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ARTICLE

MARS EXPLORATION TODAY

JEREMIE LASUE1,2

Since the beginning of the space age with the launch and success of Sputnik 1 in 1957, the exploration of our neighbouring planets has demonstrated the diversity of environments available around the Sun. More recently, the current extent of exoplanet discoveries — with more than a thousand planets confirmed by the mission in January 2015 — has lead us to a better understanding of how the planetary systems and planets within them are formed. It also made us realise that other habitable worlds may be quite common. But as we tackle the complexity of our own planet, a definite answer to the questions of how early Earth originated and evolved to support life requires a deeper understanding of the solar system as a whole, and more specifically how evolved so differently. Whether Mars harbours life or did in the past remains the most important driver of Mars exploration.

Our Most Similar Neighbour valley networks, polar caps and many others. Like Earth, it has an atmosphere with clouds and sometimes dust n our solar system, Mars is well known as the fourth storms. However, the present average atmospheric pressure planet from the Sun. It is located at the boundary is low relative to Earth’s, and the atmosphere is mostly between the realm of the rocky, terrestrial planets that I made of (95%). The polar caps make up the inner solar system and the outer gaseous and contain principally water ice, and many water related icy planets that reflect the decrease in temperature as one gets further from the central star. That boundary is best TABLE 1: Comparison Between represented by the frost line where water molecules can Earth and Mars Properties condense (estimated to be around 5 times the distance Earth Mars between the Sun and the Earth) Distance from the Sun (million km) 150 228 On top of being close enough from Earth to allow its –2 exploration by spacecrafts with relative ease, amongst all Solar irradiance (Wm ) 1367.6 589.2 the planetary objects that can be found around the Sun, Obliquity (o) 23.4 25.2 Mars is arguably the one that presents the conditions that Day length 24 hours 24h37m(=1sol) are closest to the ones found on our own planet. As Year length (days) 365.26 686.98 presented in Table 1, Mars orbital parameters, size and Diameter (km) 12756 6779 global characteristics are close to the ones of Earth. The Figure 1 illustrates the large variety of surface features on Density (g.cm–3) 5.5 3.9 Mars. These include impact craters, volcanoes, canyons, Gravity (m.s–2) 9.81 3.71 Atmospheric pressure (mb) 1000 6.36 1 IRAP, CNRS— UPS, Toulouse, France Average surface temperature (oC) 15 –63 2 OMP, CNRS— UPS, Toulouse, France N (78.1%) CO (95.3%) * Corresponding author: ! Jeremie Lasue Institut de Recherche en 2 2 Atmospheric composition O (20.9%) N (2.7%), Astrophysique et Planétologie Observatoire Midi--Pyrénées, 9 2 2 Ar(1.6%) Avenue Colonel Roche, Toulouse, FR--31028.

VOL. 81, NOS. 7–8 167 features are detected on its surface. Thus the possible Marineris canyon system about 4000 km long and up to 7 presence of liquid water and habitable environments on or km deep, named after the spacecraft. The most striking below the surface have driven interest in the planet. features involved probable formation by flowing water in Orbiting at a distance on average about 50 percent farther the planet’s past, based on identification of branching valley from the Sun than the Earth, Mars is the only other planet network systems and broad . besides the Earth that is within the solar system’s potential From 1975 through 1980, the next major spacecraft habitable zone, which ranges from 0.95 to 1.67 times the exploration from NASA involved the two Viking orbiters distance between the Sun and the Earth, where water can and two Viking landers. Figure 3 plots the position of all be found in liquid form as a potential habitable the landers that have successfully reached the surface of environment. For an introduction to the characteristics of Mars and are expected to do so in the near future on a the planet Mars the reader is suggested to refer to1. topographic map of the planet. The orbiters imaged the A interesting feature of the planet is the entire surface of Mars at a resolution of 150 to 300 meters, predominance of the ancient rocks present on its surface. and selected areas down to 8 meters. The landers were On Earth, erosion processes and plate tectonics have both successful landing respectively at and removed most of the oldest rocks, indeed just about 10% on two opposite sides of the planet. For of the total surface area of the continents is formed of the first time, they imaged the surface of Mars and Archean rocks or older (rocks older than 2.5 Gy). On the characterised the surface material at both landing sites as contrary, on Mars, about 74% of the surface of the planet iron-rich clays. Temperatures ranged from 150 to 250 K, is formed2, by and rocks, which are with a variation over a given day of 35 to 50 K. Seasonal older than 3 Gy. When comparing side to side the dust storms, pressure changes, and transport of atmospheric geological evolution of the two planets, as shown on Figure gases between the polar caps were observed. A set of four 2, one notices that Mars just like the Earth, must have had biological experiments on-aboard were designed to detect conditions favourable to the emergence of life in its early life on the soils by different means. The gas chromatograph- stages. At that time, Mars possessed abundant water that mass spectrometer experiment did not detect significant carved the extensive valley networks and probably an amounts of complex organic molecules. The gas exchange atmosphere much thicker and warmer than today and a experiment looked at gas released from incubated soil higher internal heat flux that could have provided another samples without success. The labeled release and pyrolytic energy source. After about 3.5 Gy, through the Hesperian, release experiments used the 14C isotope of carbon as a Mars suffered an important transition towards more acidic tracer of biological activity either for a nurtured or and oxidising conditions, liquid water activity at the surface incubated . An initial positive detection of 14C became more sporadic and colder conditions settled due metabolisation was not reproduced by further experiments to a thinner atmosphere and smaller geothermal heat flux. on the samples. In the end, it appears that no conclusive As a consequence, the conditions during which the initial evidence of life was found at either landing site. For a stages of life have occurred have been mostly erased on review please refer to Ref.3. However, reanalysis of the Earth, but not so on Mars. Understanding whether suitable data in the wake of the discoveries from the more recent environments for the emergence of life existed on early landers and rovers suggests that the viking results can be Mars and if life ever existed on the planet is the major explained by the presence of perchlorate (a very strong driver for the current . chemical oxidant) and martian organics in the soil samples.

