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UNIVERSITY OF OKLAHOMA

GRADUATE COLLEGE

ANALYSIS OF REQUIREMENTS OF A MARS ROVER MISSION TO ACTIVE

GULLIES

A THESIS

SUBMITTED TO THE GRADUATE FACULTY

in partial fulfillment of the requirements for the

Degree of

MASTER OF SCIENCE

LISA BILLINGSLEY Norman, Oklahoma 2010

ANALYSIS OF REQUIREMENTS OF A MARS ROVER MISSION TO ACTIVE

GULLIES

A THESIS APPROVED FOR THE SCHOOL OF AEROSPACE AND MECHANICAL ENGINEERING

Dr. David Miller, Chair

Dr. Alfred Striz

Dr. Kyran Mish

Leonard and Patricia Billingsley

Acknowledgements

I would like to express my sincere thanks to Dr. David Miller for mentoring and supporting me through my Master’s program.

I am especially indebted to my family for setting me on this path to begin with. My parents, Leonard and Patricia Billingsley, raised me in an environment of math and science. They taught me logical thinking and a love of information and discussion. My older sister, Sarah, blazed the first few steps of the path I’ve walked in college. Finally, I have to thank my younger sister, Deborah, for breaking the serious pattern and providing an atmosphere of fun and relaxation.

iv Table of Contents

Page Chapter 1: Introduction 1 1.1 The Red Planet 1 1.2 Water: Why We Care 6 1.2.1 Life As We Know It 6 1.2.2 Astrobiology 6 1.2.3 Human Use 10 1.3 Evidence for Water 11

Chapter Two: Gullies on Mars 13 2.1 Gully Detection and Statistics 13 2.2 Dry Granular Theories 18 2.3 Solid or Liquid CO 2 20 2.4 Water/Brine 21 2.4.1 Sources of Water 24 2.5 New Gully Deposits 27

Chapter Three: Planetary Rovers 30 3.1 Lunokhod 30 3.1.1 Lunokhod 1 31 3.1.2 Lunokhod 2 32 3.1.2 Lunokhod 3 34 3.1.2 Lunokhod Mobility 34 3.2 PROP-M Rover 36 3.3 Pathfinder-Sojourner 38 3.4 Mars Exploration Rovers 41 3.5 Mars Science Laboratory 44 3.6 ExoMars 47 3.7 Marsokhod 49 3.8 Nanokhod 52 3.8.1 Nanoknod Mercury 52

Chapter Four: The Martian Environment 54 4.1 Power 54 4.2.1 Solar Power 56 4.1.2 Radioisotope Thermoelectric Generators 59 4.2 Thermal Effects 63 4.2.1 Seasons on Mars 63 4.2.2 Temperature Swings 65 4.3 Atmospheric Conditions 68 4.3.1 Pressure 69 4.3.2 Atmospheric Dust 71

v Chapter Five: Landing and Movement 76 5.1 Landing Methods 76 5.1.1 Powered Descent 76 5.1.2 Airbags 77 5.1.2 Sky Crane 80 5.2 Choosing a Landing Site 82 5.2.1 Altitude Considerations 82 5.2.1 Terrain Considerations 84 5.3 Inside the Crater 89 5.3.1 Slope 89 5.3.2 Surface and Chemicals 91

Chapter Six: Straw Man Mission 93 6.1 Payload 93 6.1.1 Cameras 93 6.1.2 Chemistry 94 6.2 Rover Base 97 6.3 Power 101 6.3.1 Time 102 6.3.2 Battery Calculations 103 6.3.3 Solar Panel Calculations 104 6.4 Entry, Descent, and Landing 107 6.5 Summary 109 6.5.1 Mission Summary 109 6.5.2 Rover Summary 110

Chapter Seven: Summary and Conclusions 112 7.1 Summary 112 7.2 Future Work 115 7.2.1 Information 115 7.2.2 Hardware 116 7.3 Conclusion 117

References 119

vi List of Tables

Page 1-1 Physical properties of Earth and Mars 5 3-1 Wheel modes and their uses 51 3-2 Operational analysis of a 7 day mission 53 4-1 Properties of RTG materials 60 4-2 Length of Mars seasons 63 4-3 Solar irradiance at Mars 65 4-4 Mars’s atmospheric information 68 4-5 Mars’s atmospheric composition 69 6-1 Marsokhod specifications 98 6-2 Small Marsokhod specifications 99 6-3 Solar panel size vs. energy generated 106 6-4 Details on the rover 111

vii List of Figures

Page 1-1 Two pictures of the planet Mars 2 1-2 Topographical map of Mars 3 1-3 Comparison between Earth and Mars 4 1-4 Map of hydrogen distribution 12 2-1 An example of gullies on Mars 14 2-2 Statistics on gullies 16 2-3 Gullies on Earth and Mars 22 2-4 Gully apron over sand dunes 23 2-5 Pictures of the Barringer Crater 26 2-6 Gully streak in Terra Sirenum 27 2-7 Map showing slope streak and gully locations 28 3-1 Lunokhod 1 31 3-2 Lunokhod 2 34 3-3 Lunokhod Wheels 35 3-4 PROP-M rover 36 3-5 Sojourner rover 38 3-6 Computer model of Sojourner 39 3-7 Sojourner’s wheel 40 3-8 Mars Exploration Rover 41 3-9 Mars Exploration Rover wheel 43 3-10 Mars Science Laboratory 44 3-11 Comparison of Sojourner, MER, MSL, and Phoenix 45 3-12 Wheels from Sojourner, MER, and MSL 45 3-13 Schematic of MSL’s components 46 3-14 Elements of the ESA-NASA ExoMars Program 47 3-15 Marsokhod wheel design and ground clearance 49 3-16 Marsokhod images 50 3-17 Marsokhod wheel walking 50 3-18 Nanokhod with labeled parts 52 4-1 Lifespan versus wattage for various types of power systems 55 4-2 Effects of temperature on voltage and current 57 4-3 Two types of solar concentrators 57 4-4 Cassini’s Radioisotope Thermoelectric Generator 60 4-5 Seasonal images of Mars 64 4-6 Daytime temperature on Mars 66 4-7 Nighttime temperature on Mars 67 4-8 Mars elevation area distribution 70 4-9 Filter and capture magnets on Opportunity 74 4-10 Views of Mars with and without a dust storm 75 5-1 MER airbags 78 5-2 Opportunity with the landing petals partially opened 79 5-3 Proposed decent sequence for Mars Science Laboratory 80 5-4 Curve of atmospheric transmittance by altitude 83

viii 5-5 Altitude map of the Terra Sirenum region 85 5-6 Topographical map of the Terra Sirenum region 86 5-7 Altitude map of the Centauri Montes region 87 5-8 Topographical map of the Centauri Montes region 88 5-9 Model results for the deposit in the Centauri Montes region 90 6-1 One cell from Phoenix’s Wet Chemistry Laboratory 95 6-2 Marsokhod wheel walking 98 6-3 Comparison of Sojourner, MER, MSL, and Phoenix 99

ix Abstract

Several rover missions to Mars have been planned and executed, and most have been successful. Still, the area of the planet that has been covered by rovers has been tiny, and there is much more to learn. This thesis covers a possible mission not currently under consideration; that of visiting areas of the surface that might be geologically active at this time. The reasons for planning this mission are covered, as well as information on past rovers. The major problems faced by rovers on Mars are reviewed, from power to atmosphere to landing. Finally, a possible rover that fits the necessary mission parameters is designed using elements of other rovers and Mars missions. Only the physical makeup of the rover is covered; the electronics, including computers and communication, are not addressed.

x Chapter 1 Introduction

1.1: The Red Planet

Mars is the fourth planet from the Sun. It is commonly regarded as being the most

Earthlike of the other planets in the solar system: the nearest planet to Earth, a bit smaller than Earth, and one we might live on some day. Mars is also ‘known’ to be very cold, with lower gravity than Earth, no atmosphere, and water will boil instantly on the surface because it has no atmosphere.

That view is not very accurate. Venus is much closer to us and much nearer in size and composition, and it is easier to reach. It is simply ignored because of its high temperature and the crushing atmosphere of the surface, which is just as deadly and is much harder to compensate for than the low pressure and temperature of Mars. Due to those difficulties, Venus has long since been forgotten as anything except a gravity well to slingshot a spacecraft. All efforts towards reaching another planet are aimed at Mars.

In reality, Mars is much smaller than Earth and it does have an atmosphere, though it is a very thin one. That atmosphere permits water to stay in liquid form for at least short amounts of time, and in many places the water will freeze before it evaporates. However, some areas near the equator, in summer, do maintain temperatures in the liquid water range, and the possibilities of underground water are vast.

While there are a great many difficulties facing any attempt to colonize Mars, it is the easiest other planet for humans to live on and it is expected that the Red Planet will be the first extraterrestrial colony (not counting the Moon).

Figure 1-1: Two pictures of the planet Mars. The top picture is centered on the

Valles Marineris, while the north pole is visible in the bottom one.

Figure 1-2: A topographical altitude map of Mars. The left circle is the south pole,

while the right circle is the north pole. Note the elevation differences

between the hemispheres. [Picture from NASA JPL website; taken by

Mars Global Surveyor.]

Figure 1-3: A comparison between Earth and Mars, showing the year, gravity,

energy received from the sun, and atmospheric composition. [Picture

courtesy of P. James and NASA.]

Table 1-1: Listings of certain physical properties of Earth and Mars and

comparisons between them. Note that land surface area is actually

greater on Mars than on Earth, due to Earth’s extensive oceans.

Earth Mars Comparison (%)

Gravity (m/s 2) 9.81 3.73 38%

Land Surface Area 1.29 x 10 8 1.45 x 10 8 112%

Mass (kg) 5.98 x 10 24 6.42 x 10 23 10.7%

Oxygen (bar) 2.1 x 10 -1 7.80 x 10 -5 0.0371%

Pressure (bar) 1.00 0.006 0.60%

Not compared Average Temperature (°C) 15 -63 because not an absolute scale

Average Temperature (K) 288 210 72.9%

Volume (km 3) 1.10 x 10 12 1.64 x 10 11 14.9%

Year (Earth years) 1.00 1.88 188%

5 1.2: Water: Why We Care

By far the most research being done on Mars now is a search for water. In fact, finding water is very nearly the keystone to the planet. Why is that? There are several reasons.

1.2.1: Life As We Know It

The only life forms we are familiar with are the ones that exist on Earth, and all

Earth life is based around water. A solvent is a polar molecule that allows some solids to dissolve in it. Water is by far the most common solvent that exists on Earth and is often called the ‘universal solvent’.

Carbon is an extremely common element and it can combine into astonishingly complex, diverse, and stable compounds. Many such compounds dissolve in water, allowing the chemical flexibility of life. All life as we know it is based on carbon bonds, so carbon is an essential element that must exist for all life [Lunine, 2005] as we currently know it.

1.2.2: Astrobiology

Extraterrestrial life has been a dream that has been cherished by humans for centuries. As water is the basis for all forms of life that we know, we start there in our search for extraterrestrial life. There are few places within the solar system that might hold liquid water except for Earth, and Mars is the largest and closest.

Mars’s surface is drenched with deadly radiation, making life as we know it almost impossible to survive, but only almost. The Viking landers searched for life there, but

6 their results were contradictory and it was eventually decided that they hadn’t found any.

Still, their instruments were much more primitive than today’s are. There is still the possibility that life does exist on the surface, and a much greater possibility that it lives underground, which has never been searched. Even a few inches of regolith or rock will block most of the deadly radiation, and temperatures increase the farther below ground one goes (at least they do on Earth, and might well on Mars). Liquid water deep below the martian surface is a very real possibility.

Europa is the second major moon out from Jupiter and is also a favored candidate for having water, but it is much farther away. Jupiter is, after all is more than four AUs away from Earth, compared to Mars’s half-AU. There is also the problem of Jupiter’s extreme magnetic field, which would seriously damage any spacecraft (and presumably life) near it. One mission, Galileo, has been to Jupiter and stayed for years. Several other missions have swung by for a brief look or to use Jupiter to accelerate in what is known as a gravity slingshot. Short exposure did not damage the spacecraft, but Galileo had radiation problems when it swung too close to the gas giant.

Still, Europa has a great deal of internal heat due to the pulls of gravity of Jupiter and the other Galilean moons, and from Jupiter’s massive magnetic field; there is enough heat to maintain liquid water inside even though it is covered by an icy shell. Jupiter’s magnetic field is twisted by liquid water or an ice-water mixture inside the moon, so it is known to exist. Carbon is so common that Europa almost certainly has some, either from its initial forming or from being bombarded by meteors, or more likely both [De Pater et al., 2005].

7 So, Europa has the essential ingredients for life as we know it. The icy surface and watery core block the deadly radiation, removing that problem. What problems remain?

The major one at this point is energy. Most life on Earth requires light to thrive, but

Europa has little light. If the ice shell coating the moon is thin enough some light may leak in, but that is not a dependable source.

However, life has been found in Earth’s oceans at depths that receive no sunlight at all. That life subsists on underwater volcanoes and other sources of great heat and chemical energy. Europa certainly has heat, especially in the center, so life could possibly be based on that. Still, Europa’s oceans are probably much deeper than Earth’s, since the outer moons are usually balls of ice instead of rock, so trying to find lifeforms there could be futile, even if they exist. Between that and the distance, Mars is a much more likely candidate for life we can reach and detect [De Pater et al., 2005].

There is also the possibility that some life is not based on water. The second most common polar solvent is ammonia, which is liquid at much lower temperatures than water (195-240 K). And there is a place on our solar system where ammonia-based life might exist.

Titan is Saturn’s largest moon and is second only to Ganymede (Jupiter’s third

Galilean moon) in total size. It also has a significant atmosphere, which no other moon in the solar system does. It is actually the second densest atmosphere of terrestrial bodies in our system, since it is approximately one and a half times the density of Earth’s, though it appears to act more like Venus’s atmosphere than Earth’s. It is also the only other atmosphere in the solar system that is dominated by N2 [De Pater et al., 2005], making significant amounts of ammonia (NH 3) likely. The nitrogen atmosphere alone

8 would make it very worthy of study, but it is also fascinating from an astrobiological standpoint.

Water, if it exists, would be as solid and as hard as a rock on Titan because of its temperatures, so surface life would have to be based on ammonia, as ammonia and ammonia-water mixtures can remain liquid at temperatures as low as -100°C. Titan does have at least some liquid ammonia as well as many hydrocarbons. Titan is the likeliest example of possible ammonia-based life in the system, and it does have a solid surface, complete with plains, lakes, and mountains [De Pater et al., 2005]. Unfortunately, Saturn is twice as far from Earth as Jupiter is, so expeditions to Titan are at least as hard as those to Europa. The advantage a mission to Titan would have is that our type of life would not thrive there, so there is a far lesser chance of accidentally contaminating it with Earth life. But it will be a very long time before scientists can go there and study it in force. Mars is truly the only extraterrestrial life candidate that can be studied now.

There are also theories that life could be based on silicon rather than on carbon. This theme has been common in science fiction stories. Silicon shares many of the qualities of carbon, as it is directly below carbon on the periodic table, but free carbon is much more available on the surface of the Earth (most of the silicon is bound into rock and sand) and silicon cannot form nearly as many molecules due to its larger atomic radii, which inhibits the formation of double and triple bonds. Though since the actual number of combinations needed are unknown, that might not pose a major problem.

