Mem. S.A.It. Vol. 82, 321c© SAIt 2011 Memorie della

The exploration of Mars: past and future

A. Coradini and R. Orosei

Istituto Nazionale di Astrofisica – Istituto di Fisica dello Spazio Interplanetario, Via delFosso del Cavaliere 100, I-00133 Rome, Italye-mail: angioletta.coradini@ifsi-roma.inaf.it

Abstract. In this paper we will describe the main achievement in the Martian exploration,starting from pioneering work of Schiaparelli, and ending with the recent observation ob-tained by the space mission, from orbit and in situ. Since the time of Schiaparelli observa-tions, Mars become the most interesting object of the Solar system having in common withthe Earth several characteristics, such as the presence of geologic phenomena as volcanismand tectonics, and the presence of a thin atmosphere and polar caps. These Martian charac-teristics are now better understood, also thanks to the “in situ” exploration and this seemsto allow us to maintain the hope that - in some areas- Mars could have host a simple life.Among the most important achievements of the Mars exploration should be mentioned thediscovery of ices in both polar caps, the identification of Methane in the atmosphere, theexistence of extended deposits of sedimentary rocks, including sulphates.

Key words. History and philosophy of astronomy – Astronomical instrumentation, meth-ods and techniques – Space vehicles – Planets and satellites: individual: Mars

1. Introduction

The first observations of Mars were fromground-based telescopes. The history of theseobservations are marked by the oppositions ofMars, when the planet is closest to Earth andhence is most easily visible, which occur ev-ery couple of years. Even more notable arethe perihelic oppositions of Mars which oc-cur approximately every 16 years, and are dis-tinguished because Mars is close to perihelionmaking it even closer to Earth.

In September 1877, (a perihelic oppositionof Mars occurred on September 5), Italian as-tronomer Giovanni Schiaparelli published thefirst detailed map of Mars. Schiaparelli wasborn in Savigliano, March 14, 1835. He died in

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Milan, were he spent the most important pat ofits scientific life, July 4, 1910. Since its youngage Schiaparelli was part of the internationalcommunity of astronomers: he went to Berlinin 1854 to study astronomy under Johann F.Encke. Two years later he was appointed as-sistant observer at the Pulkovo Observatory,Russia, a post he left in 1860 when he was ap-pointed at the Brera Observatory, Milan, wherehe remained until his retirement in 1900, afterbecoming director in 1862.

He started observing systematically Mars,in 1877, during a very favourable opposition.At that time, he could start a systematic studyof planets, thanks to the installation of a morepowerful (8.6-in.) refractor at Brera. He firstwanted to test the powers of the new instru-ment, to see if it “possessed the necessary op-

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tical qualities to allow for the study of the sur-faces of the planets.” The results Schiaparelli’ssystematic observations was the most detailedmap of Mars ever published. With the ad-ditional features he filled in over the nextdecade, it became a standard reference in plan-etary cartography. Among the different fea-tures that he observed, there were the famous“canali”. The word, erroneously translated intoEnglish as “canals” instead of “channels”, ledto widespread speculation over whether the“canals” were constructed by intelligent beings(Sagan 1980).

Is the Schiaparelli work still valid? its mostimportant contribution was the first serious at-tempt to map the surface of another planet.He also started giving to the observed struc-tures original names, derived from Greek andRoman history and mythology. Some of thefeatures that he observed were associated withreal features: as an example, a light spot inthe southern hemisphere, that he called NixOlympia it is now known to be the largestvolcano in the solar system and re-nominatedOlympus Mons. The map that Schiaparelli de-veloped during the first opposition was refinedduring the opposition of 1879. During its sys-tematic work he also noticed the existence ofvariable polar caps and the presence of the at-mosphere. He interpreted these features cor-rectly, however, he also used these observa-tions in support of the existence of an ex-tended hydraulic system of channels, that un-fortunately does not exist.