The First Era of Mars Exploration The Second Era of Mars Exploration: ‘Follow the Water’ Starting in 1964 with the successful fly-bys of the 4, 6, and 7 NASA missions, close imaging of the After the disappointments of early missions to Mars planet were acquired. The first images returned new technologies were required to advance our knowledge disappointingly presented a cratered surface similar to the of the planet, and exploration stopped for almost 20 years. Moon. Further surveys showed contrasting results Since the late 1990s, a new phase of Mars exploration highlighting the geological diversity of the planet. Mariner started, named after the NASA moto of ‘follow the water’, 9 (1971-72) was the planet’s first orbiter, and it successfully indicating that if there was any chance of finding potential imaged the entire globe. These early spacecraft revealed , the best policy was first to try and determine the planet’s cratered highlands, flat northern plains, if suitable environments existed now or in the past. This immense volcanoes and huge tectonic rifts, like the Valles sustained era of exploration led to significant increases in

168 SCIENCE AND CULTURE, JULY-AUGUST, 2015 the planet history came from the near-infrared spectro- imagers on-board the ESA (OMEGA) and the NASA Mars Reconnaissance Orbiter (CRISM). Targeted at detecting spectral signatures of minerals on the surface of the planet, they have successfully shown the presence of hydrated minerals from clays such as smectites, as well as sulphates, linked with ancient areas indicating past aqueous environments in a number of locations on the planet. The respective ages of the deposits indicate the evolution of the planet from conditions suitable for life in the ancient past to the oxidising conditions seen today as represented in Figure 2. Furthermore, rovers and stationary landers, like Mars Pathfinder (1997), and Opportunity; the two Mars Figure 1 : Global view of Mars based on a image mosaic (1999-2004). Visible on top, the northern polar ice cap. At center Exploration Rovers (2004) and the mission (2008) left, the shield volcanoes , and the have performed increasingly sophisticated surface Montes. On the lower right, stretches east-west for about investigations related to the Martian geology and 4000 km (Credit : NASA/JPL-Caltech/MSSS). atmosphere. Phoenix landed near Mars’ pole (68.2° north) the types, spatial resolution, and amount of data returned to study the water ice found there and Mars’ climate. both from Mars’ orbit and its surface. Phoenix landed in an area covered by polygonal contraction cracks and detected water ice just under a few cm of dust The orbiters include Mars Global Surveyor (1996), as predicted by models and orbital measurements. The Mars Odyssey (2001), Mars Express (2003), and Mars chemical analysis of the soil showed a mildly alkaline Reconnaissance Orbiter (2005). These have delivered high- environment unlike any found by earlier Mars missions; resolution visible, thermal, and multispectral imaging; laser with small concentrations of salts that could be nutrients altimetry; radar, gravity, and magnetic sounding; and other for life. The mission discovered calcium carbonate, a measurements of the , atmosphere, and marker of effects of liquid water but also perchlorate salts, crust. In particular, the Mars Global Surveyor laser altimeter a highly oxidising chemical component that could explain global digital elevation model of Mars obtained with a the results from the Viking landers biological experiments. vertical resolution of about 1 m and a horizontal resolution The mission ended when winter temperatures and cloud around 300 m revolutionised the geological understanding cover led to depletion of solar power, as expected. of the topography of the planet (as shown in Figure 3). The cameras on-board detected the first gully patterns on Upon their arrival on Mars, the Mars Exploration steep slopes at latitudes 30° or more in each hemisphere. Rovers were able to analyse the first geological outcrops Due to the fact that the gullies are very young features on the planet, showing that mobility was key to a better usually linked with fluid Time before present (Gy) movements on Earth, they are thought to be linked to current water activity on the surface of Mars. Then, the Mars Odyssey Gamma Ray Spectrometer analysed the global chemical composition of the surface of Mars and generated a global map of water concentration in the fews tens of centimetres below the surface. The data shows an increased water content poleward, and a variety of water concentrations in the equatorial region. A fundamental Figure 2 : Comparison between the geologic timescales of Mars and Earth with eras indications and contribution in the understanding of major events in the evolution of the planets (adapted from Michalski et al4.).