Still, this theory creates four possible combinations of life: water and carbon, water and silicon, ammonia and carbon, and ammonia and silicon. We have proof that the water and carbon mixture does exist, and there is a potential world (Titan) that could use

9 ammonia for life. But as ammonia and silicon life are still purely theoretical, they will be unless and until they are found [Lunine, 2005, p. 440].

1.2.3: Human Use

Humans require a large amount of liquid water to survive, both for directly drinking it and for growing the foods we live on. Shipping water and food off Earth would be extremely expensive and time consuming, and it would be very inefficient. To form a stable, self-sustaining off-world colony, water must be present. Food can be grown, oxygen can be produced by plants, and metals can be mined, but water is an absolute that can only be used, not produced.

Recycling water helps and allows a small amount of water to work instead of a larger one, but water must still be available. The only sources out in space are on certain planets and moons and in icy asteroids that would be hard to catch. Water on or near the surface of Mars would be a great plus for colonization.

The Moon does have some water (recently found near the south pole), in the form of ice [NASA]. The Moon is a possible candidate for colonization, but as it stands humans cannot survive on the surface due to radiation, so it would most likely be an underground city. Enceladus, one of Saturn’s moons, also has water as liquid ‘lava’ coming from volcanoes, which are called cryovolcanoes. Enceladus is much to far out to consider sending humans to at this point, however.

10 1.3: Evidence for Water

The surface of Mars has been imaged extensively, by a total of 19 probes from the

Mariner 4 flyby in 1965 to Mars Phoenix lander in 2008, and there are no large bodies of water or any other liquids on its surface. The average temperatures for the planet should cause liquid water to freeze, and the average atmospheric pressure only enables water to be a solid or a gas. So just looking at ‘average’ conditions seems to rule out the possibility of liquid water.

However, those are averages only. Pressure varies at different locations and altitudes.

Temperatures change constantly, and some areas of the planet rise above the freezing point of water for days or weeks in the summer. In addition, any water present is unlikely to be pure. It is well known that dissolving salt, methanol, and other materials in water will significantly lower the freezing point; if the water on Mars is high in dissolved solids and/or other materials, it may remain liquid under at least some martian conditions.

Despite that, the search for water is now based entirely underground. Since there is no obvious water on the surface, it must be under the surface. A map of hydrogen abundances (Figure 1-4) has been made, but it only shows the top meter of the surface

[Mitrofanov et al., 2002]. This map is normally interpreted to show how much water is in various places, but there is no actual proof that the hydrogen detected is made up of water molecules.

With that as the only direct data, any evidence of liquid water underground must be inferred. There are numerous geological features on Mars, many of which appear to have been carved from water.

11 The most popular ones at the moment are the gullies that have been seen in several

Mars Global Surveyor (MGS) pictures. Some recent deposits have been photographed in places where they weren’t previously, proving that there is indeed some form of surface modification still going on on Mars. This is discussed in Chapter 2.

Figure 1-4: A map of hydrogen distribution in the top meter of the martian surface

made by the Gamma Ray Spectrometer on the Mars Odyssey orbiter

[NASA JPL website]. Note the two large areas on the equator that are

directly opposite each other; this is a possible sign that those were once

the poles and the axis of rotation has changed by 90°. The landing sites

of Viking, Pathfinder, and MER are also indicated.

12 Chapter 2

Gullies on Mars

2.1: Gully Detection and Statistics

Ever since Mariner 4 took pictures of the desolate, cratered surface of Mars, it has been believed that Mars is a dead planet, geologically speaking. It has no tectonics, volcanism, or any other forces changing the surface, aside from erosion by the wind. It has also been believed that there is no water on Mars, as it would boil immediately under the near-vacuum pressure (0.600 KPa average pressure and 227 K average temperature which would put it beneath the triple point of water on a phase diagram). Both of these assumptions made Mars seem to be an uninteresting place, with no life and not much to learn. Those assumptions have turned out to be erroneous.

Gullies were first seen on Mars by the Mars Global Surveyor (MGS) in 2000

[Heldmann et al., 2004]. In many ways they look similar to water-carved features on

Earth. This sparked a great deal of interest in the possibility of water on Mars. Since life

(as we know it) needs water, these also indicated that there might be life there after all.

In the years since, many more gullies have been spotted, and a few changes have been found between photographs, implying that there is active modification of the surface going on. This has also excited scientist and geologists, as there may be more of interest on Mars than was originally thought [Ishii et al., 2004].

Figure 2-1: An example of gullies on Mars. Alcoves are where the water emerges

and starts to flow. Channels are where the water speeds up and forms a

thin flow, cutting into the surface. The channels on Mars narrow to a

point and then stop where the water evaporates or sinks into the

surface. The aprons (also known as alluvial fans) are collections of the

material that was carved out by the flowing water. They start at the end

of the channel and spread out in a triangular form. [Picture courtesy of

Malin Space Systems/NASA]

14 Several important characteristics have been assembled about gullies on Mars [Balme et al., 2006]:

• Most gullies are found more than 30° from the equator

• 62% of gullies are found on impact crater inner walls, 16% on pit walls, 10%

on valley walls, and 10% on knob/hillslopes

• Gullies usually face the poles, not the equator

• Gullies become less common towards the poles, except for between -70° and

-80°

• Different types have different preferred directions

• Gullies tend to start well below the edge of the crater, pit, or valley wall

• Gullies usually have an initial alcove, a channel that tapers off, and an apron

at the end of the channel

• Gullies on the same wall usually taper off in length one way or the other,

producing a diagonal slant

• Note the size of the gully in Figures 2-3 and 2-4. Other gullies studied are on

similar scales.

Figure 2-2: Statistics on gullies based on latitude, elevation, and slope [Dickson et

al., 2009].

The planetary geology community has become focused on Mars again, and all sorts of hypotheses have been made on how the gullies might have formed. The fact that new deposits have been found near gullies strongly suggests that this is an active process happening today, so any theory to explain them must under with the current conditions of Mars. Here are the most common theories.

1. Dry granular flows. There are those that think there is not any liquid, either

H2O or CO 2; all activity must be dry flows of sand or rock.

2. Caused by CO 2, either liquid or solid. Many of the adherents to this theory

are ones that believe that there is not and cannot be liquid water on Mars.

Carbon dioxide (CO 2) is a possible source that changes with temperature

and season.

3. Carved by water/brine. This is mostly pursued by those that look at the

shapes and think, given their close resemblance to water-carved gullies on

Earth, that they have to be made by water. This view is the most

interesting for astrobiologists and others that want to find life on Mars.

17 2.2: Dry Granular Theories

Explaining the gullies as features of simple dry rock flows neatly avoids most of the problems with liquids on Mars. The rock is present on Mars, so there is no need to make guesses about a possible source. The lower pressure and temperature aren’t very important to the formation, either. It does not, however, answer everything.

Most studies of this theory are done by comparing the gullies on Mars to various features on Earth and on the Moon. Apparent analogues have been found in talus flows, debris flows in the French Alps caused by snowmelt, various features in Antarctica and on Iceland, and even some comparison to the Moon, though that is not a popular subject of research [Bart, 2006].

Aside from the Moon, all of these features could be influenced by water or other conditions that are not present on Mars. Aprons caused by dry flows are common on

Earth, but the other gully-related shapes—alcoves and the long, sinuous channels that taper off—are hard to explain as dry flows only, without some kind of liquid.

The alcoves can be explained by upward-propagating avalanches, but those do not normally have meandering paths. The deep, sinuous channels could not be formed by flows of dry granular material. Similar sinuous channels have been seen in dry pumice flows, but they have never been reproduced in the lab, and the primary rock on Mars is basalt, not pumice.

Incisions made by faulting or other mechanisms are not usually sinuous, and would be filled in by later eroding material; rock falls are generally straight and direct. The aprons might be made of the material eroded out of the channels, but what caused the slide to begin with seems to be missing [Ishii et al., 2004].

18 Also, the dry flow theory does not explain the location and orientation of the gullies.

Pure dry rock flows would not be greatly affected by temperature or latitude, but the extreme lack of gullies around the equator strongly suggests a relationship. There is also no reason to have many channels next to each other, all emerging at approximately the same depth of rock—this is common with subsurface aquifer-fed gullies on Earth, but not in sand flows.

A last objection, which is rarely made, is that all of these theories are based on comparing forms and attempting to say that the same processes were at work there as are known on Earth. That is a good working theory, but how similar they look alone is not proof.

19 2.3: Solid or Liquid CO 2

Amongst scientists that reject water as a possibility, carbon dioxide is a substitute that exists on Mars in abundance. It is usually assumed in these theories that the force creating the gullies is either avalanches of CO 2 frost on the surface or liquid CO 2 underground, in which case the gully is where the regolith collapsed into the CO 2 channel. CO 2 frost would also explain the lack of material at the bottom, as it would sublimate out when the temperatures rose; what caused the erosion does not stay around to be seen, and the aprons are simply the material that was once in the channel [Ishii et al., 2004].

CO 2 would also explain the location and orientation of the gullies. While water should be near the equator and dry flows should have no location preference, CO 2 is a solid only at the low temperatures that are near the poles on Mars, so there should be no

CO 2 ice along the equator. The frost avalanche theory would also get around the question of how liquids got to the knobs/hillslopes and crater walls, though liquid CO 2 is still hard to come by there unless the frost melts [Ishii et al., 2004].

There are as many problems with this theory as there are solutions. Liquid CO 2 would still have to be in some sort of flow zone (the equivalent of an aquifer, for water) or other underground holding and would need an underlying impermeable layer beneath to direct it to where it can break out in a crater or valley wall. CO 2 frost is not generally very stable, and so probably wouldn’t build up enough for an avalanche. Also, the temperature and pressure in these climatic zones are much farther out of range for liquid

CO 2 than they are for liquid water [Stewart et al., 2002].

20 2.4: Water/Brine

This theory is the most popular one, probably because it carries with it the possibility of finding life on Mars and/or with having the conditions necessary for us to colonize

Mars. It also fits in with the dominant cause of erosion on Earth.

The water is generally believed to be formed by ice melting below the surface and flowing above an impermeable layer of rock (in essence an aquifer) until it emerges in an alcove. The tapering shape of the gullies is assumed to be due to water evaporating on the way down or sinking again into permeable zones lower down in the crater/pit/valley surface. The sand or rock eroded out and carried downward while the water flowed in the gullies is left at the lower end of the gullies to form the aprons at the base of the slope [Sears et al., 2000].

The ice theory is supported by the fact that these gullies form on slopes with certain thermal commonalities. Slopes that have gullies on them have a fairly consistent amount of thermal insolation year-round as a result of their latitudes and slope facings. This suggests that they have similar temperatures to each other, and so temperature must be crucial to gully formation. Liquid H 2O is more likely than liquid CO 2 on these slopes due to their temperatures and pressures [Lanza et al., 2010]. Melting ice would produce seasonal, occasional erosion instead of constant flow, resulting in the multiple channels observed in some locations.

Figure 2-3: (Left) HiRISE image of a flow on Mars in a crater in the southern

hemisphere. The image shows the alcoves, channels, and fans of two

gullies. Note that there are several channels on the aprons, indicating

repeated flow activity.

(Right) Image of a flow on Earth in Colorado, USA. Again, there are

multiple flow channels from multiple events.

Note: Look at the scale of the images; when we discuss gullies on

Mars, we are talking about mountain-sized features, not the small (one

meter to tens of meters long) gullies that are seen every day. [Lanza et

al., 2010]

22 The water appears to explain the main features of the gullies, including the variable lengths, as wind and water volume would affect the evaporation rate. Attempts have been made to determine wind speed by the sand dunes that are under the aprons (Figure

2-4), but the samples are too small, lack the resolution needed, and the effects of wind on

Mars are not well known. Experiments on how wind speed affects the evaporation rate at

Mars pressure are currently ongoing at the University of Arkansas.

Figure 2-4: A picture showing the apron of a gully on the south-facing wall in

Nirgal Vallis (near 29.4°S, 39.1°W) covering sand dunes below it. This

is a sign that the gully is very recent, within a few centuries, or the

sand would be on top of the apron instead of below [MOC2-244].

23 2.4.1: Sources of Water

The water is commonly thought to be from melting ice because there are no other known sources of liquid water on Mars. Magmatic waters derived from cooling magmas or volcanism are known on Earth and may have been one original source for water on

Mars, but it is unlikely (given the lack of evidence for recent volcanic activity) to be a current source.

Many of the gullies imaged have multiple channels indicating multiple flow events

[Gulick et al., 2009]. This implies repeated water flow of some kind, or one event spread out over a rock shelf or similar impermeable layer (Figure 2-3).

Another possible source of water is meteoric. A theory has been put forth that Earth is constantly being bombarded by small comets, as many as 20 snow comets weighing

20 to 40 tons each that crash into the Earth's atmosphere every minute. This adds up to millions of small comets that together bring in tons of water every year [Deming, 1999;

Frank et al., 1990]. A similar bombardment could replenish the surface/near surface of

Mars with water, despite high evaporation rates.

Much of the planet is simply too cold for liquid surface water, and pools on the surface would sink into the subsurface, dry up rapidly, or freeze and then sublimate into the. Once the water is below ground, the regolith of the planet would inhibit both evaporation and sublimation, acting as a thermal ‘blanket’. The ground also appears to be rich in sulfur, which could dissolve into the water; brines and other waters with elevated dissolved solids stay liquid at much lower temperatures than pure water.

It has also been determined from spectrographic data that there is hydrogen under much of the martian surface (Figure 1-4). That hydrogen is generally believed to be in

24 the form of water ice. There is no particular relation between the locations of the hydrogen and the gullies, but the known hydrogen detectable from space is only in the top meter or so [Mitrofanov et al., 2002]. All of the gullies start far deeper below the surface than that, where the water would not be detectable by current space-based sensors.

Other theories for the gullies’ water source include underwater seepage or aquifers, the presence of brine rather than water, some kind of hydrothermal source, or occasional coldtrapping. Coldtrapping, however, would not produce the volume of water needed.

The biggest objection to this theory is that water cannot exist on Mars; it would boil immediately. This has been ‘known’ by scientists for years, but it was recently tested and found false. It was found that the extremely low temperatures (0°C or lower) on

Mars reduce evaporation by almost as much as the pressure drop increases it. It does evaporate faster than pure water on Earth, but it could certainly be on the surface long enough to make changes [Sears et al., 2000].

There are a few other objections, the next most common being that this theory does not explain any gullies on knobs or hillslopes, as there is no place for the water to come from. A possible solution to that is capillary action, which pulls the liquid from below to emerge at a higher point. This happens in places on Earth, and there is no reason to think that it is impossible on Mars.

Another objection is the location and orientation of the gullies, which is toward the poles instead of the equator, as would be expected for melting ice. The lack of any gullies near the equator is one of the biggest mysteries of this theory, though a recent theory is that the lack may be because the higher equatorial temperatures have over time

25 melted the ice needed to form the gullies; it eventually all evaporated into the atmosphere and was transported elsewhere.