2. The Mariner and Viking era

2.1. The Mariners

The real improvement in the understandingof Mars was obtained when the space erastarted. After the successful exploration of theMoon, the interest in applying the navigationtechniques developed to reach the Moon tothe study of near-by planets was developing.Nowadays we are used, when thinking to aplanetary mission, to a complex payload, thatincludes several sensors able to study differ-ent aspects of the surface and atmosphere ofa planet at different wavelengths, from X-ray

to IR. The first exploratory missions, insteadrely mainly on images. So the first observa-tions were a kind of extension of the groundbased observations, mainly obtained in the vi-sual range. Early ground-based and spacecraftvisible wavelength imaging relied upon photo-graphic techniques, often supplemented by theuse of broadband color filters. However, photo-graphic film is a highly nonlinear detector thatis extremely difficult to calibrate, so alterna-tives were sought which allowed for the deriva-tion of more quantitative information from im-ages. A major advance in spacecraft imagingafter the film systems of Lunar orbiter was thevidicon, first successfully flown on the Mariner4 flyby of Mars in 1965 (Leighton et al. 1965).Furthermore, an imaging instrument uses op-tics such as lenses or mirrors to project an im-age onto a detector, where it is converted todigital data. At the time of Mariner 4, the de-tector that creates the image was a vacuum tuberesembling a small CRT (cathode-ray tube),called a vidicon. In a vidicon, an electron beamsweeps across a phosphor coating on the glasswhere the image is focused, and its electri-cal potential varies slightly in proportion tothe levels of light it encounters. This vary-ing potential becomes the basis of the digitalvideo signal produced. Viking, Voyager, andMariner spacecraft used vidicon-based imag-ing systems. Early vidicon images (Mariners,4, 6 and 7) were crude and had low photomet-ric accuracy by today’s standards. However,they provided a dramatic first look at Mars.Improved vidicon cameras were extremely re-liable with a photometric accuracy generallybetter than 10% (Mariner 10, Viking, Voyager).The photoconductive surface was sensitive tovisible light in a range of about 350 nm to 650nm. To obtain multispectral data a filter wheelwas employed that allowed for several broad-band filters typically placed between 400 and600 nm. Color filters led to the mapping ofgross color heterogeneities on a scale of 1-10kilometers.

Mariner 4 was launched November 28,1964, on a 228-day mission to Mars. Thespacecraft passed Mars at a distance of 9,868kilometres recording and transmitting to Earththe first close-up picture of the red planet.

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In 22 pictures, Mariner’s TV camera scannedabout one percent of the Martian surface, re-vealing ancient craters of varying size: that wasa big delusion.. was then Mars very similarto the Moon? At any rate the mission com-pleted successfully the first fly-by of Mars.NASA - therefore - decided to perform the first‘”dual mission” to Mars. This is a strategy thatNASA adopted several times in the Mars ex-ploration: given the difficulty to reach success-fully the red planed, the idea was to send twotwin spacecraft, to have the guarantee that atleast one was able to survive. The twin space-craft were mariner 6 and 7. The two spacecraftwere both launched from Cape Kennedy. Themission’s goals were to study the surface andatmosphere of Mars during close flybys, in or-der to demonstrate and develop technologiesrequired for future Mars missions. Mariner 6also had the objective of providing experienceand data which would be useful in program-ming the Mariner 7 encounter 5 days later.Unfortunately, both flew over cratered regionsand missed both the giant northern volcanoesand the equatorial grand canyon discoveredlater. Their approach pictures did, however,photograph about 20 percent of the planet’ssurface, showing the dark features long seenfrom Earth, but none of the canals observedby Schiaparelli. In total 198 photos were takenand transmitted back to Earth, adding more de-tail than the earlier mission, Mariner 4. Bothspacecraft studied the atmosphere of Mars.

2.2. Mariner 9

The 1971 Mariner Mars mission was againplanned to be a dual mission. Unfortunately,the failure of Mariner 8 made it impossible.Mariner 9 then was re-programmed in orderto map 70% of the Martian surface and tostudy the variability of the Martian atmosphereand surface. The novelty of Mariner 9 wasthat it entered in orbit around Mars, becomingthe first artificial satellite of Mars. Launchedon May 30, 1971, The orbit was such thatthe spacecraft circled Mars twice each day fora full year. The payload included a camera,and infrared and ultraviolet instruments. Thespacecraft gathered data on the atmospheric

composition, density, pressure, and tempera-ture and also the surface composition, tem-perature, and topography of Mars. At the ar-rival, Mars was almost totally obscured by duststorms, which lasted for a month. When fi-nally the dust deposited, Mariner 9 revealedthe complex geology of Mars, characterized bythe presence of one that largest volcanoes ofthe Solar system and a grand canyon stretch-ing 4,800 kilometres across its surface, roughlyparallel to the equator. In honour of Mariner9 this canyon was named “Valles Marineris”Apparently ancient riverbeds were present buttotally dry. Mariner 9 provided the first com-plete photo-mapping (about 100 percent of theplanet’s surface) made the first close-up pho-tographs of the Martian moons, Deimos andPhobos.