VOL. 81, NOS. 7–8 169 of the sulphate minerals gypsum and jarosite. Gypsum usually deposits from relatively neutral water under low temperature conditions, while jarosite forms in highly acidic settings like hot springs. After 10 years and more than 40 km roving on the surface of Mars, the Opportunity continues its exploration. The vehicles currently operating on the surface of Mars are represented in Figure 4.

Mars Exploration Today: ‘Seeking Signs of Life’ The evidence for the presence of liquid water either in Mars’ ancient Figure 3 : Global view of Mars topography based on Mars Odyssey Laser Altimeter with indications past or preserved in the subsurface of the landing sites of past and future missions (Credit : NASA/JPL-Caltech/MOLA) today is the salient discovery about the planet in the last decade. Water is the key molecule understanding of the geological history. Spirit showed that because life as we know it on Earth requires liquid water.. the of Crater, the area where it If Mars once had liquid water, or still does today, it is landed, had witnessed water alteration due to hydrothermal therefore compelling to ask whether any microscopic life activity in the past and also uncovered carbonate outcrops forms could have developed on its surface. To discover that were deposited under wet, near-neutral conditions. The the possibilities for past or present life on Mars, NASA’s Opportunity rover landed at and shifted to a new strategy known confirmed in situ the hematite deposits that were detected as “Seek Signs of Life.” Finding the clues to this question from orbit in the area. Hematite is an iron oxide mineral implies a better understanding of the planet’s geologic and sometimes associated with past water activity. The climate history to ascertain whether habitable environments discovery of hematite spherules embedded in the rock existed on its surface and if they could have preserved together with voids left in the rocks apparently by mineral evidence for life during long geological periods. dissolution were strong indications of water activity in the past at the site. Furthermore, Opportunity found deposits The Mars Science Laboratory (MSL, Curiosity) rover successful landing in 2012 marks the transition to this new theme, with the overarching goal of assessing Mars’ biological potential by searching for: organic carbon compounds, the chemical building blocks of life, and biologically relevant clues. The landing area selected for this mission is located at the bottom of Crater, a crater 154 km in diameter presenting a central mountain of stratified rocks about 5 km high, Aeolis Mons (informally known as , Figure 5). The mountain is Figure 4: Comparison between the sizes of the rovers and landers on Mars: the Sojourner rover (1997), assumed to consist of fill material the Mars Exploration Rovers (2004), the Phoenix lander (2008) and the Mars Science Laboratory deposited as sediments. Gale crater (2012) (Image Credit: NASA/JPL-Caltech)

170 SCIENCE AND CULTURE, JULY-AUGUST, 2015 that the sedimentary deposits probably date back to the late Noachian - early Hesperian eras of Mars (Figure 2). From orbit, evidence of past water activity has been detected in the form of an alluvial fan ( Fan) flowing from the crater rim, across the landing ellipse, and toward a topographic low at the base of Mt. Sharp (Figure 5). Also, near – infrared spectra from CRISM onboard MRO show stratigraphical units of hematite, clays and sulphate minerals at the base of Mount Sharp Figure 5: Topography of Gale Crater and notable geological features in the MSL landing area and (Figure 5). possible MSL traverse to explore Mount Sharp. Data adapted from , Milliken et 5 6 al . and Anderson and Bell . (credit: NASA/JPL-Caltech/R. Anderson). In order to detect signs of life is located at the boundary between the older southern on Mars, Curiosity’s payload is very highlands and the more recent northern plains (Figure 3) advanced and extensive. Figure 6 describes all the 10 with an estimated age ranging from 3.5 to 3.8Gy, meaning scientific instruments on-board. Remote sensing capabilities