A less common but more solid-sounding objection is that craters do not have the kind of impermeable layers and slope that is needed. The impact of the rock tore up or broke the rock layers, and what is left is tilted so that water would run away from the crater, rather than into it [Treiman et al., 2005]. The ridge around Meteor Crater in

Arizona shows how the edges of a crater are pushed up (Figure 2-5).

Figure 2-5: Pictures of the Barringer Meteor Crater from close by and from space.

[Top photograph from the official Barringer Crater website, bottom

photo courtesy of NASA.]

26 2.5: New Gully Deposits

The most exciting fact about the gullies is that they are not simply ancient, unchanging features. Many areas with gullies have been photographed more than once, and two of them have visible changes. These changes are in the form of light-colored gully streaks on the walls of the craters (Figure 2-6).

Figure 2-6: A new gully streak on the wall of a crater in Terra Sirenum, at 36.6°S,

161.8°W [MOC2-1621, 2006]. Notice that there are no dark slope

streaks inside the crater, and the existing light-colored blotch seems to

be more defined in the second picture.

Slope streaks both light and dark are not uncommon on Mars, but all of the ones that have formed since Mars Global Surveyor started taking pictures have been dark. No new light-colored slope streaks have been seen (not counting the ones in gullies), and almost

27 all existing light-colored streaks have dark-colored ones nearby, sometimes even partially covering them [MOC2-1621, 2006].

Spirit has found that scraping away the darker surface regolith reveals lighter-colored sand below, and wind, dust devils, or dust avalanches are the commonly believed sources of most dark slope streaks.

In addition, the areas where slope streaks are usually found are not the same areas where gullies are (Figure 2-7). The two craters that were found with new gully streaks are marked on the map, and neither of them is near the areas with dark slope streaks

[MOC2-1621, 2006].

Figure 2-7: A cylindrical projection of Mars [MOC2-1621, 2006]. Brown areas

have slope streaks, while the pink areas are where gullies are found.

Because of this, and because of the lack of dark streaks in the craters, it is believed that the light gully streaks are not formed in the same way as the dark streaks found in the equatorial region. Since the craters they are found in have gullies, the streaks are

28 thought to be related in some manner [MOC2-1621, 2006]; possibly they are deposits from whatever source is creating the gullies.

Finding activity of any type on Mars that isn’t necessarily atmospheric is exciting, since the planet has long been thought to be dead. Discovering what exactly is causing this activity, and if it has any effect other than coloration, could tell us a great deal about

Mars that we do not know or even expect to find.

29 Chapter 3 Planetary Rovers

This chapter is a brief review of all of the lunar and planetary rovers that have been flown as of June 2010 (successful or not), and a few of the ones that were either not sent on a mission at all, or have not yet been flown. This paper will cover Lunokhod, the

Prop-M Rover, Pathfinder-Sojourner, the Mars Exploration Rovers, the Mars Science

Laboratory, ExoMars, Marsokhod, and Nanokhod.

In this thesis, ‘rover’ is defined as a mobile vehicle that does not have a human there with it. The later Apollo missions had a vehicle with them, called the Lunar Roving

Vehicle, but it was driven by a person that was sitting on it, as cars are, and so is not addressed here.

3.1: Lunokhod

Lunokhod means ‘Moonwalker’ in the Russian language and was the name of the

USSR’s lunar rovers. So far, they have been the only lunar rovers ever used, though several others have been considered by various agencies and universities, along with the

Google Lunar X-Prize contest, which has not yet been won.

The Lunokhod rovers were not autonomous or even semi-autonomous. They were remote-controlled vehicles, controlled from the Earth by a five-man team. Driving them was a challenge because of the more than two-second light-speed delay in round-trip communications. They were originally intended to survey and prepare sites for human exploration, but they were very successful explorers even after that part of their mission was dropped [Chronological Lunokhod 1, 2007].

30 They were tub-shaped vehicles with a convex lid and 8 independently-powered wheels. They all had a cone-shaped antenna, a helical antenna that was highly directional, and a variety of TV cameras and scientific instruments. They were powered by a solar panel that was on the inside of the lid. The lid was open during the lunar day, collecting power, and was closed at lunar night to keep the vehicle warm via a radioactive (polonium-210) heat source [Balint, 2002].

There were four Lunokhod vehicles built. The first one, which wasn’t specifically named, was destroyed when its launch rocket disintegrated early in its flight on February

19, 1969.

3.1.1: Lunokhod 1

Lunokhod 1 was the second one built. It was on the Luna 17 spacecraft and was launched on November 10, 1970, and soft-landed in the Sea of Rains in northwestern

Mare Imbrium (38.28° N, 35.00° W) 7 days later. This is further to the lunar north than any other Luna or Apollo mission of the era [Chronological Lunokhod 1, 2007].

Figure 3-1: Lunokhod 1 with the lid closed [Balint, 2002].

31 It had 4 TV cameras, two mounted in the front for navigation and two on the sides for panoramic views, and special instruments on an extendable rod to test the lunar regolith for soil density and mechanical properties. It also had an X-ray spectrometer, an

X-ray telescope, cosmic-ray detectors, and a laser device [Balint, 2002].

Lunokhod 1 was expected to last for 3 lunar days, or about 3 months, but actually lasted for 11 lunar days, until the radioactive heat source ran out and the electronics got too cold to function [Chronological Lunokhod 1, 2007].

The mission officially ended on October 4, 1971, on the 14 th anniversary of Sputnik

1 [Balint, 2002]. Based on its approximate landing location and the route map calculated from its images, Lunokhod 1 is believed to be at about 35.190° W, 38.287° N

[Chronological Lunokhod 1, 2007].

Lunokhod 1 traveled 10.54 km (6.55 miles), transmitted more than 20,000 TV images and more than 200 TV panoramas, and conducted more than 500 lunar regolith tests [Balint, 2002].

3.1.2: Lunokhod 2

More information is known about Lunokhod 2 than Lunokhod 1. It was launched on the Luna 21 spacecraft on January 8, 1973, and soft-landed in the LeMonnier Crater seven days later.

Lunokhod 2’s primary missions were to take pictures, to study light levels to determine if lunar astronomy was practical, to perform laser ranging experiments to

Earth, to observe solar X-rays, to measure lunar magnetic fields, and to study the mechanical properties of the lunar regolith.

32 Lunokhod 2 was 135 cm (4 ft, 5 in) high, 170 cm (5 ft, 7 in) long, 160 cm (4 ft, 11 in) wide, and massed 840 kg (1,850 lb). Each wheel had an independent suspension, electric motor, and brake. The rover had two speeds: ~1 km/h (0.6 mph) and ~2 km/h

(1.2 mph) [Balint, 2002].

It had three TV cameras, one of which was mounted high and used for navigation, which could return images at four different rates: 3.2, 5.7, 10.9 or 21.1 seconds per frame (notice that this is seconds per frame , not frames per second ; these were not speedy cameras). It had four panoramic cameras as well [Balint, 2002].

The scientific instruments on Lunokhod 2 were a soil mechanics tester, a solar X-ray experiment, an astrophotometer that could measure both visible and ultraviolet light, a magnetometer on the end of a 2.5 m (8.2 foot) boom in front of the rover, a Rubin-1 photo-detector for the laser detection experiments, and a French-supplied laser corner- reflector [Balint, 2002].

Lunokhod 2 operated for 4 lunar days. In that time, it covered 37 km (23 miles) of terrain, some of it quite hilly, which is almost four times more than Lunokhod 1, despite the fact that it was active for less than half as long. It sent back over 80,000 TV images and 86 panoramic ones.

On its fourth lunar day, Lunokhod 2 was accidentally driven into a crater. Its lid touched the side of the crater and some lunar dust fell onto it, partially obscuring the solar cells. This drop in power was noticeable, but did not significantly impair it.

Unfortunately, when the lid closed that night, the lunar regolith fell into the compartment and onto the thermal radiators that helped keep it cool during the day.

Lunar dust is an excellent insulator, and the rover overheated within 2 Earth days of its

33 fifth lunar day, on May 10, though the mission was not declared ended until June 4,

1973. Before it ceased operating, it was oriented so its laser reflector could still be used

[Chronological Lunokhod 2, 2007].

Figure 3-2: Lunokhod 2 with the lid partially open [Balint, 2002].

3.1.3: Lunokhod 3

There was a fourth Lunokhod vehicle built, Lunokhod 3, but it was never flown. It is at the NPO Lavochkin museum [Wikipedia, Selenokhod].

3.2.4: Lunokhod Mobility

Little has been published about the drive systems or wheels of the Lunokhod rovers.

Each had eight spoked wheels (Figure 3-3), each of which was independently powered.

Lunokhod 2 had an independent suspension, electric motor, and brake on each wheel, and could drive at either~1 kilometer/hour (0.6 miles/hour) and ~2 km/h (1.2 mph)

[Balint, 2002]. Lunokhod 1 only had one motor for each hub [Wikipedia].

34 Lunokhod 1 drove for a shorter time each lunar day due to the northern shadows

[Chronological Lunokhod 1, 2007], but it was probably also a slower vehicle since it traveled so much less on average each day (less than one kilometer per lunar day, as opposed to Lunokhod 2’s 9.25 kilometers per lunar day).

Figure 3-3: Lunokhod wheels [Wikipedia].

35 3.2: PROP-M Rover

The PROP-M rover (the acronym for the Russian phrase Mobile Robot for

Evaluation of the Surface of Mars), which has also sometimes been called Marsokhodik

(tiny Mars rover), was a very small, semi-autonomous ‘walking robot’ that was carried on four of the USSR’s Mars missions. The Mars 2 and Mars 3 missions were identical and they had both an orbiter and a lander, with PROP-M on the lander.

Figure 3-4: The PROP-M rover [NASA].

The Mars 2 orbiter was successful, but the lander descent sequence failed and the lander crashed on the surface on November 27, 1971, destroying the little rover.

The Mars 3 orbiter did slightly worse, failing to achieve the desired orbit, but the lander did better as it successfully touched down on December, 2 1971, at 45° S, 158°

W. However, 20 seconds after touchdown signals stopped coming, though it is unknown if the failure was in the lander or in the relay from the orbiter [Balint, 2002]. As PROP-

M had not moved by this time, it should still be inside the lander.

The Mars 6 and Mars 7 missions also had PROP-M rovers on board, but the landers did not even reach the planet, much less deploy the rovers.

36 PROP-M was a small rover that massed 4.5 kg (9.9 pounds). It was tethered to the lander with a 15 m (49.2 foot) cable for direct communication. The cable might also have carried power.

The rover was a squat box (Figure 3-4) with a small protrusion in the center. It had obstacle detection bars at the front, and so presumably did not have a camera. It was designed to ‘walk’ on a pair of wide, flat skis, rather than wheels or legs.

The lander had a manipulator arm that was supposed to move the rover to the surface. The rover would stay in sight of the lander’s camera, stopping to take measurements every 1.5 meters (4.9 feet). Its tracks in the martian regolith would also be recorded and analyzed for material properties [Balint, 2002].

The PROP-M rovers were the first rovers on Mars by more than two and a half decades, though the fact that none of them were ever activated means that they were not the first successful rovers on Mars. All of the failures were in the spacecraft or the landers, however, so it is possible that PROP-M would have been very successful indeed, if only their transport had functioned properly.

37 3.3: Pathfinder-Sojourner

The Mars Pathfinder (MPF) mission was a National Aeronautics and Space

Administration (NASA) mission to Mars. It was launched on December 4, 1996 and landed at Ares Vallis, in the Chryse Planitia region, on July 4, 1997, after a seven-month cruise. It carried the Sojourner rover, which was the first American rover. Sojourner was a semi-autonomous rover, with no tether to its landing vehicle, though the lander did act as the radio relay.

Figure 3-5: The Sojourner rover [NASA JPL].

Sojourner was in some senses remote-controlled from Earth, as it received commands on where to go and what to do, but it moved on its own with a basic camera and an obstacle-avoidance program.

It lasted 83 sols (Mars days: 24 hours, 39 minutes) and collected a lot of data, including most of what we know about martian ground weather. Signals from the rover

38 ended on September 27, 1997, from either rover or lander failure. Attempts to contact it stopped on March 10, 1998. Sojourner was the first successful semi-autonomous planetary rover, and as such it proved that it was possible.

Sojourner was 28 cm (0.92 feet or 11.0 inches) tall, 63 cm (2.07 feet) long, and 48 cm (1.57 feet) wide [Wang, 2007]. It massed 11.6 kg (25.5 feet), with another 4 kg (8.8 pounds) of necessary equipment on the lander, such as the transponder [Muirhead,

2004].

Sojourner had six wheels and a rocker-bogie suspension system (Figure 3-6). The front and center wheels together made the bogie, which could pivot freely at the front of the rocker. The rocker was the rear wheel, with a pivot that was close to the rover’s center of gravity and an attachment to the bogie. The pitch angle of the rover’s body was the average of the pitch angle of the two rockers, due to the differential mechanism that connected the rockers to the body [Wang, 2007].

Figure 3-6: Computer model of Sojourner, showing the rocker-bogie suspension

system [Wang, 2007].

39 The rocker-bogie suspension system enabled the rover to climb over obstacles that were one and a half times the wheel diameter, which is far better than a normal four- wheeled vehicle. There were no springs involved, which improved the traction [Wang,

2007].

The six wheels each had an independent actuator and a 2000:1 gear, which gave them enough torque to drive in soft sand. They had metal cleats to improve traction

(Figure 3-7), which extended 10 mm (0.39 in) into the sand. Each wheel had a diameter of 13 cm (5.1 in) and a width of 7.9 cm (3.1). The front wheels were independently steerable, which gave Sojourner a turning radius of 0; it allowed Sojourner to turn in place [Wang, 2007].

Figure 3-7: Sojourner’s wheel [Wikipedia].

Sojourner was a slow robot, however, with a top speed of only 0.4 meters/minute

(1.3 feet/minute), or 0.024 kilometers/hour (0.0149 miles/hour), which is far, far slower than the Lunokhod rovers’ speed of 1 or 2 km/h. [Wang, 2007]. It also could not leave sight of the lander.

40 3.4: Mars Exploration Rovers

The Mars Exploration Rovers (MER) are the best known rovers to date. They are a

NASA mission, with two rovers operating simultaneously in different locations. Spirit was launched on June 10, 2003, and landed on January 4, 2004 in the Gusev crater

(14.57° S, 175.47° E). Opportunity was launched during the night of July 7, 2003, and landed on January 25, 2004 on Meridiani Planum, in a small crater that was later named

Eagle Crater (1.95° S, 354.47° E) [NASA Facts: Mars Exploration Rover].

Figure 3-8: An artist’s drawing of the Mars Exploration Rovers [NASA].

The MER rovers were designed to operate for 90 sols (92.4 Earth days) but will continue to move as long as they are still functioning; they are currently on their sixth mission extension. They have been operational for about six and a half Earth years and have exceeded their initial expected lifespan by more than 25 times, giving them the nickname of ‘the little rovers that could’.

The MER rovers are far larger than Sojourner, with a height of 1.6 meters (5.2 feet), a length of 1.6 meters (5.2 feet), and massing 174 kilograms (384 pounds). They are often called ‘golf-cart-sized’ for this reason. Like the Lunokhod rovers, the MER rovers

41 are solar powered and have a radioisotope heater to deal with the cold [NASA Facts:

Mars Exploration Rover].

