Foreword*

The solar system has become humankind’s new backyard.It is the playground of robotic planetary spacecraft that hassurveyed just about every corner of this vast expanse inspace. Nowadays, every schoolchild knows what even thefarthest planets look like. Fifty years ago, these placescould only be imagined, and traveling to them was therealm of fiction. In just this short time in the long history ofthe human species we have leapt off the surface of ourhome planet and sent robotic extensions of our eyes, ears,noses, arms, and legs to the far reaches of the solar systemand beyond.

In the early twentieth century, we were using airplanesto extend our reach to the last unexplored surface regionsof our own planet. Now 100 years later, at the beginning ofthe twenty-first century, we are using spacecraft to extendour reach from the innermost planet Mercury to theoutmost planet Neptune, and we have a spacecraft on theway to Pluto and the Kuiper Belt. Today, there are tele-scopes beyond imagination 100 or even 50 years ago thatcan image Pluto and detect planets around other stars!Now, Sol’s planets can say “we are not alone”; there areobjects just like us elsewhere in the universe. As human-ity’s space technology improves, perhaps in the next100 years or so human beings also may be able to say “weare not alone.”

When I was a kid more than 50 years ago, I was thrilledby the paintings of Chesley Bonestell and others who puttheir imagination on canvas to show us what it might be like“out there.” Werner Von Braun’s Collier’s magazine arti-cles of 1952e1954 superbly illustrated how we would go tothe Moon and Mars using new rocket technologies.Reading those fabulous articles crystallized thoughts in myyoung mind about what to do with my life. I wanted to bepart of the adventure to find out what these places were like.Not so long after the Collier’s articles appeared, we did goto the Moon, and pretty much as illustrated, althoughperhaps not in such a grand manner. We have not senthumans to Marsdat least we have not yetdbut we havesent our robots to Mars and to just about every other placein the solar system as well.

* This foreword to the second edition has been editorially updated to be

included in the present edition.

Copyright � 2007 Elsevier Inc. All rights reserved.

This book is filled with the knowledge about our solarsystem that resulted from all this exploration, whether byspacecraft or by telescopes both in space and earth-bound.It could not have been written 50 years ago as almosteverything in this Encyclopedia was unknown back then.All of this new knowledge is based on discoveries made inthe interim by scientist-explorers who have followed theirinborn human imperative to explore and to understand.Many old mysteries, misunderstandings, and fears thatexisted 50 years ago about what lay beyond the Earth havebeen eliminated.

We now know the major features of the landscape in ourcosmic backyard and can look forward to the adventure,excitement, and new knowledge that will result from morein-depth exploration by today’s spacecraft, such as thoseactually exploring the surface of these faraway places,including the Huygens Titan lander, the Mars Explorationand the Curiosity rovers, doing things that were unimag-inable before the Space Age began.

The Encyclopedia of the Solar System is filled withimages, illustrations, and charts to aid in understanding.Every object in the solar system is covered by at least onechapter. Other chapters are devoted to the relationshipsamong the objects in the solar system and with the galaxybeyond. The processes that operate on solar systemobjects, in their atmospheres, on their surfaces, in theirinteriors, and interactions with space itself are alldescribed in detail. There are chapters on how we exploreand learn about the solar system and about the investiga-tions used to make new discoveries. And there are chap-ters on the history of solar system exploration and themissions that have carried out this enterprise. All writtenby an international set of world-class scientists usingrigorous yet easy-to-understand prose.

Everything you want to know about the solar system ishere. This is your highway to the solar system. It is as muchfun exploring this Encyclopedia as all the exploration ittook to get the information that it contains. Let your fingersbe the spacecraft as you thumb through this book visitingall the planets, moons, and other small objects in the solarsystem. Experience what it is like to look at our solarsystem with ultraviolet eyes, infrared eyes, radio eyes, andradar eyes.

ix

x Foreword

It has been almost 15 years since the first edition. Theexploration of space has continued at a rapid pace sincethen, and many missions have flown in the interim. Newdiscoveries are being made all the time. This third editionwill catch you up on all that has happened since theprevious editions, including several new chapters based oninformation from our latest missions.

I invite you to enjoy a virtual exploration of the solarsystem by flipping through the pages in this volume. Thisbook deserves a place in any academic setting and whereverthere is a need to understand the cosmos beyond our home

planet. It is the perfect solar system reference book,lavishly illustrated and well written. The editors andauthors have done a magnificent job.

We live in a wonderful time of exploration anddiscovery. Here is your window to the adventure.

Wesley T. HuntressGeophysical Laboratory,

Carnegie Institution of Washington,Washington, D.C.