2.3. Viking orbiters and landers searchfor life

The Viking Mission to Mars was composed oftwo spacecraft, Viking 1 and Viking 2, eachconsisting of an orbiter and a lander. The pri-mary mission goals were to gather high resolu-tion images of the Martian surface, character-ize the structure and composition of the atmo-sphere and surface, and search for evidence oflife.The Orbiters imaged the entire surface ofMars at a resolution of 150 to 300 meters, andselected areas at 8 meters.The Orbiter imageshave been converted to digital image mosaicsand maps, allowing the first geological inter-pretation of the geologic evolution of the sur-face.

The two lander, instead, were specificallyequipped to identify existing life. Viking 1 waslaunched on August 20, 1975. On July 20,1976 the Viking 1 Lander separated from theOrbiter and touched down at Chryse Planitia.The landing was successful, despite the factthat both areas, very flat when seen from or-bit, was instead, characterized by the pres-ence of large boulders. Viking 2 was launchedSeptember 9, 1975 and entered Mars orbit onAugust 7, 1976. The Viking 2 Lander toucheddown at Utopia Planitia on September 3, 1976.The Viking Landers transmitted images of thesurface, took surface samples and analysed

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them for composition and signs of life, stud-ied atmospheric composition and meteorology,and deployed seismometers.

The two Viking landers each carried fourtypes of biological experiments to the surfaceof Mars in the late 1970s. The landers used arobotic arm to put soil samples into sealed testcontainers on the craft. The two landers wereidentical, so the same tests were carried out attwo places on Mars’ surface, Viking 1 near theequator and Viking 2 far enough north to seefrost in winter. The experiment specially de-voted to the life were: Gas chromatograph -Mass spectrometer (GCMS). This instrumentwas able to detect several molecules, includingorganic ones. However the quantity of organicmolecules detected was very limited. Gas ex-change experiment (GEX)was devoted to seeif there were indication of metabolism whenadding water to the Martian soil. Also this ex-periment gave negative results. The LabeledRelease (LR) experiment gave somehow am-biguous results. In the LR experiment,on asample of Martian soil was damped with a dropof very dilute aqueous nutrient solution, in-oculated. The nutrients were tagged with ra-dioactive 14C. Then the evolution of 14C inthe air above the soil was monitored.The airabove the soil was monitored for the evolu-tion of radioactive 14CO2 gas in order to seeif the was an indication that micro-organismsin the soil had metabolised one or more of thenutrients. The results seem to indicate that astream of radioactive gases were released bythe soil immediately following the first injec-tion.Unfortunately the subsequent injectionsdid not confirm the results. So also this ex-periment was considered inconclusive. Finallythe Pyrolytic Release was based on a simi-lar concept: the Martian soil was exposed tolight, water and CO atmosphere. The CO gaseswere made with 14C, as in the previous exper-iment. If there were organisms able to oper-ate the photosynthesis, they would incorporatesome of the carbon as plants and cyanobacteriaon Earth do. Some activity was detected, butit was interpreted as chemical one. The con-clusions were that - at least with this kind ofinstruments - no life was detected.