Part of the extra size and mass of these rovers is due to their biggest difference from

Sojourner. Sojourner needed its lander for its main communication and calculating functions, along with a platform for its camera. The MER rovers carry all of these functions on board; the landers were done as soon as the rovers left them, and have not been needed since [NASA Facts: Mars Exploration Rover].

The MER rovers have six wheels and a rocker-bogie suspension system, both adopted from the successful Sojourner. Their extra weight and higher center of mass, however, means that the suspension system has to be in the back of the rovers to be most effective [Mars Exploration Rover Mission: Technology]. The suspension is designed so that they can drive over obstacles that are 25 centimeters (10 inches) in height, or almost as large as the wheel diameters [Rover Wheels, NASA].

The MER wheels were machined from one solid piece of aluminum, minimizing the useless extra mass (called scar mass) of joins and bolts that would be needed to hold together and strengthen an assembled wheel. They are anodized (covered with a black exterior coating) to add strength [Wheels in the Sky].

The wheels are about 26 centimeters (10.2 inches) in diameter. They are filled

(Figure 3-9) with an orange substance called Solimide. It is a solid, cut to fit in the grooves of the spiral flecture, and is there to fill in the open spots in the wheels to prevent dust and gravel from getting into them and interfering with the driving and steering actuators. Solimide was used because it maintains its flexibility even at the lowest Mars temperatures [Mars Exploration Rover Mission: Technology].

Figure 3-9: A Mars Exploration Rover wheel [Mars Exploration Rover Mission:

Technology].

The wheels have cleats to add to traction, but they can still slip on sand, even sliding backwards when going uphill. Because of that, they use their cameras and Visual

Odometry software system to gauge distance more accurately than simple wheel odometers [Mars Exploration Rover Mission: Technology].

The rovers are designed so that they can handle a full 45° tilt without tipping over, but their hazard software is designed to prevent them from ever exceeding a 30° tilt.

They have a top speed of 5 cm/s (1.97 in/s), 3.0 meters/minute (9.84 feet/minute), or

0.18 km/hour (0.112 miles/hour). This is about seven and a half times faster than

Sojourner’s top speed, but still only a twentieth of Lunokhod’s top speed. Their total distance traveled is much less than their speed would suggest, due to the amount of time they have to spend stopped in order to assess the terrain and so science. 1.0 cm/s (0.34 in/s) or 0.036 km/s (0.0224 miles/hour) is still more than Sojourner’s top speed, but not by much [Rover Wheels, NASA].

43 3.5: Mars Science Laboratory

The next NASA martian rover will be Mars Science Laboratory (MSL), now named

Curiosity. It is scheduled to be launched in the fall of 2011 (having missed its original

2009 deadline), so it is not yet complete and not much has yet been published about it.

NASA’s website contains most of the information, however.

Figure 3-10: An artist’s drawing of Mars Science Laboratory [NASA].

MSL will have the same basic wheel system and rocker-bogie suspension as the previous rovers, though it is far larger and heavier than they were. It is, in fact, the size of a compact car, rather than a golf cart (Figure 3-11). Its wheels are correspondingly larger (Figure 3-12) [NASA, Mars Science Laboratory].

Figure 3-11: A comparison of Sojourner, the Mars Exploration Rovers, Mars

Science Laboratory, and the Phoenix lander [NASA].

Figure 3-12: Two comparisons of the wheels from Sojourner, the Mars Exploration

Rovers, and Mars Science Laboratory [NASA, Mars Science

Laboratory].

MSL has a ground clearance of more than 60 centimeters (24 inches) [NASA, Mars

Science Laboratory], and it should be able to climb over obstacles 75 centimeters (29 inches) high [NASA Facts: Mars Science Laboratory]. It is also designed to withstand a tilt of 45°, but should never have one of more then 30°.

Figure 3-13: A schematic of MSL’s components [Wikipedia].

MSL is supposed to have a top speed of 4 cm/s (1.575 in/s), or 144 meters/hour (472 feet/hour) [NASA: Mars Science Laboratory], which is faster than MER but still much slower than Lunokhod. Its average speed will be 30 meters/hour (98.4 feet/hour), due to the terrain and its power levels [NASA, Mars Science Laboratory]. It is expected to do about 200 meters/day (660 feet/day) on average [NASA Facts: Mars Science

Laboratory].

While MSL inherited many characteristics from MER, including the wheels, suspension, and basic structure, it will be powered via nuclear power, rather than solar power, as it is such a large rover that Mars’s weak sunlight would not be able to sustain it. It will use a radioisotope power system, as the Voyager spacecraft had, using plutonium [NASA, Mars Science Laboratory].

46 3.6: ExoMars

The last Mars mission that is currently planned is ExoMars, by the European Space

Agency (ESA) and supported by NASA. The ExoMars program will be in two launches,

2016 and 2018 (Figure 3-14). In 2016, an orbiter and a lander will go to Mars. The lander is an Entry, Descent, and Landing (EDL) Demonstrator, not a rover, so it will do measurements from a stationary position. [ESA-NASA ExoMars 2016-2018]

Figure 3-14: Elements of the ESA-NASA ExoMars program [ESA-NASA ExoMars

2016-2018].

In 2018 the second half of ExoMars will be launched; two rovers, one by the ESA and one from NASA. They will do some exploration on their own, including drilling and sampling. No Mars rover has done any deep drilling yet, though MER did use a Rock

Abrasion Tool (RAT) on surface rocks.

47 ExoMars is being primarily designed as an astrobiology mission, meaning that it will look for signs of current and past life. It will also look at past water and at current atmospheric gases.

Another function of ExoMars will be to collect and prepare rocks for a sample return mission. This will be significant, since no samples have ever been returned from Mars, and Earth-based laboratories can do many tests that rovers cannot [ESA-NASA ExoMars

2016-2018].

The ESA’s rover will be solar powered and highly autonomous, traveling up to 100 meters per sol with only infrequent contact with Earth. It has six wheels and a rocker- bogie suspension system, a design that has been proven to work. Each wheel is individually steered and motorized, and they can be pivoted to adjust the rover’s angle to the local surface. They can do a limited amount of walking (wheel walking is discussed below, with the Marsokhod) [ESA-NASA ExoMars 2018].

The rover’s navigation instruments include stereo cameras on the mast, inclinometers, gyroscopes, and Sun sensors. The Sun sensors can be used to determine the absolute attitude of the rover compared to the martian surface, along with the direction towards Earth.

Ground-penetrating radar will be used to determine which sites to drill at. The drill can reach a depth of up to 2 meters, and it can investigate the borehole as well as the sample collected. There is a small laboratory inside the rover with four different instruments that can examine the crushed powder retrieved from the borehole. The powder will be examined through chemical, physical, and spectral analysis [ESA-NASA

ExoMars 2018].

48 3.7: Marsokhod

Marsokhod means ‘Mars-walker’ in the Russian language and was Russia’s (after the

USSR broke up) second attempt at a Mars rover. It was developed by the Mobile

Vehicle Engineering Institute, St. Petersburg, Russia, and it was intended to fly on the

Russian Space Agency Mars-96 mission, which is also sometimes called Mars 8. It was not sent, however, which is just as well as the fourth-stage rocket failed to burn and the spacecraft broke up as it fell back into Earth’s atmosphere.

Marsokhod wound up being used for a number of planetary rover simulations on

Earth, including some by NASA as they were developing Sojourner. The Automation

Technology Laboratory of the Helsinki University of Technology is currently using the

Marsokhod basic design and modifying it slightly to build a new rover [Hakenberg,

2008].

Marsokhod was a six-wheeled, skid-steered rover with 6 hollow conical wheels. Its maximum wheel diameter is 35 cm (13.78 inches or 1.15 feet). The wheels have wheel blades for enhanced turning efficiency in weak ground [Hakenberg, 2008].

Figure 3-15: Marsokhod wheel design and ground clearance [Hakenberg, 2008].

Figure 3-16: (Left) Original prototype rover with conical wheels.

(Middle) Marsokhod at the Laboratoire d'Analyse et d'Architecture des

Systèmes.

(Right) Marsokhod at Silver Lake. [Hakenberg, 2008]

Marsokhod also has wheel-walking capacity; that is, each of the axles is a ‘leg’ of sorts, and can move (Figure 3-17). This means that it has even more wheel mobility and climbing ability than the rocker-bogie system and can climb a 25-30° slope instead of just a 20° slope (Table 3-1). Wheel-walking also prevents the wheels from ripping up the ground.

Figure 3-17: Helsinki University of Technology Marsokhod rover in snow and

climbing over a log [Hakenberg, 2008].

50 Table 3-1: Wheel modes and their uses [Hakenberg, 2008]. Wheel Mode Conditions Areas with accumulations of stones, slightly With minimum base rugged terrain. Maneuvering under hindered conditions. Turning on weak grounds. Principal mode of motion on mean rugged With nominal base terrain. With increased base Benches, slopes. Increase in course stability. Overcoming of high obstacles. Hanging of With forced bending of joints motor wheels which failed. Considerable lifts on granular grounds, including maximum equal to angle of repose of Wheel walking mode of motion ground (30-35 degrees). Dangerous obstacles and their combinations. Wheel walking mode of motion Overcoming of fractures, accumulations of with forced reconfiguring of frame stones.

51 3.8: Nanokhod

Nanokhod (meaning ‘small rover’ or ‘small walker’ in Russian) was another initially

Russian design that started in the 1990’s, though they never flew it.

Nanokhod was proposed for a NASA Discovery mission [Squyres, 2005], but that mission was not selected. Later, it was flown on the ESA’s Beagle 2 Mars lander, but the lander crashed and the rover was lost.

More recently, the small rover has been proposed to be used on Mercury.

3.9.1: Nanokhod Mercury

Nanokhod is a very small rover. It is only 25 x 16 x 6.5 cm (9.8 x 6.3 x 2.6 inches), and it masses only 3.2 kg (7.04 pounds). This is even smaller than the PROP-M rover. It can overcome obstacles up to 10 cm (3.9 inches) in height, though, and it travels at 2.7 m/hr (8.9 ft/hr), or 0.0027 km/hour (0.00168 miles/hour) [Hoerner, 2008].

Nanokhod is a tracked vehicle, not a wheeled one. It is to be tethered to its landing vehicle and receive power and communication through the extendable tether. The mission lifetime is only estimated to be 14 days, so this small rover is not expected to match the current Mars rovers. [Hoerner, 2008].

Figure 3-18: Nanokhod with labeled parts [Nanokhod Rover].

52 The mission is aimed to land during Mercury’s night. Nighttime on Mercury lasts far longer than the expected lifetime of the rover, so it will be designed only to work in darkness and in cold. Mercury at night has surface temperatures of -183° Celsius

(-297.4° Fahrenheit), a very finely powdered regolith surface, and a landing shock of

200 gees in only 20 milliseconds. It will have as much as 5.7 W of power at peak, and

1.3-3.4 W at other times (Table 3-2) [Hoerner, 2008].

Table 3-2: Operational Analysis of a 7 day mission [Hoerner, 2007]. Duration Energy Consumed Activity Description Hours % Total Wh % Total Checkout System test on lander 8 5% 13 6% Exit Lander perform first Deployment 9 5% 17 9% measurement cycle Move between two Movement 14 8% 76 38% measurement sites Measurement 1 measurement with each 57 34% 95 47% cycle instrument Idle No activity 111 66% 0 0%

Nanokhod is being designed to do in-situ geochemical and geological studies. The payload cab that holds the instruments (Figure 3-17) has two degrees of freedom

[Hoerner, 2008].

While no landing missions to Mercury are currently being considered, the features that would make this rover good for Mercury would also make it good for working on the Moon during lunar night [Nanokhod Rover].

53 Chapter 4 The Martian Environment

Mars has a harsh climate, even for machinery, and any rover designed to go there must be able to withstand it. Three major areas are looked at in this chapter: powering the rover, Mars’s seasons and temperature, and the atmospheric conditions. It then examines how those areas affect the design of Mars rovers.

4.1: Power

The power sources that have been used or considered for rovers and spacecraft are battery power, solar power, fuel cells, nuclear reactors, and radioisotope thermoelectric generators (RTGs). All of these sources have factors that recommend them and ones that do not. Batteries have been used for short-lifespan probes, but when used as the main power source, they do not last long enough for most uses (though all solar-powered craft do have batteries in them). Most spacecraft use solar power or RTGs, depending largely on where in the solar system they operate. Both nuclear reactors and fuel cells have been used on spacecraft, but they have not actually been used in any rovers.

In this section, we will look at the two main systems that have been used for powering rovers in the past: solar power and RTGs. The current Mars Exploration

Rovers are powered by solar energy. NASA’s next rover, Mars Science Laboratory, will be powered by an RTG.

Solar power is simpler, cheaper, and usually lower-mass than RTG systems, but it is also highly inefficient and relies on a good source of sunlight. It is the system that has been used on the most spacecraft as well as on most of the rovers so far.

54 RTGs are more reliable than solar arrays, but they are even less efficient. They are also bulky, more likely to cause problem with associated instrumentation, in some ways more dangerous, and have a finite lifespan. They are also politically controversial, since they involve radioactive materials. They are most often used on spacecraft that are going beyond Mars, and MSL will be the first rover to be powered by RTGs.

Figure 4-1: Lifespan versus wattage that can be expected for various types of

power systems [Griffin et al., 2004]. Solar arrays and RTGs both have

long lifespans but give less power than fuel cells or nuclear reactors.

Notice how the amount of power is essentially independent of lifespan

for both of those systems.

4.1.1: Solar Power

Solar power is a form of photovoltaic power, where light is converted to electrical energy. The amount of power gained is a function of the intensity of the light, the angle of the light, the efficiency of the solar cells, the temperature, the area of the array, and the amount of time that the array is in the sunlight.

A solar array is a number of solar cells connected together in series and parallel. The number of cells connected together and the way in which they are connected can provide any desired combination of current and voltage.

Solar cells can be made in a variety of shapes and sizes. Most cells are rectangular to make it easier to pack them together as densely as possible, since the higher the density the smaller the actual area of the array has to be. Since some area must be allotted for spacing requirements and electrical connections, the highest densities so far achieved have been about 90%, and that is unlikely to be improved on much [Griffin et al., 2004].

Solar cells are essentially semiconductors, and are thus very sensitive to temperature.

As a general rule, voltage decreases as temperature increases. Current decreases as temperature drops, changing by about 10% of the magnitude that voltage changes (see

Figure 4-2). The net result is an increase in available power as temperature falls, and

Mars is cold.

All photons falling on a solar cell are absorbed within the first ten micrometers, so solar cells can be very thin. Modern solar arrays are usually capable of being rolled up or folded up (depending on how stiff the final form must be). Most spacecraft have deployable arrays, to prevent damage and save space on launch [Griffin et al., 2004].

Figure 4-2: Effects of temperature on voltage and current [Griffin et al., 2004].

Ways to make smaller solar arrays are constantly being pursued, and one way that has been discovered is solar concentrators. Such concentrators collect light from a wider area and concentrate the light onto the solar cells, effectively giving a solar array with the size of the concentrator, but only the mass of the smaller number of solar cells.

Figure 4-3 shows two types of solar concentrators.

Figure 4-3: Two types of solar concentrators. The top one is normally used with

gallium arsenide cells, and the bottom one is a simpler, flat

concentrator design [Griffin et al., 2004].