Chapter 19

Mars: Landing Site Geology,Mineralogy, and Geochemistry

Matthew P. GolombekJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

Harry Y. McSweenDepartment of Earth & Planetary Sciences, University of Tennessee, Knoxville, TN, USA

Chapter Outline

1. Introduction to Mars Exploratiion 397

2. Landing Sites on Mars 400

3. Mars Landing Sites in Remotely Sensed Data 404

3.1. Surface Physical Properties 404

3.2. Global Compositional Units 407

4. Landing Site Geology 409

4.1. Introduction 409

4.2. Rocks 409

4.3. Outcrops 410

4.4. Soils 410

4.5. Eolian Deposits 411

4.6. Craters 412

5. Landing Site Mineralogy and Geochemistry 412

5.1. Rocks 412

5.2. Soils 416

6. Implications for the Evolution of Mars 417

6.1. Origin of Igneous Rocks 417

6.2. Chemical Evolution and Surface Water 417

6.3. Weathering on Mars 417

6.4. Eolian Processes 418

6.5. Geologic Evolution of the Landing Sites and Climate418

6.6. Implications for a Habitable World 419

Bibliography 419

1. INTRODUCTION TO MARSEXPLORATIION

Most of our detailed information about the materials thatmake up the Martian surface comes from in situ in-vestigations accomplished by seven successfully landedspacecraft (Table 19.1). The focus of these spacecraft andthe era in which they explored Mars have varied, but allhave been preceded by orbiters that acquired remotesensing data that helped frame the questions they addressedand the locations where they landed. The first successfullandings were the Viking spacecraft in 1976, part of twoorbiter/lander pairs that were launched in 1975. Theselanders were preceded by a number of failed Soviet andUnited States spacecraft, several successful “flyby” mis-sions, and the Mariner 9 orbiter that provided basic imagingand spectral information that gave an early view of thesurface and atmosphere of Mars. The overriding impetusfor the Viking landers was to determine if life existed on

Mars. Both immobile, legged landers carried sophisticatedlife detection experiments as well as imagers, seismome-ters, atmospheric science packages, and magnetic andphysical properties experiments. The Viking mission wasdone in the post-Apollo era (after 1972) and involved amassive mobilization of engineering and scientific talent(as well as a budget befitting a major mission). Both landerscarried arms with scoops that collected soil and fed theminto the life detection experiments. No unequivocal evi-dence for life was found in the soil (although gases releasedfrom the soil suggested a significant oxidizing component),but the spacecraft imaged the landing sites, determined thechemistry of the soils, and provided a long record of surfacemeteorology.

The successful landings and operations of the orbiters(that lasted years) set the stage for the systematic studyof Mars that fueled our modern view of the Red Planet.The spacecraft also left a legacy for landing using aer-oshields and supersonic parachutes that have been

Encyclopedia of the Solar System. http://dx.doi.org/10.1016/B978-0-12-415845-0.00019-0

Copyright � 2014 Elsevier Inc. All rights reserved.

397

employed by all subsequent landers. Orbiter and landerdata defined the atmosphere and basic geology of Mars.By mapping morphologic units and crater density, threemain eras of Martian geologic history were defined:Noachian (>3.6 billion years ago), Hesperian(3.6e3.0 billion years ago), and Amazonian (since3.0 billion years ago). The mapping showed that Marswas very geologically and tectonically active during theNoachian, with decreasing activity into the Hesperianand Amazonian. The Viking orbiters returned images ofvalley networks and eroded ancient craters in Noachianterrain that suggested an earlier wetter and possiblywarmer environment and the onset of freezing conditionsin the Hesperian, leading to the present climate in theAmazonian that is generally too cold and thin (and dry)to support liquid water (current atmospheric pressure andtemperature are so low that water is typically stable onlyin solid and vapor states).

The Mars Pathfinder (MPF) mission, launched 20 yearslater in 1996, was an engineering demonstration of a low-cost lander and small mobile rover and on landing onJuly 4, 1997, ushered in our modern era of Mars explora-tion. The spacecraft was a small free flyer that used aViking-derived aeroshell and parachute, but employednewly developed robust airbags surrounding a tetrahedrallander, rather than retrorockets and legged landers as didViking. The lander carried a stereoscopic color imager(Imager for Mars Pathfinder (IMP)), which included amagnetic properties experiment and wind sock and an at-mospheric structure and meteorology experiment. The10-kg, microwave-size rover (Sojourner) carried engi-neering cameras, 10 technology experiments, and an AlphaProton X-ray Spectrometer (APXS) for measuring thechemical composition of surface materials, and conducted10 technology experiments. The MPF lander and rover