2.4. A new image of Mars

At the end of this intense period, in whichseveral probes investigated the red planet anew image of Mars emerged. First of all Marsis characterized by an extended crustal di-chotomy. The problem of Martian dichotomywas not solved in that period, but only posed.The Martian dichotomy is a global feature sep-arating the cratered southern highlands andsmooth northern lowlands. Parts of the bound-ary are defined by steep slopes with elevationdifferences of 2-6 km. Older theories foreseenthe presence of a giant impact (Wilhelms &Squyres 1984), or impacts. Arguments againstare that the geology of the northern lowlands(Vastitas Borealis) is not consistent with aone impact hypothesis (Frey & Schultz 1988).Moreover, the lowlands are not radial in shape(Smith et al. 1999), and there is no evidenceof a crater rim. To achieve this results wasneeded the use of laser altimeter, that per-mitted to define the Martian topography. TheMars Orbiter Laser Altimeter (MOLA), an in-strument on the Mars Global Surveyor space-craft, has measured the topography, surfaceroughness, and 1.064-µm reflectivity of Marsand the heights of volatile and dust clouds(Smith et al. 2001). Another possible expla-nation was that the northern plains were gen-erated by the erosion of an ancient shal-low ocean. Topographic profiles across theMars dichotomy are not consistent with an-cient shoreline (Withers & Neumann 2001).Possible shoreline slopes are not orientated inthe correct direction. Therefore these shore-lines were most likely created by compressivetectonic stress.

The differences between the Northern andSouthern hemisphere’s can be interpreted us-ing analogues to Earth’s plate tectonics. Seafloor spreading continuously forms new crustat rift margins and a new crust will be smotherand thinner then old crust. Possibly, subductiondestroys the new crust, which provides the en-ergy and material needed for volcanic activity.This could be the Tharsis origin. Convergencewill produce contraction features along the“plate boundary”. Due to the smooth natureof the northern lowlands “sea floor spreading”

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must have occurred relatively fast. A quanti-tative estimate of a full plate spreading rate is80 mm/yr. The northern lowland crust wouldhave formed rather quickly, and plate tectonicsmay not have lasted that long. Plate tectonicscould aid in cooling the interior of Mars, beinggenerated by convective motion in the interior.Therefore, the interpretation of this dichotomyis related to the thermal history of the planet.Recently (Guest & Smrekar 2005) modeled ofthe relaxation of the crust locally, taking intoaccount of an internal thermal history assum-ing convection in presence of a “stagnant-lid”model of the interior. If so, the steep slopes arethe product of faulting due to the relaxation ofthe boundary by lower crustal flow. This relax-ation requires that the lower crust must be atleast 10 km thick and have a viscosity of ≈ 1021

Pa s during the first 100-300 Myr of relaxation.

2.5. Volcanism

The Northern Hemisphere is also character-ized by an extended volcanism, by the signof a past extended tectonic activity, that wasgenerating the largest rift in the solar Systemnamed “Valles Marineris” . The extended vol-canism produced large volcanoes, arranged insequences, as seen on the Earth in the Hawaiianislands.

The volcanic features on Mars are verysimilar in shape, but not in dimensions, to thosefound on Earth, and they probably formedby similar processes. Martian volcanism ex-tended for a large time-span and the volcanoeswere generated on terrains of variable ages.Numerous volcanic landforms can be found inthe older cratered highlands and in the youngervolcanic plains surrounding them. However,the most impressive volcanic landforms are as-sociated with the extensive, hotspot-related up-lifts of Tharsis and Elysium plateaus.

The large scale of the Tharsis shield volca-noes suggests that they formed from massiveeruptions of fluid basalt over prolonged peri-ods of time. Similar eruptions on Earth are as-sociated with flood basalt provinces and man-tle hotspots. However, on Earth the source re-gion for hotspot volcanism moves in respect tothe crust, due to the plate tectonics. On Mars

an extensive plate tectonics never developed,and we speak now, about a “one-plate” tec-tonics. Therefore, the Martian surface remainsabove the plume source so that huge volumesof lava will erupt from a single central ventover many millions of years of activity. A sin-gle shield volcano of enormous volume is thengenerated. The most spectacular volcanic fea-tures on Mars are the isolated, giant basalticshield volcanoes called Montes. The largest ofthese are four giant shield volcanoes associatedwith Tharsis uplifted region The largest of thefour is Olympus Mons, the largest volcano inthe solar system, with a base diameter of 600km and 25 km of relief from the summit to theplains surrounding it abrupt basalt scarp.

Smaller volcanoes are called Tholi andPeterae characterized by smaller volcanicvents. A Tholus volcano is an isolated moun-tain with a central crater,and a Patera volcano isdominated by an irregular or complex calderawith scalloped edges, surrounded by very gen-tle slopes.

At present, the processes that gave rise tothe volcanism are not any more active. Onlylocally, and at small scale, volcanism couldbe still present. This is very important for thesearch of life, since the presence of hydrother-mal volcanism, some micro-organism couldsurvive.