Different solar cells have different efficiencies. The higher the efficiency of the cell, the higher the percentage of energy is converted. Even the most efficient solar cells are wasteful, however, in that they can only convert about 20% of the energy they receive into useful power. The most expensive solar cells have 30% efficiency, but the other

70% is still wasted, no matter how the cell is made.

Another drawback is simply that the solar energy must be available, and the solar flux drops off compared to the square of the distance from the sun. Mars, which is one and a half times as far from the sun as the Earth is, receives only half as much solar energy as we do. Solar power is also unavailable at night, so power can be collected only half of the time. Batteries can store power for use during that time, but more cannot be generated. Any form of shade, including rocks, interferes with energy production.

Other drawbacks include the fact that solar panels work best when lined up perpendicular to the light, cells can become dirty or scratched over time, atmosphere screens out light, weather can block light almost completely, and long-term radiation degrades solar cells [Griffin et al., 2004].

For rovers on Mars, clouds are not an issue, but dust is. Mars’s atmosphere is dusty at the best of times, and the dust accumulates on the panels. If the wind had not cleaned them several times, the MER rovers would have stopped working long ago. Methods of preventing or removing dust buildup will be discussed later.

The terrain on Mars is also very uneven, preventing the rovers from always pointing their panels at the sun. There is the possibility of making the panels movable and capable

58 of tracking with the sun, but this is still not perfect and it has its own complications in both hardware and software.

Mars has less atmosphere than Earth does, but there is still enough to reduce the light levels even on the clearest days. This is made worse by the fact that almost all gullies are at 30° latitude or higher, and so the light is highly angled, especially in winter. Gullies are often in craters as well, which could potentially shade the rovers and increase the problems due to cold.

Still, despite all of the problems, tests on Earth indicate that solar power can keep batteries charged during the day despite being constantly moving [Miller, 2003].

4.1.2: Radioisotope Thermoelectric Generators

A radioisotope thermoelectric generator is a power source that uses internally generated heat, not sunlight, as the power source. The heat source is surrounded by an array of semiconductor thermocouples connected in series and parallel to give the desired voltage and current to the spacecraft. Because the power source is independent of the sun, RTGs have the great advantage of being able to work in shade, including in deep craters.

Figure 4-4: Cassini’s RTG [jpl.nasa.gov].

The heat source for a RTG is a radioactive isotope of an element. The radioisotope decays at a predictable rate and gives off the heat that is needed. Due to the way that isotopes decay, the amount of heat given off will always fall off over time, even if the rest of the RTG still works perfectly. Choosing the appropriate isotope for the desired lifespan of the spacecraft is an important feature of designing a RTG.

Table 4-1: Properties of RTG materials [Griffin et al., 2004].

Property 210 Po 238 Pu 144 Ce 90 Sr 242 Cm Half-life, years 0.378 86.6 0.781 28.0 0.445 Watts/gram, thermal 141 0.55 25 0.93 120 $/Watt, thermal 570 3,000 15 250 495

Table 4-1 shows the properties of five different radioisotopes: polonium-210, plutonium-238, cesium-144, strontium-90, and curium-242. Polonium-210, cesium-144,

60 and curium-242 clearly have very short half-lives; far shorter than any multiple-year mission could use, since their power output would fall by half within less than one year.

Plutonium-238 and strontium-90 are the only real choices for use in RTGs, despite the fact that a larger mass must be used. Strontium-90 would be far cheaper to use than plutonium-238, in terms of Watts generated per cost of material, but it has a shorter half- life and gives off much more damaging radiation. Because of that, most RTGs to date have used plutonium-238 as their power source [Griffin et al., 2004].

Cheaper , however, does not mean cheap . The radioisotope for an RTG is far more expensive than solar arrays would be, in order to generate the same power. In addition, the supply of plutonium-238 is low, and NASA is currently trying to get permission to make more of it.

RTGs also give off waste heat and radiation that can damage electronics and affect the sensors and equipment on the spacecraft. Due to this, RTGs are usually located as far away from the rest of the craft as possible, usually on a deployable boom of some kind.

For a rover, much more emphasis will have to be placed on shielding from these effects.

Thermoelectric conversion is a very inefficient process, even more inefficient than solar power. The thermocouples can, on average, convert only 10-11% of the heat energy into power. Additional losses are caused by conductance drops, internal resistance, and material effects, dropping the true efficiency of a RTG down to only 6-

7%. The other 90+% has to be radiated away to avoid overheating the spacecraft [Griffin et al., 2004]. It is easier to lose heat through convection than through pure radiation, however, so heat problems are easier to manage on a planet.

61 Another problem with RTGs is the potential for radioactive contamination in the event of a failed launch or landing. RTGs are well designed to prevent such contamination if a launch fails, explodes, or reenters atmosphere, and at least one RTG has been recovered and reused after a launch failure [Griffin et al., 2004].

Still, failure of those measures cannot be completely ruled out. Failure on landing would have considerably higher velocities than failure on launch, so it is possible that a bad landing would contaminate Mars no matter how well designed it is.

The Cassini mission was designed so that there would be less than 10 6 (one chance in a million) that radioactive materials would be released upon Earth impact [Cassini

Environmental Impact Statement]. Other RTG missions should have similar risks.

62 4.2: Thermal Effects

Mars is, on average, very cold, and cold affects both electrical and mechanical parts.

It would also affect the efficiency of the power source.

4.2.1: Seasons on Mars

A martian day, or sol, is 24.66 hours (24 hr, 37 min, and 22.65 sec) long; slightly longer than an Earth day. The year is 686.98 Earth days long [SEDS, 2003] and 668.6 sols long. The Mars calendar is not measured by days, as Earth’s is, but in the number of degrees it has moved in its orbit (similar to longitude measurements). This is called heliocentric longitude, and abbreviated Ls. The zero point is at the northern vernal

(spring) equinox, with summer solstice at Ls 90, autumnal equinox at Ls 180, and winter solstice on Ls 270.

Mars has a much more eccentric orbit than Earth does (0.0935 instead of 0.0167

[Mars Fact Sheet]). Its distance from the sun ranges from 1.38 to 1.67 AU, so the seasons vary in length considerably more than they do on Earth (Table 4-2) [SEDS,

2003]. Its tilt of 25.19° is slightly larger than Earth’s tilt of 23.45° [Mars Fact Sheet], so the seasons are more severe than seasons on Earth [Caplinger, 1994].

Table 4-2: Length of Mars seasons [SEDS, 2003].

Season Length (Earth days) Length (sols) Northern spring/southern fall 199.6 194.3 Northern summer/southern winter 181.7 176.8 Northern fall/southern spring 145.6 141.7 Northern winter/southern summer 160.1 155.8 Total 687.0 668.6

Mars’s shortest season is its northern fall/southern spring (Table 4-2), but it is actually closest to the sun during its northern winter/southern summer (Figure 4-5), as it is only at 1.36 AU on that solstice (Ls 270). Because of this, summer in the south has more in the way of solar power than summer in the north, but it is also shorter.

Figure 4-5: Seasonal images of Mars. The seasons shown are those of the northern

hemisphere [Caplinger, 1994].

64 On average, Earth receives 1367.6 W/m 2 from the sun at 1 AU. Mars’s semi-major axis is 1.524 AU from the sun, so on average Mars receives 1/(1.524) 2 = 43.07% as much sunlight as Earth, or 589.1 W/m 2. Mars’s high eccentricity makes this value less useful, however, than knowing the values at various points in its orbit (Table 4-3).

Table 4-3: Solar irradiance at Mars.

% of Earth’s Solar irradiance AU sunlight (W/m 2) Perihelion 1.381 52.4% 716.7 Semi-major axis 1.524 43.1% 589.1 Aphelion 1.666 36.0% 492.7

Northern spring/southern fall 1.56 41.1% 562.0 Northern summer/southern winter 1.65 36.7% 502.3 Northern fall/southern spring 1.45 47.6% 650.5 Northern winter/southern summer 1.38 52.5% 718.1

4.2.2: Temperature Swings

Because of Mars’s orbit, summer in the south is warmer and shorter than summer in the north. It also means that southern winter is both long and far from the sun, so seasonal extremes are greater in the south than in the north.

Figure 4-6 shows the temperatures on one particular day in late summer in the northern hemisphere. Note that temperatures on that day reached close to 0° Celsius, and this was in the cooler north and not on the hottest day of the year.

Figure 4-6: The daytime temperature on Mars at Ls 104 during one year. Picture

courtesy of Arizona State University [http://tes.asu.edu/].

Figure 4-7 shows the temperatures during that night. This is late winter in the southern hemisphere, and temperatures dropped to below -100° C (173 K). In the latitudes of interest (30-70° S), they dropped as low as -120° C (153 K), and this is not the coldest temperature possible.

Figure 4-7: The nighttime temperature on Mars on the same day and year: Ls 104.

Picture courtesy of Arizona State University [http://tes.asu.edu/].

Any rovers intended to last more than a few months would have to be able to deal with temperatures lower than anything encountered on Earth. Even a short-term rover would have to deal with nighttime temperatures in summer, and the hottest area of the north this particular night was around -55° C (218 K).

The area that was warmest during the day dropped to close to -85° C (188 K), so temperature swings of 85° C do happen, and it is possible that the temperature could change by 100° C or more in a single day. Extreme temperature swings are harder on machinery than just extreme temperatures, adding yet another level of difficulty.

67 4.3: Atmospheric Conditions

Mars’s atmosphere is very, very different than Earth’s. It is, on average, between 4.0 and 8.7 millibars (Table 4-4), which is 0.5-0.8% that of Earth. It is also almost pure carbon dioxide, with little nitrogen or oxygen and almost no water vapor (Table 4-5)

[Mars Fact Sheet], so chemical reactions are very nonstandard. Rust, for example, is the interaction of iron and oxygen with water acting as a catalyst, so rusting is very unlikely on Mars.

Table 4-4: Mars’s atmospheric information [Mars Fact Sheet].

6.36 mb at mean radius, variable from 4.0 to 8.7 mb Surface pressure depending on the season (6.9 mb to 9 mb (Viking 1 Lander site)) Surface density ~0.020 kg/m 3 Scale height 11.1 km Total mass of atmosphere ~2.5 x 10 16 kg Average temperature ~210 K (-63 C) Diurnal temperature range 184 K to 242 K (-89 to -31° C) (Viking 1 Lander site) 2-7 m/s (summer), 5-10 m/s (fall), 17-30 m/s (dust Wind speeds storm) (Viking Lander site) Mean molecular weight 43.34 g/mole

Mars’s atmosphere is 1.6% Argon (16,000 ppm) [Mars Fact Sheet], while Earth’s is only 0.95% Argon (9,340 ppm) [Earth Fact Sheet]. Argon has a low thermal conductivity (K GAS ), which is why it is used as an insulator in dry suits. Carbon dioxide is a well-known greenhouse gas, with a similar thermal conductivity to Argon [Maiken,

2006], so almost 97% of Mars’s atmosphere is good at conserving heat. At Mars’s distance from the sun, this is important.

68 Table 4-5: Mars’s atmospheric composition [Mars Fact Sheet]. Partial pressures are

calculated based on an average pressure of 6.32 mb.

Parts per million Partial Molecule % by volume (ppm) pressure (mb) Carbon Dioxide (CO 2) 95.32% 6.024 Nitrogen (N 2) 2.70% 0.171 Argon (Ar) 1.60% 0.101 Oxygen (O 2) 0.13% 8.22E-3 Carbon Monoxide (CO) 0.08% 5.06E-3 Water (H 2O) 210 1.33E-3 Nitrogen Oxide (NO) 100 6.32E-4 Neon (Ne) 2.5 1.58E-5 Hydrogen-Deuterium- 0.85 5.37E-6 Oxygen (HDO) Krypton (Kr) 0.3 1.90E-6 Xenon (Xe) 0.08 5.06E-7

4.3.1: Pressure

Pressure affects many mechanical things, from lubricants to tire rubber outgassing.

Very few studies seem to have been done on how Mars atmosphere affects materials, but there have been many studies done over the years as to how vacuum will affect various materials [Campbell et al., 1990].

Mars is not a vacuum, but since any rover sent there will be in vacuum for several months on its way, materials must be chosen with that in mind. The difference between vacuum and Mars atmosphere will only matter once it reaches the Red Planet and starts functioning. There are also studies and experience based on the missions already sent to

Mars that show how well the hardware has worked in those environments, but most of that is at the lower elevations.

69 Pressure on Mars varies with altitude quite a bit, and the higher you go the lower the pressure would be. All NASA missions to Mars so far have landed below ‘sea level’ by more than a full kilometer, including the current rovers (Figure 4-8). Opportunity is the highest landing so far, at approximately -1.4 km, and Spirit is the next highest at -1.9 km

[Braun et at., 2006]. Mars Science Laboratory’s proposed landing site is at 2 km above sea level, but as it has not landed yet, there is still no actual experience with machinery at that altitude on Mars.

Figure 4-8: Percentages of Mars’s surface at all elevations and the altitudes of

previous Mars missions, along with MSL’s proposed landing elevation

and the landing elevations of the two sites of interest for this mission.

Modified from [Braun et at., 2006].

The two craters of interest in this thesis are in Terra Sirenum and the Centauri

Montes region (see next chapter for reasons). The crater in Terra Sirenum is about 3.3 km above sea level: almost five kilometers higher than either MER rover and more than one above the proposed MSL site. The Centauri Montes region, however, is at about

-2.25 km [MOLA Elevation Map], which is only slightly above Pathfinder’s -2.5 km landing site [Braun et at., 2006], and lower than either Spirit or Opportunity.

Because of that, a rover sent to Terra Sirenum could run into low-pressure difficulties that have not yet been dealt with, while a rover in the Centauri Montes region would be at an altitude that we have data on and experience with.

4.3.2: Atmospheric Dust

Mars is extremely dusty compared to Earth, and the dust has a very different composition. It is red due to iron oxides (rust), and the particle sizes are extremely small.

Dust can be separated into three main categories, based on particle size and method of transport: airborne, settled, and saltating.

Airborne dust is dust with a radius of 2 m or less. This dust is so small that it stays in the air for long periods of time; days, weeks, or even months [Fernandez et al., 2007].

It is primarily composed of composite silicates with some magnetite (Fe 2O3) [Madsen et al., 2005], and as such it is magnetic.

Settled dust has a radius of between 10 and 80 m. This dust is lifted into the air by wind and dust devils, but they settle out fairly quickly [Fernandez et al., 2007].

71 Saltating dust has a radius of greater than 80 m. Saltation is the process of moving particles through fluid flow; in this case, strong winds blowing the dust around. Saltating particles have been noted to reach at least a meter into the air [Fernandez et al., 2007].

There are several problems that dust can cause a rover. The most obvious problem is if it covers up a solar panel, reducing the light and the power that the rover receives.

Over time, this will render a rover nonfunctional. The Mars rovers would already have reached that point if wind hadn’t swept the dust off the panels [NASA]. Dust can also block the cameras or other sensors, making them useless even if the rover is still working.

The Pathfinder rover had what was called the Mars Adhesion Experiment; an experiment designed to quantify how quickly dust built up. The rate depends on both the amount of dust in the air and the angle of the solar panels. Pathfinder, with horizontal solar panels and an airborne dust opacity of 50%, had an accumulation rate of 0.18% per sol. This means that 0.18% of the light would be blocked with every martian day that passed [Fernandez et al., 2007].