operated on the surface for about 3 months (well beyondtheir design lifetime) and the rover traversed about 100 maround the lander, exploring the landing site and charac-terizing surface materials in a couple of hundred squaremeter area. Rocks analyzed by the APXS appeared rela-tively high in silica, similar to andesites; tracking of thelander fixed the spin pole and polarmoment of inertia thatindicates a central metallic core and a differentiatedplanet, and the atmosphere was observed to be quite dy-namic with water ice clouds, abruptly changing near-surface morning temperatures, and the first measurementof small wind vortices or dust devils. The mission capturedthe imagination of the public, garnered front-page head-lines during the first week of operations, and became one ofNASA’s most popular missions as the largest Internet eventin history at the time. Much of the flight system, lander, androver design were used for the next two successful landings.

Launching before MPF, but arriving later, was the MarsGlobal Surveyor (MGS) orbiter, which was a partial reflightof instruments on the Mars Observer orbiter that was lostwhen attempting to enter into orbit around Mars in 1993.This spacecraft defined the global topography and magneticfield and identified different rock types and minerals thatmake up the surface. It also identified layered sedimentaryrocks in high-resolution images suggesting deposition instanding bodies of water and fresh gullies suggesting recentflow of liquid water. Mars Odyssey (2001) followed MGSand the failed Mars Climate Orbiter and Mars Polar Landerlaunched in 1999. Instruments on Odyssey identifiedground ice at high latitudes, produced the highest resolutionglobal image mosaic (100 m/pixel) to date, and with MGSimproved our knowledge of the atmosphere and globalphysical and mineralogical properties of surface materialsby measuring their thermal properties and infrared spectralcharacteristics.

TABLE 19.1 Landing Sites on Mars

Site Latitude (Degree þN) Longitude (Degree þE) Elevation (km, MOLA) Region

Viking Lander 1 (VL1) 22.27 311.81 �3.6 Chryse Planitia

Viking Lander 2 (VL2) 47.67 134.04 �4.5 Utopia Planitia

Mars Pathfinder (MPF) 19.09 326.51 �3.7 Ares Vallis

MER Spirit (SPI) �14.57 175.47 �1.9 Gusev Crater

MER Opportunity (OPP) �1.95 354.47 �1.4 Meridiani Planum

Phoenix (PHX) 68.22 234.25 �4.1 Vastitas Borealis(high northern plains)

MSL Curiosity (MSL) �4.59 137.44 �4.5 Gale Crater

MOLA, Mars Orbiter Laser Altimeter on Mars Global Surveyor.

398 PART | IV Earthlike Planets

The Mars Exploration Rover (MER) mission landedtwin golf cart-sized rovers in early 2004 that have exploredover 40 km of the surface at two locations. Each rovercarried a payload that contains multiple imaging systemsincluding the color, stereo Panoramic Camera (Pancam)and Miniature Thermal Emission Spectrometer (Mini-TES)for determining mineralogy. The rovers also carried an armthat can brush and grind away the outer layer of rocks (theRock Abrasion Tool (RAT)) and can place an APXS,Mossbauer Spectrometer (MB), and Microscopic Imager(MI) against rock and soil targets (Table 19.2). The roverand payload partially mimics a field geologist, being able toidentify interesting targets using the remote sensing in-struments (a field geologist’s eyes), rove to those targets(legs), remove the outer weathering rind of a rock (equiv-alent to a rock hammer), and identify the rock type(equivalent to a geologist’s hand lens and analysis in thelaboratory) using the chemical composition (APXS), ironmineralogy (MB), and rock texture (MI). These rovers havelasted years (well beyond their 3-month design lifetime)and returned a treasure trove of basic field observationsalong their traverses as well as sophisticated measurementsof the chemistry, mineralogy, and physical properties of therocks and soils encountered. They have returned

compelling information that indicates an early wet andlikely warm environment on Mars.

Mars Express, the first European Space Agencymission, also carried the British Beagle 2 exobiologylander to Mars, arriving in late 2003. Although the landerwas not successful, the orbiter has observed Mars foralmost 10 years. Mars Express carries imagers, imagingspectrometers, radar sounders, and atmosphere andexosphere sensors. Stereo color images have refined thegeologic history of Mars and the first visible to near-infrared imaging system discovered clay minerals thatformed by alteration of primary volcanic minerals inneutral waters in the ancient terrains in agreement with anearly warmer and wetter Mars.