Geomorphic features visible in orbital im-ages obtained during the Viking missions ofthe late as well as the infrared spectral dataobtained from the floors of rifted basins, oron Mars are suggestive of hydrothermal ac-tivity. OMEGA (the IR Mapping Spectrometerof Mars Express) data have permitted to iden-tify deposits of sulphates, indicative of pasthydrothermal processes on Mars. Another ev-idence for hydrothermal activity on Mars de-rives from studies of SNC meteorites, objectsbelieved to have come from Mars. SNC me-teorites comprise a geochemically and iso-topically related group of objects that havebulk compositions similar to terrestrial basalts(Newsom et al. 1999). In these meteorites, arepresent minerals of primary hydrous ,includ-ing amphiboles and micas contained withinglassy of primary igneous origin, as wellas post-crystallization mino-, sulfates, carbon-

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ates,halides and ferric oxides formed throughinteractions with late-stage aqueous. solutions.

2.6. Tectonics

The formation of large volcanic edifices hasproduced also a large tectonic activity. Thiswas probably produced by mantle processessuch as solid-state mantle convection. Becauseof the large-scale up-warping at Tharsis, frac-ture systems either radial or concentric tothe Tharsis bulge have been identified. Thisis a network of interconnected grabens cen-tred on the non-volcanic part of the Tharsisuplift and next to the western edge of theValles Marineris. It is clear that the older,north-trending fractures were overlaid by theyounger, more chaotic system. Lithosphericdeformation models show that loading over thescale of Tharsis (large relative to the radiusof the planet) produces the concentric exten-sional stresses around the periphery and the ra-dial compressional stresses closer in that areneeded to explain the radial grabens and riftsand concentric wrinkle ridges. According toGolombek (2005) models based on present daygravity and topography can explain the ob-served distribution and strain of radial and con-centric tectonic features, implying that the ba-sic lithospheric structure of Tharsis has prob-ably changed little since 3.7 Ga. This Tharsisload appears to have produced a flexural moataround it, which shows up most dramaticallyas a negative gravity ring (ibid). Many ancientfluvial valley networks, which likely formedduring an early wetter and likely warmer pe-riod on Mars, flowed down the present large-scale topographic gradient, further arguing thatTharsis loading was very early. If the load iscomposed of magmatic products as suggestedby fine layers within Valles Marineris, wa-ter released with the magma would be equiv-alent to a global layer up to 100 m thick,which might have enabled the early warm andwet Martian climate (Golombek et al. (2001),Golombek (2005)). The largest tectonic fea-ture on Mars is Valles Marineris. Recentlya new interpretation of Valles Marineris ori-gin has been proposed by Montgomery et al.(2009). They conclude that the generally lin-

ear chasmata of Valles Marineris reflect ex-tension, collapse, and excavation along frac-tures radial to Tharsis, either forming or re-activated as part of one lateral margin ofthe Thaumasia gravity-spreading system. Thecompressional mountain belt defined by theCoprates Rise and Thaumasia Highlands formsthe toe of the “mega-slide”. Topographic ob-servations and previous structural analyses re-veal evidence for a failed volcanic plume be-low Syria Planum that could have providedfurther thermal energy and topographic poten-tial for initiating regional deformation, (ibid).Higher heat flow during Noachian time, orgeothermal heating due to burial by Tharsis-derived volcanic rocks, would have contributedto flow of salt deposits, as well as formation ofgroundwater from melting ice and dewateringof hydrous salts. According with these authors(Montgomery et al. 2009) connection of over-pressured groundwater from aquifers near thebase of the detachment through the cryosphereto the Martian surface created the outflowchannels of Echus, Coprates, and Juventaechasmata at relatively uniform source eleva-tions along the northern margin of the “mega-slide”, where regional groundwater flow wouldhave been directed toward the surface. Thishypothesis provides a unifying framework toexplain the relationships between the rise ofthe Tharsis volcanic province, deformation ofthe Thaumasia Plateau, and the formation ofValles Marineris and associated outflow chan-nels (Montgomery et al. 2009).