Ways to overcome dust buildup are being studied, and they fall into two broad categories; active and passive. Passive methods try to prevent the dust from settling in the first place, giving the rovers a longer potential lifespan. One method of doing this is to put a cover over the panels or sensors, which is only removed when they are being used. For solar panels, this would mean covering them at night to cut the rate of dust accumulation in half.

Another possible means of passive dust prevention is to use a permanent magnetic field of some sort to repel the dust. This depends heavily on the dust being magnetic

72 enough to react to the field, however, and a strong magnetic field could affect some of the instruments on the rover as well [Fernandez et al., 2007].

Passive methods are helpful, but in the end, the dust will still cover the panels. And if some of the sensors are extremely sensitive to dust, passive methods won’t be enough.

Active methods are ways to remove the dust after it has settled. Two of the ways currently being studied are using physical brushes (‘windshield wipers’) or using changeable electromagnetic fields.

Brushes are a proven technology, but how much dust they remove depends on how fine the brushes are, or how clean the wipers are. Dirty windshield wipers, after all, leave behind streaks when they are used. A finely bristled brush is good at removing dust, but bristles wear out or fall out over time, so the brush would get less effective with age.

Brushes made of Teflon, Titanium, and Kapton have been tested in a Martian Dust

Simulation Chamber (MDSC) [Fernandez et al., 2007], so they are possible on Mars.

Brushes also involve motors and other items that must move correctly, and any loss of motion could disable the brushes for good. Covers have similar problems. And even when they are working, motors and actuator take up mass, and brushes take up volume.

Electromagnetic fields would not need much in the way of moving parts, which might make them less likely to fail, but they also depend on dust the dust being magnetic. Electrodynamic fields also require high voltages, which might require high- mass shielding to protect any instruments the rover might be carrying [Fernandez et al.,

2007].

The Mars Exploration Rovers both carried Magnetic Properties Experiments designed to test the magnetism of Mars dust. The Sweep Magnet Experiment consisted

73 of a magnetic ring with aluminum inside and outside of it. The magnet was strong enough to attract almost all magnetic particles, so the magnetic dust would settle on the ring and the only dust accumulation on the aluminum would be nonmagnetic. The difference in dust settling inside and outside of the magnet was also of interest. The nonmagnetic cores had very little dust on them by sol 173, indicating that the vast majority of martian dust is magnetic. Spirit’s core was cleaner than Opportunity’s

[Madsen et al., 2005], which might indicate a difference in the dust at the two different locations.

The Capture and Filter Magnet Experiment had two magnets (Figure 4-9); the capture magnet was a very strong magnetic core placed inside a nonmagnetic aluminum ring, while the filter magnet was a weaker magnet that was designed to attract only the most magnetic particles, effectively sorting the dust by how magnetic it was. The two types of dust would then be studied and compared to each other [Madsen et al., 2005].

Figure 4-9: The filter and capture magnets on Opportunity after 180 sols, as seen

by the Panoramic Camera (PanCam) [Madsen et al., 2005].

Based on the magnetic experiments, most of the dust on Mars would respond to magnetic fields, so such fields are a possibility for both passive and active means of preventing dust accumulation.

Both Mars and Earth occasionally have dust storms, but Mars’s dust storms are far more severe than Earth’s. On some occasions they have been known to completely blanket the planet in dust (Figure 4-10), which severely restricts any platform that relies on solar power. Even after the winds die down, it can take weeks or months for the dust to settle completely [Mars Rovers Battle Severe Dust Storm].

Figure 4-10: Two views of Mars, one without a dust storm and one with. The dust

storm picture was taken three months after the start of the storm [The

Perfect Dust Storm Strikes Mars].

75 Chapter 5 Landing and Movement

Choosing where a mission will land can be difficult, and depends on many factors. A good choice is essential, however, because the rover will be limited to the nearby area and cannot study anything too far away.

5.1: Landing Methods

The rover must land as close to the study area as possible, to minimize travel time and possible terrain issues, but it must also land in an area wide enough and flat enough so that it lands safely. The smallest target landing area so far was for MER, and that was an ellipse of 80 km by 12 km [Braun et al., 2007].

All NASA landings on Mars have started the same: a heat shield aeroshell that protects the lander from the heat of atmospheric friction as it slows from interplanetary velocities, followed by a parachute to slow it further.

After slowing down, two forms of landing have been tried: airbags and powered decent. MSL is envisioned as going in with a third system: powered to a point, and then using a skycrane to finish. This is an interesting idea, but it is untested so far.

5.1.1: Powered Descent

Both Viking landers and Phoenix used powered descent all the way to the surface.

While this is a proven technology, it is not very accurate; Viking had a landing ellipse of

280 x 100 km, and Phoenix had one of 260 x 30 km. This requires a vertical velocity at

76 touchdown of less than 2.5 m/s, and a horizontal touchdown velocity of less than 1 m/s

[Braun et al., 2007].

It also needs the area to be flat and clear of rocks. Viking had only a 20 cm rock clearance; if either lander had had rocks larger than that under it, or had been tilted on landing by more than 15°, it would probably have failed.

Viking required four expensive Doppler radar beams [NASA Facts: Viking] and throttled engines, though Phoenix used cheaper canted multi-beam radar and off-pulsed engines [Braun et al., 2007]. Neither mission needed an accurate or rocky landing area, so this method was acceptable.

5.1.2: Airbags

Mars Pathfinder and the Mars Exploration Rovers used a different system; a parachute, then solid rocket motors to slow to low velocities, followed by a bouncing landing. They were protected on landing by being completely encased in spherical airbags while they bounced to a stop [How to Land Softly on a Hard Planet]. Pathfinder bounced fifteen times [NASA Facts: Pathfinder], while the MER landers bounced twenty-six and twenty-seven times [NASA Facts: Mars Exploration Rovers].

Each of the four sides of the lander had a single airbag with six lobes, rather than six separate airbags (Figure 5-1). The airbags are made up of many layers of Vectran TM , which is almost twice as strong as Kevlar. The airbags were held together by ropes called tendons, and were connected to the lander by ropes instead of being directly attached [How to Land Softly on a Hard Planet].

Figure 5-1: MER airbags [NASA JPL]. Note the size of the airbags compared to

the people nearby.

The Pathfinder airbags allowed a vertical landing velocity of 16 m/s and a horizontal landing velocity of up to 22 m/s, a rock clearance of 50 cm, and a surface tilt of up to

30°. The MER rovers were larger and heavier than Pathfinder (539 kg instead of 360), so the airbags had to be toughened to work. As a result, they could handle landing velocities of up to 26 m/s [Braun et al., 2007].

After landing, they deflated the airbags and unfolded the petals of the lander, revealing the rover (Figure 5-2). The way the petals were designed, the lander could be

78 on any of the four sides and would right itself as it unfolded, though Pathfinder was lucky enough to land right-side-up in the first place [NASA Facts: Pathfinder].

Figure 5-2: Opportunity with the landing petals partially opened [NASA images].

Note how much smaller the lander is without the airbags.

An airbag landing is somewhat more accurate than a powered one, with landing ellipses of 200 x 100 km (Pathfinder) or 80 x 12 km (MER), but this is still far from pinpoint accuracy [Braun et al., 2007]. Still, Spirit landed only ten kilometers from the center of its area, and Opportunity was within 25 km of its center [NASA Facts: Mars

Exploration Rovers].

Rough terrain, such as sharp rocks or ridges, could rupture the airbags or otherwise damage the lander before it comes to a stop, and so must be avoided.

5.1.3: Sky Crane

Airbags have a limit, however; they can handle only small- or medium-sized rovers.

MER was about as large as airbags can land without rupturing, so larger rovers (such as

MSL) need a different method.

One problem with powered descent is that the thrusters can blast a pit in the ground or throw rocks and dirt up onto the lander. Because of this, most powered-descent landers were designed to land as quickly as possible and with as high a velocity as possible, in order to minimize landing time. This has consequences when it comes to ground clearance and slopes, however [Braun et al., 2007].

MSL has a new landing system that is designed to get around this problem, which has been dubbed a ‘sky crane’ (Figure 5-3) after a helicopter with a similar system.

Figure 5-3: The proposed decent sequence for Mars Science Laboratory, from

parachute to powered slowing to final sky crane [Braun et al., 2007].

Instead of putting the engines and propellant tanks below the lander, it will have them above. The ‘lander’ in this case will actually use thrusters to slow to a hover above the ground, coming to an almost complete stop, and then it will lower the rover to the ground using the sky crane. The rover will actually be lowered and placed upright on its own wheels, instead of being in a lander and then driving out [NASA Facts: Mars

Science Laboratory].

The sky crane landing allows it to have a much lower landing velocity (0.75 m/s vertically and less than 0.5 m/s horizontally). It also has a rock height capability of 100 cm, instead of 20 or 50 [Braun et al., 2007].

The landing slope is still restricted to less than 15°, though MSL can handle a tilt of

45° without tipping over [NASA Facts: Mars Science Laboratory]. The lander will be able to hover for much longer than is normal, and so will be able to be much choosier when it comes to a landing spot. MSL is planned to have a landing ellipse of only 20 x

20 km, which is far smaller than that of any other lander on Mars [Braun et al., 2007].

81 5.2: Choosing a Landing Site

Since gullies occur only on slopes and are thus usually on hills or in craters, getting a rover to them presents a special challenge. Not only will the rover have to be capable of driving on such slopes, but it will also have to be able to land in the area to begin with.

The first thing to do when considering the terrain a rover will have to face is to determine where it will go. In this case, the choices are very limited.

So far, light-colored gully streaks have been found in five places: an unnamed crater in Terra Sirenum (36.6°S, 161.8°W) [MOC2-1618], an unnamed crater in the Centauri

Montes region (38.7°S, 263.3°W) [MOC2-1619], on the northeast wall of Hale Crater

(35.5°S, 35.4°W), on the south wall of an unnamed crater on the northern plains

(59.0°N, 277.7°W), and on the walls of a pit in a filled crater in Noachis Terra (47.2°S,

355.8°W) [MOC2-1620].

Only the craters in Terra Sirenum and the Centauri Montes region have shown new gully streaks, however. The other three areas were photographed with the streaks already there, and no changes have been seen in them. So there are only two known places to go to study active areas near gullies.

5.2.1: Altitude Considerations

As mentioned in the previous chapter, those two regions have vastly different elevations. Terra Sirenum is 3.3 km above ‘sea level’, while the Centauri Montes region is at about -2.25 km [MOLA Elevation Map]. The height difference leads to vastly different conditions.

82 A rover in Terra Sirenum would have more power than one in the Centauri Montes region, since there would be less atmosphere blocking the sunlight (Figure 5-4). On the other hand, both pressures and temperatures will be lower, possibly causing their own complications.

Figure 5-4: A curve of atmospheric transmittance by altitude at two different

wavelengths on March 7, 1987 [Korablev et al., 2003]. While this does

not have data near the surface, it shows that a few kilometers can make

a big difference.

Altitude also makes a difference on landing. The higher the landing, the less atmosphere there is and the less time the lander takes to travel through it. This means a lot less time to slow down; parachutes probably would not be very effective at high altitudes unless they were extremely large and bulky. Slowing to a stop would require

83 more deceleration from thrusters instead, which would mean more fuel and more weight.

A higher thrust would probably also be needed since they could not fire for as long, making the thrusters themselves larger and heavier as well.

Since Pathfinder and MER were solar-powered rovers at about the same altitude as the Centauri Montes region, much more is known about conditions there than conditions higher up, making it a simpler engineering challenge from landing to movement to power.

5.2.2: Terrain Considerations

That is not the only consideration, however. Another, perhaps more important one, is whether or not the rover can even reach the gullies to study them. Cratered terrain tends to be rough, making both the landing and the drive to the crater difficult.

A further complication is that few areas on the planet have been studied in great detail, and those do not include Terra Sirenum or the Centauri Montes region. Publicly available maps (Figures 5-5 though 5-9) have scales measured in tens of kilometers or in degrees of the planet’s surface.

Since Mars’s radius is 3,390 km [NASA: Mars Fact Sheet], one degree of surface calculates out to be 59.2 km. This is far too great to see obstacles that could stop a rover, and boulders of one meter or more can seriously affect a landing. Neither site can be completely ruled as safe, then, unless and until better maps are taken.

Figure 5-5: An altitude map of the Terra Sirenum region around the crater

[Explore Mars]. The terrain looks rough even at the wide scale shown.

Figure 5-6: A topographical map of the Terra Sirenum region around the crater

[USGS]. Notice that this covers a full 30° of the surface, or 1,770 km

across.

Figure 5-7: An altitude map of the Centauri Montes region around the crater

[Explore Mars]. The scale is the same as the one on the Terra Sirenum

map; still far too large to be certain the smooth-appearing terrain is as

easy to travel on as it looks.

Figure 5-8: A topographical map of the Centauri Montes region around the crater

[USGS]. This has the same enormous scale as the topographic map of

Terra Sirenum; it is 1,770 km across.

The deposit in Terra Sirenum is more interesting than the one in the Centauri Montes region, since there is another light flow in the same crater, but its higher altitude and rougher-appearing terrain make it a far riskier place to put a rover. Because of that, the rest of this thesis will concentrate on the Centauri Montes region.

88 5.3: Inside the Crater

In order to study the new deposits inside the craters, the rover must be able to get inside the crater and up to the deposit. This requires that it go over the lip of the crater, travel down the slope, and stop at the deposit. To study the gullies as well as the deposit, it will probably have to go inside the gully channel itself to take samples. This presents some very challenging terrain.

5.3.1: Slope

The slope around the Centauri Montes deposit varies from 15° to 35° (Figure 5-9), depending on how high you are on the crater wall [Pelletier et al., 2007]. If the rover avoids the rock outcrop at the top it might be able to find a way in that only requires a slope of 30°, but it is very unlikely that it will be able to reach the deposit without having to navigate a steep slope.

NASA’s rovers have attempted to stay on level terrain, and they are not built to handle this kind of slope. Most of the rovers mentioned in chapter 3 were also designed for level surfaces, and so would not be suitable to go inside craters.

Marsokhod, however, has been tested and can handle both rough terrain and slopes of up to 30-35° (Table 3-1) [Hakenberg, 2008]. Marsokhod has not been flown, but it was tested thoroughly, and its base would make a good platform for this mission.

Figure 5-9: Two-dimensional model results for the deposit in the Centauri Montes

region [Pelletier et al., 2007]. It shows a slope contour map, a slope

color map, a shaded relief map, and a section of the High Resolution

Imaging Science Experiment (HiRISE) image (PSP_001714_1415)

that shows the deposit.

90 5.3.2: Surface and Chemicals

Another challenge is the regolith in and around the crater. Soft sands are dangerous to rovers; Spirit has been stuck in sand since March 2009, and will never move again

[Brown et al., 2010]. If the gullies and deposits are caused by dry granular flow, the surface of the crater is probably unstable and is likely to avalanche; catching the rover is a sand-slide which could disable it before it has a chance to do any science. This is less likely if they are caused by a liquid, but it cannot be ruled out.