The Mars Reconnaissance Orbiter (MRO) waslaunched in 2005 and carries imagers capable of resolvingmeter-size features on the surface (25 cm/pixel), imagesthat cover broad regions at 6 m/pixel, and a higher reso-lution (18 m/pixel) visible and near-infrared spectralimager. It has confirmed widespread deposits of clayminerals in the ancient highlands, refining our under-standing of water activity on Mars. It has also sounded theatmosphere to provide a much better understanding of itstemperature, pressure, and density variations with

TABLE 19.2 Instruments Used to Process and Analyze Rocks and Soils at Spacecraft Landing Sites

Alpha Particle X-ray Spectrometer (APXS) on Mars Exploration Rovers and MSL: measures rock elemental chemistry using

interactions of alpha particles with the targetAlpha Proton X-ray Spectrometer (APXS) on MPF: measured rock elementary chemistry, using interactions of alpha particles and

protons with the targetChemCam on MSL: fires a laser and analyzes the elemental abundances of vaporized areas on rocks and soilsChemMin on MSL: a powder X-ray diffraction instrument used to identify mineralsGas Chromatograph/Mass Spectrometer (GCMS) on Viking: instruments that analyzed chemical compounds in soilsIMP : a lander-mounted digital imaging system for stereo, color images and visible near-infrared reflectance spectra of mineralsMars Hand Lens Imager (MAHLI) on MSL: a camera that provides close-up views of the textures of rocks and soilMast Camera (MASTCAM) on MSL: a digital imaging system for stereo color images and visible near-infrared reflectance spectra of

mineralsMI on Mars Exploration Rovers: a high-resolution camera used to image textures of rocks and soilMicroscopy, Electrochemistry, and Conductivity Analyzer (MECA) on Phoenix: includes a wet chemistry laboratory, optical and

atomic force microscopes, and a thermal and electrical conductivity probeMini-TES on Mars Exploration Rovers: identifies minerals via thermal infrared spectral characteristics produced by crystal lattice

vibrationsMB on Mars Exploration Rovers: identifies iron-bearing minerals and distribution of iron oxidation states by measuring scattered

gamma raysPancam on Mars Exploration Rovers: digital imaging system for stereo color images and visible near-infrared reflectance spectra of

mineralsRAT on Mars Exploration Rovers: brushes or grinds rock surfaces to reveal fresh interiorsSAM onMSL: suite of three instruments (mass spectrometer, gas chromatograph, tunable laser spectrometer) used to identify carbon

compounds and to analyze hydrogen, oxygen, and nitrogenSampling System (SA/SPaH) on MSL: includes a drill, brush, soil scoop, and sample processing deviceSurface Stereo Imager (SSI) on Phoenix: digital imaging system for stereo color images and visible near-infrared reflectance spectra

of mineralsThermal and Evolved Gas Analyzer (TEGA) on Phoenix: furnace and mass spectrometer to analyze ice and soilX-ray Fluorescence Spectrometer (XRFS) on Viking: instrument that analyzed elemental composition of soils

399Chapter | 19 Mars: Landing Site Geology, Mineralogy, and Geochemistry

altitude, which has dramatically improved our knowledgeof the atmosphere that is important in landing spacecraft.

The Phoenix lander was a low-cost refly of a landeroriginally developed to be launched in 2001 that landed inthe high northern plains in 2008. It carried a variety ofimagers and meteorology instruments, but its main goalwas to measure the chemistry of the soil and shallowground ice believed to be in equilibrium with the present-day climate. It did find ice several centimeters beneaththe surface and found a surface that is heavily modified bythe ice. The instruments discovered low levels of calciumcarbonate and perchlorate salts in the soils, both arguing foraqueous processes in the past.

The Mars Science Laboratory (MSL) rover is a majormission designed to determine if Mars was habitable in thepast. MSL is a mobile laboratory with remote sensing in-struments and in situ instruments that can be placed againstrocks and surface materials. MSL carries a drill designed tofeed material to sophisticated laboratory instruments thatmeasure the mineralogy and geochemistry of surface ma-terials and, for the first time since Viking, organic mole-cules. It landed on Mars in 2012 in Gale crater and isdesigned to last several years and traverse tens of kilome-ters. It is the first spacecraft that used aeromaneuvering andentry guidance during flight on the aeroshell to dramati-cally reduce the size of the landing ellipse (25 kmcompared to >100 km for all previous landers). The smalllanding ellipse (the uncertainty from entry, descent, andlanding to a targeted location) and long roving capabilitymake this mission the first to consider “go to” landing sitesin which landing occurs in smooth, flat terrain next to areasof prime scientific interest (that are too hazardous to land).As of this writing, the Curiosity rover is in the middle of itssurface exploration, but has already discovered conglom-erates that formed in surface running water, sandstones andmudstones deposited in streams and lakes, and clays,indicative of a habitable environment.