Valles Marineris underwent a complex evo-lution, due to erosion processes. The VallesMarineris walls in the Tharsis region of Marshave a relief up to 11 km in the centralparts of a 4000-km-long system of troughsthat lie just south of the Martian equator.Lucchitta (1979) attributes the present configu-ration of the Valles Marineris walls to erosionalscarp retreat, recognizing two major types ofwalls (Lucchitta et al. 1992): spur-and-gullymorphology, landslide scars, and small-scaletalus slopes. Gullying probably implies somekind of vertical erosion and longitudinal wastetransport by fluids or viscous interstitial mate-rial, probably ice Lucchitta (1979), related tothe widening of the Central Valles Marineris

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troughs during the late Hesperian (Lucchitta etal. 1992), and to the emplacement of interiorlayered deposits.

The relative age of different parts of theMartian surface was estimated through cratercounting, as older surfaces have been exposedlonger to meteoric bombardment and have thusa higher crater density. Three broad epochshave been identified in the planet’s geologictimescale, which were named after places onMars that belong to those time periods. Theprecise timing of these periods is not knownbecause there are several competing modelsdescribing the rate of meteor fall on Mars (seee.g. Hartmann & Neukum (2001)), so the datesare approximate. From oldest to youngest,these periods are the Noachian epoch (namedafter Noachis Terra), in which the oldest extantsurfaces of Mars formed between 4.6 and 3.5billion years ago; the Hesperian epoch (namedafter Hesperia Planum), marked by the forma-tion of extensive lava plains 3.5 to 1.8 billionyears ago; and the Amazonian epoch (namedafter Amazonis Planitia), from 1.8 billion yearsago 1.8 to present.

2.7. The polar caps

Mars has two permanent polar ice caps. Duringa pole’s winter, it lies in continuous darkness,chilling the surface and causing 25-30% of theatmosphere to condense out into thick slabsof CO2 ice (dry ice) (Mellon et al. 2004).When the poles are again exposed to sunlight,the frozen CO2 sublimes, creating enormouswinds that sweep off the poles as fast as 400km/h. These seasonal actions transport largeamounts of dust and water vapour, giving riseto Earth-like frost and large cirrus clouds. Bothpolar caps show spiral troughs, which are be-lieved to form as a result of differential so-lar heating, coupled with the sublimation ofice and condensation of water vapour (Pelletier2004).

2.8. Water on Mars

Viking Orbiters caused a revolution in ourideas about water on Mars by discover-

Fig. 1. An example of chaotic terrain and outflowchannels in Aromatum Chaos, Mars (1.09◦S 317◦E).The outflow channel feeds into Hydroates Chaos, ul-timately extending onto the plains of Chryse Planitiathrough Simud Vallis and Tiu Vallis. Illumination isfrom the right. Viking Mars Digital Image Modelmosaic.

ing many geological forms that are typicallyformed from large amounts of water. Hugeriver valleys were found in many areas. Theyshowed that floods of water broke throughdams, carved deep valleys, eroded grooves intobedrock, and traveled thousands of kilometres(Carr 1996). Many craters look as if the im-pactor fell into mud. When they were formed,ice in the soil may have melted, turned theground into mud, then the mud flowed acrossthe surface (Carr 1996). Normally, materialfrom an impact goes up, then down. It doesnot flow across the surface, going around ob-stacles, as it does on some Martian craters.Regions, called “chaotic terrain”, seemed tohave quickly lost great volumes of water whichcaused large channels to form downstream (anexample can be seen in Fig. 1). The amount ofwater involved was almost unthinkable - esti-mates for some channel flows run to ten thou-sand times the flow of the Mississippi River(Carr 1996). Underground volcanism may havemelted frozen ice; the water then flowed awayand the ground just collapsed to leave chaoticterrain.

Estimates of the amount of water outgassedfrom Mars, based on the composition of the at-mosphere, range from 6 to 160 m, as comparedwith 3 km for the Earth. In contrast, large flood

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features, valley networks, and several indica-tors of ground ice suggest that at least 500 m ofwater have outgassed (Carr 1987). The two setsof estimates may be reconciled only if early inits history, Mars lost part of its atmosphere byimpact erosion and hydrodynamic escape (Carr1996).