Once the rover reaches the deposit, it has a new challenge: how to get inside without getting stuck or damaged. The deposit at the Centauri Montes region is about twenty meters wide at its narrowest point (Figure 5-9). The best place to sample for a chemical analysis would be close to the center of the flow, but that requires driving across it.

Since we do not know what it is made of, the rover must be designed to handle as many of the likely conditions as it possibly can. Sampling several times as it moves would be a good idea as well, so we know what the conditions are at more than one point.

If the flow is dry, the main risks come from the soft sands and possible slides. The rover must have good traction, and must be able to judge if it is too dangerous to go further. If it is, it will have to sample where it can reach, then back off to keep from being lost. This is a judgment that will have to be made by either the rover or the controllers, no matter what the source is.

If the flow is from liquid or solid CO 2, the risks (other than those stated above) are mostly chemical. There is the possibility of finding liquid, though that is extremely unlikely; liquid CO 2 requires pressures far higher than those found on Earth, much less

91 on Mars. More likely, the regolith could be impregnated with CO 2. This could change the properties of the sand, and the evaporation could also leave the sand porous and unstable, and likely to collapse in on itself. The rover’s underside must also be made of a material that would not react with carbon dioxide or carbonic acid in any way.

If the flow is from liquid water, there is the chance of it still being wet; liquid water usually evaporates or freezes on Mars, but under the right conditions (see chapter 2) it can last for some time. But since the deposits were formed years ago (the second pictures dates from 2006) and might not have changed since, the main danger would probably be in what the water might have carried with it. It is entirely possible that the white color comes from salts or other dissolved substances that were left behind when the water evaporated. Salts are most likely, since liquid water on Mars is probably brine, but there could be other chemicals as well.

Finally, craters can have significant shadows inside them. A solar-powered rover probably would not have enough energy to be active for the entire day.

92 Chapter 6 Straw Man Mission

Based on the information presented so far in this thesis, it is possible to put together a rover that would accomplish the mission. While further optimization could be done, this rover should meet all of the requirements listed previously.

6.1: Payload

The payload is the most important part of any mission; without it, the rest is completely irrelevant. Because of that, it should always be chosen first, and the rest of the mission should be built around it. It is not always done this way, but it should be.

6.1.1: Cameras

This rover will require four cameras. Two can be simple black-and-white cameras, arranged so as to give stereoscopic vision for the purpose of navigation. MER has a similar system. Another needs to be a color camera that can take wide-angle and panoramic images. It has to be color because color could give important information about the deposit and the gullies.

The fourth camera needs to be a microscopic imager, which needs to be a multispectral imager (a wide spectrum B&W imager with a filter wheel in front of it). Multi- spectral imagers are often used to do mineralogy. It must be on an arm so it can be extended and lowered close to the ground, and it needs to have extremely high magnification at close range. Reasons for this are given below.

93 6.1.2: Chemistry

The chemistry package for this mission will be based on the one that the Phoenix lander used. This package was intended to test ice and icy regolith in the polar regions

[Smith et al., 2007], but it should also work for warmer areas since the temperature of the samples was not important.

One major instrument is the Thermal and Evolved Gas Analyzer (TEGA). A small amount of regolith is delivered to a tiny oven, which bakes it to about 1,000° C. Water vaporizes at 100° C and most carbonates between 200 and 300° C, so the chemicals of interest will become gaseous. The gases are piped into a mass spectrometer along with a carrier gas and analyzed [Smith et al., 2007].

Another instrument provides a Microscopic, Electrochemical, and Conductivity

Assessment (MECA) of the regolith. Tiny grains of less than 0.2 mm diameter are examined microscopically to try and determine how they were placed. Electrical and thermal conductivity of the regolith is also measured [Smith et al., 2007]. This should help answer whether the gully streaks were caused by wind, dry avalanches, or fluids.

This is the reason to include the microscopic imager.

There is also a Wet Chemistry Laboratory (WCL) incorporated in MECA. It has four single-use independent cells (Figure 6-1), each of which is made of two parts. The upper part holds leaching solution (water and the first calibrants for the sensors), a sample drawer that holds a 1 cc sample, and a reagent dispenser. The reagents are solid and include another calibrant, an acid, and some barium chloride that is used to determine sulfates [Young et al., 2009].

94 The lower part of the cell consists of several electrochemical sensors that measure pH, Eh, conductivity, cyclic voltammetry, chronopotentiometry, and the dissolved ionic

+ + + + + + - - - - - content for Na , K , Mg 2 , Ca 2 , Ba 2 , NH 4 , Cl , Br , I , and NO 3 /ClO 4 , using ion selective electrodes (ISE) [Young et al., 2009].

Figure 6-1: One cell from Phoenix’s WCL [Young et al., 2009]. The left picture

shows the whole cell, while the right one shows the inside of the lower

half, including the ion selective electrodes.

The WCL operates by putting the leaching solution with the first calibrant (10 -5 mol

+ + + + + + of nitrate, bicarbonate, or chloride salts: Na , K , Mg 2 , Ca 2 , Ba 2 , and NH 4 ) into a container. Measurements are taken, providing the first calibration point. The second, solid calibrant is then added and dissolved, bringing the concentration of salts up to

3.4x10 -5 mol. A second calibration point is taken [Young et al., 2009].

After it has been calibrated, the regolith sample is added and stirred in. The chemical sensors will then test the water chemistry, including salt content, salt composition, and

95 the concentrations of certain trace minerals [Smith et al., 2007]. Since the concentrations of the calibrants are known, they can be subtracted in order to get accurate information on the regolith sample.

Since there are only four WCL cells, it is especially important to get good samples.

The first sample should be of the regolith in the crater but outside of both the deposit and the gullies; this is a control measurement. The second sample should be of the deposit, and should come from near the center in order to get the best readings; this is why the rover should be able to drive out into the deposit.

The third sample could be of the deposit as well, if the second one was uncertain, but it would be better if it could be saved for the gullies. Ideally, the rover could get one sample from the channel of a gully and one from the apron.

Of course, each additional WCL cell that could be included would be extremely useful; four might be enough for a stationary platform, but a rover really needs more if it is possible.

96 6.2: Rover Base

As mentioned in the last chapter, the rover will have to be able to handle slopes of up to 30° or even 35°. Rovers are generally designed for fairly level terrain, just as almost all cars are.

Designing a new rover is well outside the scope of this thesis, so an existing rover base (wheels, suspension, and chassis) will be chosen to handle the payload. The choice will be made from the rovers covered in Chapter 3.

None of NASA’s rovers have been designed for slopes as steep as this one will have to face, though Mars Science Laboratory comes close as it can move at any angle up to about 30° [NASA Facts: Mars Science Laboratory]. Still, that is the extreme case and not something it is expected to do regularly. So none of the NASA rover bases will do.

ExoMars uses the same wheel arrangement and rocker-bogie suspension that the

NASA rovers have used [The ESA-NASA ExoMars Programme: Rover, 2018], and so is equally unsuitable.

Nanokhod is a small rover; much too small to handle even this modest payload.

PROP-M is even smaller. By process of elimination, this leaves Lunokhod and

Marsokhod.

Lunokhod was a fast, fairly large rover. It is certainly big enough, but it was a remote-control rover driven from Earth, though this could probably be changed. Still, information about how it handled slopes is elusive. Because of this, it must be assumed that Lunokhod was not designed for steep slopes. It was also designed for very different conditions and would not be able to function in Mars gravity [Kemurdjian, 1991].

97 That leaves Marsokhod. Fortunately, Marsokhod was specifically designed to handle steep slopes (up to 30-35°) and rough terrain (Figure 6-2) [Hakenberg, 2008;

Kemurdjian, 1991].

Figure 6-2: Helsinki University of Technology Marsokhod rover in snow and

climbing over a log [Hakenberg, 2008].

There have been several different versions of the Marsokhod rover. The initial

Russian plans involved two different rover sizes; an initial plan with the rover weighing up to 450 kg (Table 6-1), and a later, smaller rover of only 70 kg (Table 6-2)

[Kemurdjian, 1991].

Table 6-1: Marsokhod Specifications [Kemurdjian, 1991]. Parameter Unit Value Mass kg 350-450 Mass of scientific equipment kg 60-70 Wheel diameter m 0.5 Clearance — None Velocity m/s 0.5 Maximum obstacle to overcome m 1.2 Slope of free-flowing ground to overcome degrees 30-35

98 Table 6-2: Small Marsokhod Specifications [Kemurdjian, 1991]. Parameter Unit Value Mass kg 70 Mass of scientific equipment kg 15 Wheel diameter m 0.35 Velocity m/s 0.14 Maximum obstacle to overcome m 0.75 Slope of free-flowing ground to overcome degrees 30-35

Finding actual masses for the instruments is difficult, but Phoenix massed 410 kg total (Fig 6-3). This did not include wheels, as Phoenix had none, but it did include the chemistry package and solar panels that were large enough to provide power even at the poles. Therefore, the assumption is being made that the same basic chemistry package can fit on a rover with a base of 450 kg.

Figure 6-3: A comparison of Sojourner, the Mars Exploration Rovers, Mars

Science Laboratory, and the Phoenix lander [NASA].

99 While the basic Marsokhod design will be used, it will not necessarily be the full 450 kg rover mentioned above. The reference above shows that there is a version of

Marsokhod that could carry this science package, not that it is the only version that would do so. Field tests have been successfully performed on rovers with various masses from 100-180 kg [Hakenberg, 2008], and 70-450 kg [Kemurdjian, 1991]. Marsokhod’s basic structure is clearly quite adaptable to change and resizing.

Marsokhod does have disadvantages, including the fact that it is more massive and uses more energy than NASA designs, but its sturdiness and ability to handle slopes outweighs the disadvantages for this particular mission.

100 6.3: Power

Now that we know a payload and rover base that could work, we know the power requirements and can design a power source. As was mentioned in the last chapter, the source will be solar power. While this has some drawbacks, it is the best known technology for rovers, and should be adequate for a rover like this one.

Exact calculations of the size of the solar panels depend on the bus voltage and the peak power usage. The Helsinki University of Technology rover has an average bus voltage of 24 V and a peak bus voltage of 27 V. The highest current comes during difficult driving, and never exceeds 5 Amps. The peak power load can thus be calculated as 24*5 = 120 W. Since experiments can be assumed to take place only when the rover is stationary, additional drain from science can be neglected.

Calculations also need to be made for the size of the batteries the rover will carry.

The batteries are meant to sustain the rover when it is not receiving solar power, and it will not be doing much during those times. There are certain station-keeping power loads, possibly including communication, that will happen, but nothing as draining as actually driving.

The MER rovers require at least 400 W-hr per sol to be able to drive, and they use more than 138 W-hr every sol just staying alive with no activities, not even communicating [NASA Mars Rovers Braving Severe Dust Storms]. 161 W-hr per sol appears to be enough to replenish batteries and to communicate [Spirit Rover

Recuperating after Dust Storm].

Because of the above numbers it will be assumed that, when not driving, this rover can function on 175 W-hrs a sol (because of Marsokhod’s higher power requirements).

101 A margin of 20% more will be assumed, taking the minimum battery storage up to 210

W-hr. Since the batteries can only be depleted by 70%, they must hold at least 210/0.7 =

300 W-hr of power, to survive one sol with no sun.

6.3.1: Time

One of the things that has to be known is time: how long will the mission take? This depends on how far the landing site is from the study site (how far the rover has to go), how long it will take to get down into the crater and to the deposit, and how long the actual science will take.

The landing (see next section) will be in a 20 x 20 km circle, which could mean the rover is up to 20 km from the best spot. There is also the question of how far the landing zone is from the crater. Those questions are impossible to answer with much accuracy at this point (see Chapter five for a discussion of scale), so an assumption will be made that the maximum distance the rover will have to drive to reach the crater is 30 km.

The next question is, how fast is the rover? The MER rovers are quite slow; in the last six years, Opportunity has only driven 20 km, and Spirit has driven only about half that distance [NASA]. A large part of this is because they have frequently stopped to do science; they would have traveled much farther if all they had been doing was driving.

The speed of the Marsokhod rover varies, depending on the version used and the source cited. Hakenberg, 2008 gives the speed at 0.17 m/s, or 0.612 km/hr. Lamboley,

1995 gives the slightly smaller number of 0.15 m/s (0.540 km/hr), while Wettergreen,

1999 gives a much slower number of 0.07 m/s (0.252 km/hr). This thesis will assume an average value of about 0.5 km/hr on good terrain. The terrain in the area to be traversed will probably be bad, however, so that number should be halved to 0.25 km/hr.

102 If it is assumed that the rover will only drive while it has good sunlight, that will be only about four hours a day (though there will be time to accumulate more power during the morning and evening), which equates to one kilometer per day. It can do others things during this time, such as take some pictures and panoramas, but it will not stop for long periods. As such, it will take perhaps a month to reach the crater.

After it reaches the crater, some time will be required to reach the deposit and the gullies, and some for the actual science. Based on previous rovers, it could take weeks to safely navigate into a crater, so perhaps another month will be spent driving.

If you schedule the science part of the mission to take one month, at most, that gives a total mission time of three months. The science shouldn’t take that long, but some more travel will be required to reach various locations.

There will be good sunlight for at least that long, based on distance from the sun and changing sun angle (see below), so there should be plenty of power for the rover.

6.3.2: Battery Calculations

Assumptions:

Nickel-Hydrogen (Ni-H2) batteries.

ebat = battery energy density = 65 W-hr/kg [Griffin et al., 2004]

DoD = allowed Depth of Discharge = 70%

mbat-c = mass of each Ni-H2 battery cell = 400 g = 0.40 kg [Griffin et al., 2004]

Calculations:

Es = energy storage needed = 300 W-hr

mbat-t = total mass of batteries needed to store enough energy = E s/e bat = 4.62 kg

103

By these calculations, the rover needs an absolute minimum of 4.62 kg of batteries, and more if there is a chance of surviving more than one sol without sunlight. This does not qualify as part of the payload, but rather part of the power subsystem. Marsokhod has hollow wheels, so the batteries are usually placed there.

6.3.3: Solar Panel Calculations

While the rover will only drive for four hours, when the light is best, there will be sunlight for several hours before and after that. The calculations below reflect the changing sun angle and the way that it generates progressively less power on either side of zenith.

Since the summer in Mars’s southern hemisphere is 155.8 sols long [Table 4-2] and

Mars’s tilt is 25.19° [Mars Fact Sheet], the sun angle changes 12.595° between summer solstice and the equinox, or 0.0808 °/sol. Given Mars’s elliptical orbit, a calculation by year gives a smaller number (0.0754°/sol, on average), but it changes more quickly than that in the summer.

A mission of three months (approximately 90 sols) during the summer means the rover will need to be there for 45 sols on either side of summer solstice. As a result, the sun will be 3.64° away from the solstice maximum at both the beginning and end of the mission. All power calculations are made from that point, since the beginning and end of the mission will receive the least power.

Given the latitude of 38.7° and the planetary tilt of 25.19°, the best sun angle possible is 13.51° on the solstice. At the beginning and end of the mission, the best sun angle is only 17.15°. Solar power available at Mars is 718.1 W/m 2 on summer solstice

104 [Table 4-3], but drops off on either side. These calculations will use only 700 W/m 2, since a more exact number is not available.