Two missions are presently under development that willcontinue the exploration of Mars. The low-cost MAVENorbiter, launched in 2013, will study the upper Martianatmosphere to determine atmospheric escape rates as a clueto how the atmosphere evolved from a possibly warmer andwetter (thicker) state early on to its current cold and dry(thin) state. Finally, the low-cost Interior Exploration UsingSeismic Investigations, Geodesy, and Heat Transport(InSight) mission will land a seismometer, heat flowprobe, and precision tracking station in 2016 to measure theoverall structure of the interior to better understand theaccretion and differentiation of the rocky planets.

2. LANDING SITES ON MARS

The seven landing sites (Table 19.1) that constitute the“ground truth” for orbital remote sensing data onMars were

all selected primarily on the basis of science and safetyconsiderations. Because a safe landing is required for asuccessful mission, the surface characteristics must meet theengineering constraints based on the designed entry, descent,and landing system. The most important factor controllingthe selection of the seven landing sites is elevation, as alllanders used an aeroshell and parachute to slow them downand sufficient atmospheric density and time are required tocarry out entry and descent. This favored landing at low el-evations is shown in Figure 19.1, which illustrates the lo-cations of the landing sites on a topographic map of Mars.

The map shows that the southern hemisphere is domi-nated by ancient heavily cratered terrain estimated to bemore than 3.6 billion years old (Noachian). The northernhemisphere is dominated by younger (Hesperian andAmazonian), smoother, less-cratered terrain that is on anaverage 5 km lower in elevation. Astride the hemisphericdichotomy is the enormous Tharsis volcanic province,which rises to an elevation of 10 km above the datum,covers one quarter of the planet, is surrounded by tectonicfeatures that cover the entire western hemisphere, and istopped by five giant volcanoes and extensive volcanicplains (active during the Hesperian and Amazonian). Theelevated Tharsis province and the cratered highlands havebeen too high for landing of existing spacecraft. The Vikinglanders landed in the northern lowlands, as did MPF andPhoenix; the Mars Exploration Rovers and MSL landed atrelatively low elevations in the transition between thehighlands and lowlands. The next most important factor inlanding site selection is latitude, with low latitudes (�30�)favored for greater solar power (Pathfinder, Spirit, andOpportunity) and thermal management (Curiosity).

Landing site selection for the seven landers includedintensive periods of data analysis of preexisting andincoming information. The Viking lander/orbiter pairs werecaptured into Mars orbit and the orbiter cameras started aconcentrated campaign to image prospective landing sites(at tens to hundreds of meters per pixel) selected on the basisof previous Mariner 9 images. A large site selection sciencegroup assembled mosaics (using paper cutouts pastedtogether by hand) in real time and, after waiving off severallanding sites on the basis of rough terrain and radar scatteringresults (and missing the intended July 4th landing), Viking 1landed on ridged plains in Chryse Planitia. The site isdownstream fromMaja and Kasei Valles, giant catastrophicoutflow channels that originate north of Valles Marineris,the huge extensional rift or canyon that radiates fromTharsis(Figure 19.1). The site’s low elevation and proximity to thechannels suggested that water and near-surface ice mighthave accumulated there, possibly leading to organic mole-cules and life. Viking 2 was sent to the middle northernlatitudes where larger amounts of atmospheric water vaporwere detected, thereby ostensibly improving the chance forlife. Landing was deferred for Viking 2 as well, as the site

400 PART | IV Earthlike Planets

selection team analyzed images and thermal observationsbefore landing in the midnorthern plains, just west of thecrater Mie (Figure 19.1). Although predictions of the sur-faces and materials present at the Viking landing sites wereincorrect (likely due to the newness of the data and the coarseresolution of the orbital images), the atmosphere was withinspecifications and both landed successfully.

The MPF site selection effort involved little new datasince the Viking mission 20 years earlier, but there was amuch better understanding of how the two Viking landingsites related to the remote sensing data acquired by theViking orbiters. The site selection effort took place over atwo-and-a-half-year period prior to launch and includedextensive analysis of all existing data as well as theacquisition of Earth-based radar data. An Earth analog inthe Ephrata fan near the mouth of a catastrophic outflowchannel in the Channeled Scabland of western and centralWashington State was identified as an analog and studied asan aid to understanding the surface characteristics of theselected site on Mars. Important engineering constraints, inaddition to the required low elevation, were the narrowlatitudinal band 15� N� 5� for solar power and the largelanding ellipse (300 km by 100 km), which required arelatively smooth flat surface over a large area. This and therequirement to have the landing area covered by high-resolution Viking Orbiter images (<50 m/pixel) severelylimited the number of possible sites to consider(wapproximately seven). The landing site selected forMPF was near the mouth of a catastrophic outflow channel,