2.9. The atmosphere, past and present

The atmosphere of Mars is relatively thin, andpressure on the surface varies from around 30Pa on Olympus Mons’s peak to over 1,155Pa in the depths of Hellas Planitia, with amean surface level pressure of 600 Pa, com-pared to Earth’s sea level average of 101.3 kPa.However, the scale height of the atmosphereis about 11 kilometers, somewhat higher thanEarth’s 7 kilometers. The atmosphere on Marsconsists of 95% carbon dioxide, 3% nitrogen,1.6% argon (Owen 1992).

The existence of liquid water on the sur-face of Mars requires both a warmer andthicker atmosphere. Atmospheric pressure onthe present day Martian surface only exceedsthat of the triple point of water (6.11 hPa) in thelowest elevations; at higher elevations watercan exist only in solid or vapor form. Annualmean temperatures at the surface are currentlyless than 210 K, significantly less than what isneeded to sustain liquid water. However, earlyin its history Mars may have had conditionsmore conducive to retaining liquid water at thesurface.

Early Mars had a carbon dioxide atmo-sphere similar in thickness to present-dayEarth (Carr 1999). Despite a weak early Sun,the greenhouse effect from a thick carbondioxide atmosphere, if bolstered with smallamounts of methane (Squyres & Kasting 1994)or insulating effects of carbon dioxide iceclouds (Forget & Pierrehumbert 1997), wouldhave been sufficient to warm the mean surfacetemperature to a value above the freezing pointof water. The atmosphere has since been re-duced by sequestration in the ground in theform of carbonates through weathering (Carr1999), as well as loss to space through sput-tering (an interaction with the solar wind due

to the lack of a strong Martian magnetosphere)(Kass & Yung 1995).

3. The latest generation of missionsto Mars

For almost twenty years after the Vikingmission no probes were sent to Mars, per-haps because of the failure of Viking landersto find any evidence of existing life. MarsGlobal Surveyor (MGS) was the first success-ful NASA mission launched to Mars sincethe Viking mission in 1976, arriving at Marson September 12, 1997 and operating untilNovember 2006. Perhaps the most importantresults of MGS is the discovery that liquid wa-ter episodically flows over the surface of theplanet even today, through release from subter-ranean reservoirs along sun-facing scarps andcliffs (Malin & Edgett 2000). Although thequantity of water released in this way is verysmall, compared to that required to carve out-flow channels and valley networks, the pres-ence of liquid water implies the potential exis-tence of biological habitats in the Martian sub-surface.

Another important finding of MGS is thedetection of patches of residual magnetizationin the Martian crust (Acuna et al. 1998). Thisimplies that Mars, once, had a magnetic field,and thus an internal dynamo like the Earth,which stopped in the mid-Noachian, about fourbillion years ago, for causes that are still beingdebated (Lillis et al. 2008). Without the shield-ing from the solar wind afforded by the mag-netic field, the radiative environment at the sur-face of the Earth would be too harsh for life, asit is today on Mars, whereas the presence of amagnetic field would have made it possible forlife to evolve on a once wetter Mars.

A final discovery of MGS that influencedthe exploration strategy for the determinationof the biologic potential of Mars is that ofa fossilized and exhumed delta, which wasformed most likely by the flow of water, inEberswalde crater (Malin & Edgett 2003). Theexistence of the delta provides unambiguousevidence that bodies of water existed in thepast. Although it is currently deemed that thedelta likely formed not in a stable long-lived

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lake but over the course of a small num-ber of shorter lacustrine episodes (Lewis &Aharonson 2006), Eberswalde crater is one ofthe potential landing site for NASA’s futureMars Science Laboratory rover, whose objec-tive is the search for signs of ancient life.

Following the discoveries made by MGS,NASA’s 2001 Mars Odyssey was launched onApril 7, 2001, and arrived at Mars on October24, 2001. It is still in operation, and it isthus the longest-serving spacecraft at Mars.Its mission is to search for evidence of pastor present water and volcanic activity. Thanksto the gamma ray spectrometer on board, itwas found that the upper layers of Martiansoil poleward of 55◦ of latitude contain up to50% water ice by weight (Boynton et al. 2002).Because this sensor can analyse only the topmeter of soil, it is not known if ice fills avail-able pore space below this depth. It is estimatedthat total amount of water that could be buriedin the soil would be equivalent to a global layer0.5 to 1.5 km deep (Boynton et al. 2002).

