the overlying water ice to vaporize (from below) but if this process is incomplete it might temporarily leave behind a "remnant material". De- pending on what kind of material could be stored under the ice, the surface may have a lower albedo, creating the darker lines seen on the CTX and HiRISE images, Figures 4 – 8. To find out if these black lines might change the color of the surface over time, images from the same solar longitude but in MY 29, have been studied from CTX and HiRISE. Unfortunately the images taken by HiRISE do not show this area of the crater and the theory can therefor not be confirmed. Looking at CTX images from spring at the same solar longitude as above, the black lines can be seen. This might indicate that the black lines are not responsible for how the surface albedo changes. If the change of References: [1] Armstrong J. C. et. al (2004) LPS the crater surface depends on solar longitude, a daily XXXV Abstract #1744. [2] Russell P. S. et al. (2004) variation or due to temperature changing within the LPS XXXV Abstract #2007. [3] McEwen A. S. et. al. crater remains to be solved. (2007), JGR, 112, doi:10.1029/2005JE00265. Conclusions: By monitoring and analyzing the im- [4] Malin M. C. et. al. (2007), JGR, 112, ages of craters in the Martian northern polar region, doi:10.1029/2006JE002808. [5] Hajigholi M. et. al. especially ice covered craters like Korolev, a deeper (2009), LPSC XXXXI, this meeting. [6] Armstrong, understanding of how water behaves on the Martian J.C. and Titus, T.N. (2005) Icarus 174, 360–372. northern polar cap and what kind of influence it has on [7] Brown A. J. et. al. (2007) LPS XXXXI, Abstract the CO2 cycle between the northern and southern he- #2262.