Ares Vallis, that drained into the Chryse Planitia lowlandsfrom the highlands to the southeast (Figure 19.2). AresVallis formed during the Hesperian (after the early warmand wet period) and involved outpourings of huge volumesof water (roughly comparable to the water in the GreatLakes) in a relatively short period of time (a few weeks).The surface appeared acceptably safe, and the site offeredthe prospect of analyzing a variety of rock types from theancient cratered terrain and intermediate aged ridgedplains. Surface and atmospheric predictions were correctand Pathfinder landed safely.

Landing site selection for the Mars Exploration Roverstook place over a two-and-a-half-year period involving anunprecedented profusion of new information from theMGS (launched in 1996) and Mars Odyssey (launched in2001) orbiters. These orbiters supplied targeted data of theprospective sites that made them the best-imaged, best-studied locations up to that time in Mars explorationhistory. For comparison, most of the ellipses were coveredby w3 m/pixel Mars Orbiter Camera (MOC) images,whereas the MPF ellipse was covered by w40 m/pixelViking images.

All major engineering constraints were addressed bydata and scientific analyses that indicated that the selectedsites were safe. Important engineering requirements forlanding sites for these rovers included relatively lowelevation, a latitude band of 10� N to 15� S for solar power,and landing ellipse sizes that were ultimately less than100 km long and 15 km wide. Because of the smaller

FIGURE 19.1 Mars Orbiter Laser Altimeter on Mars Global Surveyor (MOLA) topographic map of Mars showing the seven successful landing sites.

Elevations are reported with respect to the geoid (or geopotential surface) derived from the average equatorial radius extrapolated to the rest of the planet

via a high-order and high-degree gravity field. The resulting topography faithfully records downhill as the direction that liquid water would flow.

Longitudes are measured positive to the east according to the most recent convention. The locations of the landers, their elevations, and their three-letter

acronyms are reported in Table 19.1. Prior to MOLA, which provided excellent global topography and an accurate cartographic grid, elevations and

locations were poorly known for landing spacecraft on Mars. The map shows three fundamental terrains of Mars: the southern highlands, northern

lowlands, and Tharsis, an enormous elevated region of the planet (located southwest of VL1 on the map). Tharsis is surrounded by a system of generally

radial extensional tectonic features (including the huge Valles Marineris canyon that extends to the east of Tharsis) and generally concentric compres-

sional tectonic features that both imprint the entire western hemisphere of the planet. Located at the edges of Tharsis and the highlandelowland boundary

are the catastrophic outflow channels that funneled huge volumes of water into the northern plains (including Chryse Planitia where the VL1 and MPF

landing sites are located) intermediate in Mars history (during the Hesperian). Note that all of the landing sites are at low elevation and many are near the

equator.

401Chapter | 19 Mars: Landing Site Geology, Mineralogy, and Geochemistry

ellipse size compared to Pathfinder, w150 sites wereinitially possible from which high-science-priority siteswere selected for further investigation. Both sites selectedshowed strong evidence for surface processes involvingwater to determine the aqueous, climatic, and geologichistory of sites where conditions may have been favorableto the preservation of prebiotic or biotic processes. The siteselected for the Spirit rover was within Gusev crater, anancient 160-km-diameter impact crater at the edge of thecratered highlands in the eastern hemisphere. The southernrim of Gusev is breached by Ma’adim Vallis, an 800-km-long branching valley network that drains the ancient cra-tered highlands to the south (Figure 19.3). The smooth flatfloor of Gusev was interpreted as sediments deposited in acrater lake, so that the rover could analyze fluvial sedimentsdeposited in a lacustrine environment (Figure 19.4).

The site selected for the Opportunity rover is in Mer-idiani Planum in which thermal infrared spectra fromorbiting Thermal Emission Spectrometer (TES) instrumentindicated an abundance (somewhat unique) of dark, graycoarse-grained hematite, a mineral that typically forms inthe presence of liquid water. Layers associated with thehematite deposit in Meridiani Planum suggested a sequenceof sedimentary rocks that could be interrogated by therover. Meridiani Planum is a unique portion of the ancientheavily cratered terrain in western Arabia Terra that wasdownwarped in response to the formation of Tharsis andheavily eroded early in Mars history and thus stands at alower elevation than the adjacent southern highlands(Figure 19.5). The atmospheric and surface characteristicsinferred from the extensive remote sensing data were cor-rect for both, and Spirit and Opportunity landed safely.