Imaging and Remote Analysis MARDI - descent imaging/movie Mastcam - dual primary cameras 440-1035 nm spectral range ChernCam - remote chemical analysis and spot imaging; 240-800 nm spectral range; ~350-550 microns laser spot size. Environmental Analysis REMS - winds, temperatures, humidity RAD - radiation environment DAN - neutron detection of soil hydrogen Contact Analysis APXS - alpha particle X-ray chemical analyzer MAHLI - ‘hand-lens’ detail imaging; primary LEDs at 380-680 nm spectral range, plus UV LEDs at 365 nm Internal Laboratory SAM - spectrometry, chromatography, thermal analysis CheMin - mineralogy by X-ray diffraction and fluorescence

Figure 6: The Mars Science Laboratory (2012) instruments (Image Credit: NASA/JPL-Caltech)

VOL. 81, NOS. 7–8 171 detectors of radiation and energetic particles (RAD, DAN). Mounted on the robotic arm are a microscope (MAHLI) and an Alpha Particle X- ray Spectrometer (APXS) able to determine the abundances of the elements composing rocks and soils. The arm can also drill into selected rocks and collect samples. Finally the bulk of the analysis is done with a suite of instruments located inside Curiosity’s body, where an X-ray diffraction and fluorescence instrument (CheMin) identifies the minerals contained in a sample, and the laboratory (SAM) detects volatiles and carbon-rich molecules in selected samples and in the atmosphere. Figure 7: a) panorama of the Mount Sharp showing the stratigraphy at its base. b) Curiosity’s self- Results from Curiosity so far portrait at Yellowknife Bay showing polygonal fractured bedrock, ridge-forming veins and erosion of the upper unit. c) Curiosity’s first drilling sample at John_Klein showing the grey colour of the material have shown evidence for fluvial contained within the rock (Image Credit: NASA/JPL-Caltech/MSSS) activity within the crater with the of the rover include the cameras (MastCam, MARDI) and presence of numerous exposures of a Laser-Induced Breakdown Spectroscopy instrument pebble conglomerates. A potentially habitable environment (ChemCam) that uses a pulsed laser to analyse the chemical was found at a location called Yellowknife Bay near the composition of samples at a distance. Environmental landing site, where a few centimetres under the red assessments include a meteorological station (REMS) and oxidising surface of the rocks, grey mudstone samples contain clays and sulphates (the drill picture with grey material is shown in Figure 7). Liquid water with relatively neutral pH, low salinity and low temperatures must have been present to create this sedimentary deposit. Therefore, the lowest part of the crater must have sustained at one point in the past conditions suitable for life. Furthermore, analysis of the samples drilled at Yellowknife Bay by SAM indicates the presence of indigenous martian carbonaceous molecules in the rock. However, the carbon-containing molecules Figure 8: Payload of the Indian Mars Orbiter Mission currently in orbit around Mars (Image Credit : ISRO) are degraded by perchlorates