Mars’s rotation means that the angle changes by 5° every 0.3425 hours. The calculations below start at the highest sun angle and drop by 5° for every calculation afterwards. The calculations at zenith are done for 0.3425 hours, but the others are done for 0.685 hours, since they occur twice a sol: morning and afternoon.

Assumptions:

ηGa = solar cell efficiency for Ga-As cells = 18% [Griffin et al., 2004]

2 Is = solar illumination intensity at Mars = 700 W/m

ηpack = how closely solar cells can be packed = 90% [Griffin et al., 2004]

τ = average optical depth of the martian atmosphere = 0.4 [Miller et al., 2003]

Calculations (iterative):

ηtemp = efficiency change due to temperature = 1 - (0.005/K)*(273 K - 301 K)

[Griffin et al., 2004] = 114%

PLdrive = peak load used when driving = 24*5 = 120 W

Tdrive = the amount of time the rover will be driving each day = 4.0 hours = 14,400 s

Edrive = total energy used when the rover is driving = P Ldrive *T drive = 480 W-hr

Eneed = total energy needed for one day = E drive +E n = 690 W-hr

2 If A Mars = size of array needed under Mars conditions = 1.80 m

2 Afact = size of array under factory conditions (Earth temp) = A Mars *ηtemp = 2.05 m

2 Acells = area of all cells needed under factory conditions = A fact *ηpack = 1.85 m

Pgen = power generated by the solar array = I S*A cells *ηGa = 233 W

105

θ1 = sun angle at zenith = 17.15°

t1 = length of time it is at the maximum angle = 0.3425 hours

ηangle1 = reduced efficiency due to off-normal sun angle = cos(17.15°) = 95.6%

E1 = energy generated while at the highest sun angle = (P gen *ηangle1 *e^(-τ/ηangle1 ))*t 1

[Miller et al., 2003] = 50.1 W-hr

θ2 = second-to-highest sun angle = 22.15°

t2 = length of time it is at the second-to-highest angle = 0.685 hours

ηangle2 = reduced efficiency due to off-normal sun angle = cos(22.15°) = 92.6%

E2 = energy generated while at the second-to-highest sun angle = (P gen *ηangle2 * e^(-τ/ηangle2 ))*t 2 = 95.9 W-hr

If you continue the calculations, dropping the sun angle by 5° each time, and sum up all of the individual energies, you get a total energy per sol of 692.98 W-hr for a solar panel size of 1.8 m 2. Since 1.79 m 2 produced less than the needed 690 W-hr, the rover needs 1.8 m 2 of solar panels.

Table 6-3: Solar panel size vs. energy generated. Solar Panel Size (m 2) Total Energy Generated (W-hr/sol) 1.78 685.28 1.79 689.13 1.80 692.98 1.81 696.83 1.82 700.68

The calculations above ignore extra-dusty days or other problems and conditions outside the average for Mars.

106 6.4: Entry, Descent, and Landing

Finally, based on the weight of all of the above, a method for landing must be chosen. This mission will start with an aeroshell and a parachute, similar to the ones all

Mars landers have used.

After that, it gets more complicated. Neither powered descent nor airbags are suitable for this mission. They require too large a landing area, and the landing site will probably have large (and possibly sharp) rocks.

Instead, we will use a modified form of the MSL landing system, as it is the only way (out of the methods already flown or at least scheduled) to get the precision required. The initial part, that of using thrusters to slow to a stop and choose a landing spot, is very similar to normal powered descent and so can be considered a tested technology.

The sky crane part is untested and seems risky, as it requires lowering the rover directly onto its wheels. Marsokhod is a more compact, solid, and robust design than

NASA has been using (which does have drawbacks, as mentioned above), but even so it was not designed for impact and might be damaged if it landed on uneven rocks or was lowered too forcefully.

Instead, it should be possible for the hovering craft to lower or drop a small cage with the rover in it. This would look much like the landing craft of the MER rovers, but it would not need to handle impacts nearly as strong.

If the cage is dropped, it would have airbags on all sides, but they would be much smaller than the MER airbags. They would probably also be designed differently;

MER’s airbags were solid and did not deflate on their own, which meant that they

107 bounced a lot and was part of the reason for their large landing area. Car airbags, on the other hand, have small holes so they start deflating quickly. Airbags with holes would provide sufficient cushion, and they would also prevent bouncing. How large the holes would be, and how numerous, is difficult to say without further study.

If it is lowered on a tether, only the bottom would need to be cushioned; probably with perforated airbags as described above. This would be safer than simply dropping it, as there would be less risk of landing against a rock that would prevent the petals from unfolding, but it would also be more difficult.

In either case, however, the landing ‘cage’ could be much lighter than MER’s was.

The impacts would be relatively low, even if it was dropped from five meters up, so the petals could be made of a light latticework. MER’s petals were made of a light composite, strong but not too heavy, and these could be made even lighter.

After the cage has come to rest, the petals would unfold. Even if it is leaning against a rock, at least one of the petals should provide a ramp that the rover could drive down, as the MER rovers did.

Admittedly, lowering a cage would give a higher total weight than just lowering a rover, but the advantages should outweigh the drawbacks in this case. Perhaps this will change after MSL lands and the sky crane has proven itself, but not at this point.

108 6.5: Summary

Here is a summary of the mission, the rover, and the payload it will carry.

6.5.1: Mission Summary

This mission is to go and study the light-colored deposit in a crater in Centauri

Montes (38.7°S, 263.3°W), near some of the martian gullies. This area is about 2.2 km below Mars’s ‘sea level’, and so has a relatively thick atmosphere. It has very rough terrain, however.

The first thing any rover must do on Mars is to land, and landing is always difficult.

It is impossible to know exactly where a landing can be made without much more detailed pictures of the area, but the landing zone is estimated to be estimated 20 x 20 km, and must be flat (no more than 15° tilt) and largely clear of rocks greater than 50 cm high. The landing method chosen (powered descent to a sky crane) gives the landing craft time to choose a good spot within the landing zone, so a few areas outside of proper conditions should not be disastrous.

Phase 1 of the mission (after the landing) is the driving phase: the time it will take to reach the crater. There will also be pictures taken during this time, but no science will be done aside from photography. The estimated 20 x 20 km landing zone gives a minimum distance of 20 km and a maximum of 40 (assuming one edge of the landing zone touches the crater). At an estimated speed of 1 km per day, phase 1 will last 20 to 40 days.

Phase 2 of the mission is to get the rover inside the crater and down to the study area.

This is a much shorter distance, but involves far worse terrain. Without further study, the amount of time it will take is difficult to pin down, but an assumption can be made that it

109 will take two to four weeks. So phase 1 and phase 2 will probably take no more than two months to complete.

Phase 3 involves the science portion of the mission, and will last as long as the rover has power and instruments. Since the rover has a 90-sol designed lifespan, phase 3 should last about one month. The actual science will take much less time, but the rover will have to drive around between areas inside the crater, and that will take time.

6.5.2: Rover Summary

The rover will use four cameras (two for navigation, one panoramic, and one microscopic), the chemistry package from Phoenix, the Marsokhod rover base, 4.62 kg

2 of nickel-hydrogen (Ni-H2) batteries, solar power with 1.8 m worth of solar panels, and a landing method that is a cross between that of the Mars Exploration Rovers and the

Mars Science Laboratory.

110

Table 6-4: Details on the rover and the source of each item (if applicable). Item Name Source Details Two black-and-white Navigation cameras MER stereoscopic cameras One color panoramic Cameras Panoramic camera MER (360°) camera One black-and-white Microscopic imager --- multispectral imager Thermal and Evolved Gas Oven and mass Phoenix Analyzer (TEGA) spectrometer Microscopic, Electrochemical, Uses the microscopic and Conductivity Assessment Phoenix imager to examine the Chemistry (MECA) placement of the regolith package Measures the water Wet Chemistry Laboratory chemistry (salt content & Phoenix (WCL) composition, concentrations of trace minerals) The basic base that the Rover Marsokhod base Marsokhod various Marsokhod versions have used Nickel-Hydrogen Batteries --- 4.62 kg of Ni-H2 batteries Power 1.8 m 2 of Ga-As solar Gallium-Arsenide Solar Panels --- panels Viking- Aeroshell and a parachute Entry and Descent MSL to slow down Entry, A platform that slows to a Descent, Sky Crane MSL stop in midair and lowers and something on a tether Landing A small cage with the Landing MER rover inside, padded by airbags

111 Chapter 7 Summary and Conclusions

This thesis has covered a possible rover mission to Mars, from the reasons for the mission to a possible rover that could accomplish it. This chapter will summarize what has been stated, as well as looking at what more needs to be done.

7.1: Summary

Features have been seen on Mars that have been called gullies, since features on

Earth that look like them are usually channels carved by running water. The idea that there might have been running water on Mars in the recent past, or that it might still be present, is an exciting one, since water is a prerequisite for life as we know it. If this could be proven, Mars as a current or past habitat for life—and a possible future home for human settlement—becomes much easier.

If the gullies are not from water or brine, they are probably caused by dry sand avalanches or by carbon dioxide in solid or liquid form. Sand avalanches would not be very interesting, from a geological or astrobiological standpoint, since they would likely be caused by the atmospheric effects that are already known. Still, the locations and patterns of gullies would not match any of the known theories, so there would be at least a few things to be learned.

CO 2 would not carry with it the possibility of life as we know it, but it would reveal information about subsurface Mars that we have not been able to get in other ways. CO 2 aquifers are purely theoretical, as they do not occur on Earth, so very little is known about how they would form or behave.

112 There is no way to tell which of the theories is correct simply from pictures; a mission must actually go to the gullies and take samples in order for these questions to be answered. And no matter what the answer is, it will reveal mysteries that should be studied. In that sense, it is impossible for such a mission to have a truly disappointing ending.

Even more interesting than the fact that the gullies exist is the fact that they aren’t simply relics from the past. Bright streaks have been found inside several craters with gullies; such bright gully streaks have not been seen elsewhere. More importantly, two of the craters have developed new gully streaks in the time we have been observing them. Whatever is causing them, there is clearly some active force on Mars today that is capable of making changes. Mars is not a geologically dead world, as it has long been thought.

Unfortunately, this would be a challenging mission. There are many risks and problems when it comes to space missions, to landings, and to driving a rover. Putting all of those together makes a rover mission to Mars very difficult, and the terrain here makes this one even more difficult than the ones that have happened already.

Most of the risks due to the environment have been faced and overcome with earlier rovers. That does not mean that the risks are not there; they exist, but methods for dealing with them have been found. We can be reasonably confident of making a rover that can handle Mars’s atmosphere and gravity, generate enough electrical power to do its job, and that can communicate with Earth.

The biggest problem we have not mastered yet is that of the locations. The places we would like to go are extremely limited, and none of them is in an area that would be easy

113 to reach. All landings on Mars to date have aimed at flat, clear areas to make the landings safer, and there are no areas near the gullies that are suitable. A rover would either have to land far from the site it is to study or land in difficult terrain.

Both choices have their risks. Driving for a long distance on a strange planet, through what is clearly rocky and broken terrain, would be well beyond the abilities of any rover made to date. It’s not easy to drive in such areas on Earth, much less Mars.

The fact that most rovers are slow and limited to the decision-making abilities of their onboard computers makes it even harder.

But landing near the gullies would not be easy, either. It is difficult to find a safe landing spot that is large enough, and a bad landing could mean that the rover would never move at all. The newest landing technologies, which will be pioneered by Mars

Science Laboratory, are the best chance of landing safely. Those technologies are untested as of yet, and might not be good enough even so, but they are the best that we have to work with.

114 7.2: Future Work

There is obviously a great deal of work to be done before this mission should launch, both with information and with hardware.

7.2.1: Information

All of the information on gully streaks and their locations is old. Nothing has been posted on them since 2006; there is no recent information on them, despite the fact that it has been almost four years. If any more gully streaks have been found, or if new pictures have been taken of these areas, the information is not available. But the possibility should be considered before a final location is actually selected, since other new gully streaks might be easier to reach.

Even if the location stays in the Centauri Montes region, the data available on the area is extremely limited. The best resolution currently available is in ten or hundreds of kilometers (Figures 5-4 and 5-5), when rocks that could make a landing fail are measured in centimeters.

Resolution of that scale is not available on any of the orbiters currently circling

Mars, but the Mars Reconnaissance Orbiter (MRO) has a High Resolution Imaging

Science Experiment (HiRISE) camera that is capable of distinguishing objects as small as one meter in size [MRO website]. Images should be made with that camera for the entire chosen landing zone, if possible, as well as the path to the deposit and the gullies.

The images should be studied for possible problems so solutions to them might be found before they happen.

115 7.2.2: Hardware

The rover presented in this thesis was assembled from pieces of other rovers and landers that have been sent to Mars, or at least been planned to go to Mars. This is certainly far quicker and cheaper than building a new rover and new instruments from scratch, but it also means that it will not, perhaps, be the best rover possible.

There have been other missions sent out with such leftover technology, such as the

Magellan spacecraft, that performed well. The assemblage should certainly be checked for problems and incompatibilities, but there is no immediate reason to think that it would not work.

If time and money are short, the proposed rover could be built, tested, and sent off as is, though that would sacrifice the chance to make a lighter, faster, or better rover. It would certainly be far better, from an engineering standpoint, to take the time to choose exactly the right combination of elements. Still, it is not nearly as necessary as getting more detailed information from Mars is.

116 7.3: Conclusion

In the 1990s, NASA tried to send several large, complex spacecraft to Mars. Some, like Mars Global Surveyor, were quite successful and long-lived. Others, though, failed; most notably Mars Climate Orbiter. Its loss was disastrous both to science and to

NASA’s reputation.

As a result of those losses, NASA adopted the ‘faster, cheaper, better’ philosophy of sending less complex spacecraft more often. That approach has been extremely successful; several of the spacecraft sent in the last decade have arrived safely and completed—even exceeded—their missions.

Most of the rovers sent to Mars so far have been large and complicated, with a diverse array of instruments and missions. So far none of the NASA rovers have been lost, but that is in part due to the decision to avoid risky areas and stick to the safer (and more scientifically boring) terrain. The rovers have gotten even larger and more complex every time, as well as more expensive; sooner or later one will fail, taking years worth of work and millions or billions of dollars with it.

Perhaps it is time to rethink the way rovers are made, as happened with orbiters.

Many smaller rovers, with fewer instruments, could do all of the science and more that one large, complex rover could, and without putting all of the eggs into one basket. This would increase the total mass, due to the extra platforms, but it would also increase redundancy and lessen the chance of a single, catastrophic failure.

Simpler rovers designed from almost off-the-shelf components would be more suitable for missions to risky areas, since they would be far quicker to build and less

117 would be lost if the mission failed. This would allow some of the more interesting terrain to be covered and studied, as well as enabling us to see more areas of the planet.

On Earth, learning about one area is nearly useless when it comes to understanding another, because our planet is so incredibly diverse. Mars appears to be more uniform than Earth, but right now ‘appears’ is the important word. We know very little about the planet, when it comes down to it, and we are using that small amount of knowledge to make large generalizations.

Spreading smaller rovers out across the planet would mean that we would learn less from each rover and each area, but we would learn far more overall about Mars. This is one case where a lot of superficial knowledge would be better than a little in-depth know-ledge. But we must change our methods before we can increase our knowledge.

118 References

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