The Phoenix lander was designed to land at the northernpolar region (65�e72� N) where ground ice overlain by afew centimeters of soil had been detected by instruments onthe Mars Odyssey orbiter. In addition to the low elevationof the northern plains, landing sites were initially identified(before MRO was operational) that had low rock abun-dance, low slopes, and a calm atmosphere. Once MRObecame operational, about 1 year before Phoenix launched,the High-Resolution Imaging Science Experiment(HiRISE) began imaging these areas at about 25 cm/pixel.To everyone’s surprise and dismay, most of the highnorthern plains are covered by areas with dense boulderfields that could not be avoided with the large landing el-lipse of Phoenix (w100 km long) and were far too rocky tosafely land. Because of the low Sun angle of the HiRISEimages, large rocks cast long shadows, which could beeasily measured. An anxious search for rock-free areasusing HiRISE ultimately identified a suitable landing sitejust before launch. Phoenix landed safely in an ellipsecompletely covered by HiRISE images and the sitematched expectations with few rocks, a smooth flat plain,and a few centimeters of soil over ground ice. The missionlasted several months until the darkness and cold of polarwinter encased the lander in ice.

The selection of Gale crater as the MSL landing sitetook over 5 years with prospective landing sites heavilytargeted by MRO instruments. Engineering constraintsincluded low latitude for thermal management of the roverand instruments, low elevation and relief, and low rockabundance. Science criteria important for the selectionincluded the ability to assess past habitable environments,which include diversity, context, and potential biosignature

FIGURE 19.2 Regional enhanced color mosaic of

Chryse Planitia, Ares Vallis, and the MPF landing ellipse.

Viking mosaic shows catastrophic outflow channels cutting

the heavily cratered (ancient) terrain to the south and

flowing to the lower northern plains. Ares Vallis is about

100 km wide and 2 km deep and by analogy with similar

features on Earth formed in about a 2-week period when

roughly the volume of water in the Great Lakes carved the

valley in about 2 weeks. Note streamlined islands produced

during the flooding. The MPF landing ellipse shown is

200 km by 100 km and lies about 100 km north of the

mouth of the channel where it exits the highlands and thus

was interpreted to be a depositional plain composed of

materials dropped by the flood. Characterization of the

surface after landing supports this interpretation.

402 PART | IV Earthlike Planets

(including organic molecules) preservation. Over 50 pro-spective landing sites were studied and downselected tofour finalists (three of which were “go to” sites), all ofwhich have layered sedimentary rocks with spectral evi-dence for clays. All four sites were covered with unprece-dented imaging data, dominantly from MRO, includingspectral and stereo images and derived high-resolutionHiRISE-derived topographic and rock maps that were

FIGURE 19.3 Viking regional color mosaic of Ma’adim Valles and

Gusev crater. The 800-km-long Ma’adim Valles, one of the largest

branching valley networks on Mars, drains the heavily cratered terrain to

the south and breaches the southern rim of Gusev crater. Gusev crater,

which formed much earlier, is 160 km in diameter and the smooth flat floor

strongly suggests that it was a crater lake that filled with water and sedi-

ments. Spirit did not identify any sediment associated with Ma’adim

Valles. The cratered plains are composed of basalt flows modified by

impact and eolian processes and so represent a late volcanic cover. Rocks

in the Columbia Hills have been altered by water, but cannot be related to

deposition in a lake associated with Ma’adim discharge.

FIGURE 19.4 Mosaic of Gusev crater showing the landing ellipse,

landing location for the Spirit rover, and the extensive data sets that were

obtained to evaluate the Mars Exploration Rover landing sites. Ma’adim

Valles breaches the southern rim and hills immediately downstream have

been interpreted as delta deposits. The blue ellipse is the final targeted el-

lipse and the red X is the landing location. Background of mosaic is Viking

230 m/pixel mosaic, overlain by MOLA elevations in color. Thin image

strips mostly oriented to the northenorthwest are MOC high-resolution

images typically at 3 m/pixel. Wider image strips mostly oriented to the

northenortheast are Mars Odyssey THEMIS-visible images at 18 m/pixel.

Mosaic includes 13� Se16� S latitude and 174� E�177� E longitude; solid

black lines are 0.5� (w30 km) and dashed black grid is 0.1� (w6 km).

FIGURE 19.5 Regional setting ofMeridiani Planum inMars Orbiter Laser

Altimeter shaded relief map (w850 km wide). Note that smooth lightly

cratered plains on which Opportunity landed (cross), which bury the under-

lying heavily cratered (ancient) terrainwith valley networks to the south.Note

that large degraded craters in the smooth plains indicate the sulfate rocks

below the basaltic sand surface are very old (>3.6 billion years). In contrast,

the lightly cratered basaltic sand surface that Opportunity has traversed is

young. Opportunity has traversed 35 km to the large crater, and Endeavour to

the southeast of the landing location.