172 SCIENCE AND CULTURE, JULY-AUGUST, 2015 TABLE 2: The Future Mars Space Missions

Name Space Agency Launch window Arrival Goals

MAVEN NASA (USA) Nov. 2013 Sep. 2014 Analysis of Mars’ atmosphere composition and its evolution MOM ISRO (India) Nov. 2013 Sep. 2014 Global imaging of the surface. analysis of the exosphere, detection of methane Insight NASA (USA) Jan.-Apr. 2016 Nov. 2016 Seismic and heat flow investigation TGO Schiaparelly ESA (Europe) Trace Gas Orbiter to detect, characterise the spatial and lander Roscosmos (Russia) Jan.-Apr. 2016 Nov. 2016 temporal variation of atmospheric minor gas species. Landing technology demonstrator ESA (Europe) Exomars rover Roscosmos (Russia) Apr.-May. 2018 Feb. 2019 Rover designed to detect biosignatures Mangalyaan-2 ISRO (India) Apr.-May. 2018 Feb. 2019 Orbiter and rover Mars Science Detect ancient habitable environments and biosignatures. Laboratory 2020 NASA (USA) Jul.-Sep. 2020 May 2021 Prepare a cache of returnable martian samples contained in the soil, making it difficult to determine their Due to the possible implications of methane detection for original molecular structure. life and its influence on the atmospheric and climate processes on the planet, confirming the sporadic release of After this discovery, Curiosity proceeded towards and its global distribution will be one of Mount Sharp. In September 2014, it reached the Pahrump the major goals of the current and next missions to Mars. Hills region which forms the first outcrop of a rock unit that belongs to the base of Mt. Sharp. The drilled samples The Next Steps in Mars Exploration there present the specific signature of hematite indicating probable water deposition but under more oxidizing With the confirmation of the presence of liquid water on a diversity of environments in current and ancient Mars conditions than at Yellowknife Bay. This measurement can and strong evidence for the presence of indigenous carbon- be linked to the orbital detection of hematite at the base bearing molecules in the near-surface, the current goal of of Mount Sharp. Evidence of thin laminations and cross- the exploration to find more complex molecules and bedding deposition of sediments at the Pahrump Hills whether life ever existed is most relevant. The current outcrops also indicates flowing and standing bodies of water mission and a number of future missions to Mars will also interpreted as formed by deltas in a lacustrian deposit inside involve an increasing number of space faring nations. Table Gale crater. 2 summarises the next space missions that will explore the In December 2014, NASA reported that the Curiosity red planet. rover had measured a tenfold spike in the methane (CH) NASA will continue its current architecture for content of the atmosphere during its traverse to Mount exploration. The MAVEN orbiter successfully reached Mars Sharp, which lasted a couple of months and abruptly in September 2014, and it will explore how the climate disappeared. This measurement seems to confirm previous and environment changed, and why the atmosphere is so methane detections made in 2006 but that could not be thin today. The same month, India has also successfully repeated at later times. The methane molecule is an inserted in orbit the Mars Orbiter Mission (MOM), which important key to understanding the possible habitability of demonstrated its interplanetary exploration capability. Mars since, on Earth, 90% of methane is biological in Figure 8 details the instruments on-board MOM. Of special origin. Also this molecule is quickly destroyed in the interest to the understanding of the atmospheric methane , with an expected lifetime of about content is the Methane Sensor for Mars, which should 300 years. It is also important because it is a powerful detect low levels of methane and measure its global greenhouse gas, and if produced in large quantities in the distribution. The next set of European space missions will past, could have substantially increased the surface also analyse in detail the composition of the atmosphere temperature of Mars. Methane can be produced biologically of Mars with the Trace Gas Orbiter spacecraft. The next or non-biologically, by the interaction of water and rocks. generation of rovers will pursue the analysis of the surface The fact that its presence is so variable is probably due to geology of ancient terrains and find out new habitable relatively localised sources and sink processes yet unknown. environments and prepare the next stage of the exploration.

VOL. 81, NOS. 7–8 173 India announced its plan to send a martian rover by 2018, /nssdc.gsfc.nasa.gov and more specifically for the Mars while China is developing its Mars exploration program Science Laboratory Mission at http://mars.nasa.gov/msl/. as well. The European ExoMars rover scheduled for 2018 is designed to find biosignatures on the surface while the References Mars Science Laboratory 2020 rover will retrieve a set of 1. N. G. Barlow; Mars-An introduction to its interior, surface, and samples for possible return to Earth and further analysis atmosphere: Cambridge, (2008), 264 p. (Cambridge University in the laboratory. The next set of missions are poised to Press). make ground breaking discoveries about our neighbour 2. K. L. Tanaka, J. A. Jr. Skinner, J. M. Dohm, R. P. III, Irwin, E. J. Kolb, C. M. Fortezzo, T. Platz, G. G. Michael, and T. M. Hare, planet. Geologic map of Mars: U.S. Geological Survey Scientific Investigations Map 3292, scale 1:20,000,000, pamphlet 43 p., Acknowledgments http://dx.doLorg/10.3133/sim3292, (2014). 3. Hugh H. Kieffer, B. M. Jakosky, C. W. Snyder and M. S. The author gratefully acknowledges support from the Matthews. “Mars” (1992) (University Arizona Press). French Space Agency CNES, NASA/JPL and improvements 4. J. R Michalski, J. C., P. B. Niles, J. Parnell, A. D. Rogers and suggested by J. Crisp. S. P. , Nature Geoscience, DOl: 10.1038/NGE01706. (2013). On the Internet 5. R E. Milliken, J. P. Grotzinger and B. J. Thomson, Geophysical Research Letters, DOl:10.1029/2009GL041870 (2010). Additional information and images are available on 6. R. B. Anderson and J. F. Bell III Mars, (2010) DOl: 10.1555/ Web sites for the National Space Science Data Center http:/ mars.2010.0004.

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