403Chapter | 19 Mars: Landing Site Geology, Mineralogy, and Geochemistry

used to run detailed landing simulations that indicated allfour sites were safe. In addition, the traversability of thelanding sites and target areas outside of the ellipse wereevaluated, indicating that all are trafficable and that “go to”sites could be accessed within the lifetime of the mission.The Gale crater site (Figure 19.6) has a 5-km-high moundadjacent to the landing site that has layered strata thatcontains clays and sulfates at its base, which will be studiedby the Curiosity rover after traversing out of the landingellipse. The landing site explored so far is consistent withexpectations from remote sensing data.

3. MARS LANDING SITES IN REMOTELYSENSED DATA

3.1. Surface Physical Properties

Understanding the relationship between orbital remotesensing data and the surface is essential for safely landingspacecraft and for correctly interpreting the surfaces andkinds of materials globally present on Mars. Safely landingspacecraft on the surface of Mars is obviously criticallyimportant for future landing missions. Understanding thesurfaces and kinds of materials globally present on Mars isalso fundamentally important to deciphering the erosional,

weathering, and depositional processes that create andaffect the Martian surface layer. This surface layer orregolith, composed of rocks and soils, although likelyrelatively thin (of order meters thick), represents the keyrecord of geologic processes that have shaped it, includingthe interaction of the surface and atmosphere through timevia various alteration (weathering) and eolian (wind-driven) processes.

Remote sensing data available for selecting landing siteshave varied for each of the landed missions, but most usedvisible images of the surface as well as thermal inertia andalbedo. Thermal inertia is a measure of the resistance ofsurface materials to a change in temperature and can berelated to particle size, thermal conductivity, bulk density,and cohesion. Albedo is a measure of the solar reflectance ofa surface in which the viewing geometry has been taken intoaccount. A surface composed mostly of rocks will changetemperature more slowly, remaining warmer in the eveningand night, than a surface composed of fine-grained loosematerial that will change temperature rapidly, therebyachieving higher and lower surface temperatures during thewarmest part of the day and the coldest part of the night,respectively. As a result, surfaces with high thermal inertiawill be composed of more rocks or cohesive, cementedmaterial than surfaces with low thermal inertia. Thermalinertia can be determined by measuring the surface tem-perature using a spectrometer that measures the thermalinfrared radiance at several times during the day or by fittinga diurnal thermal model to a single radiance-derived tem-perature measurement. Thermal observations of Mars havebeen made by many orbiters, including theMariners, Viking,MGS, and Mars Odyssey, with increasingly higher spatialresolution. Thermal inertia data have been used to map areasof the surface covered by high-inertia materials or rocksfrom areas covered by lower inertia materials or soil.

Global thermal inertia and albedo data combine in waysthat reveal several dominant surface types. One has highalbedo and very low thermal inertia and is likely dominatedby substantial thicknesses (centimeters to a meter or more)of high albedo, reddish dust that is neither load bearing nortrafficable. These areas have very few rocks and have beeneliminated for landing solar-powered or surface missionsinterested in investigating rocks or outcrop. Regions withmoderate to high thermal inertia and low albedo are likelyrelatively dust free and composed of dark eolian sand and/or rock. Regions with moderate to high thermal inertia andintermediate to moderately high albedo are likely domi-nated by cemented crusty, cloddy, and blocky soil units thathave been referred to as duricrust with some dust andvarious abundances of rocks. Coarse-resolution globalabundance of rocks on Mars, derived by thermal differ-encing techniques that remove the high-inertia (rocky)component, shows that the high-albedo, low-inertia type ofsurface has almost no rocks and the other two types of

FIGURE 19.6 Regional setting of Gale crater, the MSL landing ellipse,

and the landing location. Gale crater is 150 km in diameter with a 5 km

high mound of material (Mount Sharp) in its interior. The landing ellipse is

on smooth cratered plains to the northwest. Final landing ellipse (black) is

20 km by 7 km and the rover landed at the yellow X. Dark material in the

southeastern part of the ellipse are active basaltic sand dunes and the

layered rocks of clays and sulfates at the base of the mound are due south

of the landing ellipse. As a result, this is a “go to” site in which the landing

ellipse is on the smooth flat terrain nearby and the rover must traverse to

the material of greatest interest by leaving the ellipse.

404 PART | IV Earthlike Planets