After decades in which Mars explorationwas advanced through American probes (whileSoviets made many attempts that mostly metfailure), the European Space Agency (ESA)launched its first mission to Mars, called MarsExpress, on June 2, 2003. Mars Express con-sists of two parts, the Mars Express Orbiter andthe Beagle 2, a lander designed to perform exo-biology and geochemistry research. Althoughthe lander failed to land safely on the Martiansurface, the Orbiter is successfully perform-ing scientific measurements since early 2004,namely, high-resolution imaging and miner-alogical mapping of the surface, radar sound-ing of the subsurface structure, precise deter-mination of the atmospheric circulation andcomposition, and study of the interaction of theatmosphere with the interplanetary medium.

The importance of water for both the ge-ologic, climatic and potentially exobiologi-cal evolution of Mars was recognized alreadyin Viking times, but after MGS and MarsOdyssey the search for water became one ofthe key drivers of scientific exploration. Fromthe early mapping observation of the perma-nent ice caps on the Martian poles, the north-ern cap was believed to be mainly composed

of water ice, whereas the southern cap wasthought to be constituted of carbon dioxideice. The OMEGA spectrometer on board MarsExpress achieved the first direct identificationand mapping of both carbon dioxide and wa-ter ice in the Martian high southern latitudes,showing that this south polar cap containsperennial water ice in extended areas (Bibringet al. 2004).

After OMEGA revealed that the South po-lar layered deposits of Mars were ice-rich, theMARSIS subsurface sounding radar on MarsExpress probed them penetrating to depths ofmore than 3.7 kilometers. It was found that thecharacteristics of radar echoes were compati-ble with a composition of nearly pure waterice down to the bottom of the deposits, beyondwhat OMEGA could detect. Because MARSISwas able to map the thickness of the layereddeposits over the entire South polar area, it waspossible to estimate that the total volume of icecontained in them is 1.6×106 cubic kilometers,which is equivalent to a global water layer ap-proximately 11 meters thick (Plaut et al. 2007).Although this is below most estimates of thevolume of water needed to carve geologic fea-tures such as outflow channels and valley net-works, it is the largest known reservoir of wateron the planet.

Perhaps the most important discoverymade by Mars Express is the detection ofmethane in the atmosphere by the PFS spec-trometer (Formisano et al. 2004). Methane isnot stable in the Martian atmosphere, becausesolar UV radiation causes its photodissociationover times of perhaps several centuries. Thistime is long enough for any release to distributeevenly around the planet, but careful analysisshowed that methane was localized in specificareas. Its concentration there increased fromMartian spring through summer, then dropped.This argues for destruction by UV-producedperoxides or other rapid chemical oxidants onthe surface or entrained on airborne dust. Mostimportantly, it also indicates real-time releaseof methane from an active source.

There are only a few processes thought tobe able to produce methane on Mars, by anal-ogy with the Earth. Volcanism is one possi-bility, but there has been no detection of on-

330 Coradini and Orosei: Exploration of Mars

going volcanic activity on Mars in spite ofmany thermal infrared sensors searching for it.Another potential source could be a reactionof olivine rock with groundwater and subsur-face heat (serpentinization), but it is unclear ifthe necessary temperatures can be reached inthe Martian subsurface. The third possibility issubterranean microbial life. Because of the im-mense significance of the discovery of extantlife on Mars, the determination of the originof methane has become the new key driver ofscientific exploration, and both NASA’s MarsScience Laboratory (to be launched in 2011)and ESA’s ExoMars (whose launch is plannedfor 2018) will carry instruments to detect andanalyze Martian methane.

Connected to the problem of the habitabil-ity of Mars, OMEGA was able to completea thorough mapping of the spectral proper-ties of the Martian surface, which led to theidentification of hydrated minerals requiringfor their formation a much wetter environmentthan today’s Mars. This discovery reinforcedthe theory that liquid water was present on theMartian surface in the past, and eventually ledto the proposal of an alternative timeline forMars based upon the correlation between themineralogy and geology of the planet. Thisproposed timeline divides the history of theplanet into three epochs; the Phyllocian, theTheiikian and the Siderikan (Bibring et al.2006).

– The Phyllocian (named after the clay-rich phyllosilicate minerals that character-ize the epoch) lasted from the formation ofthe planet until around four billion yearsago. In order for the phyllosilicates to forman alkaline water environment would havebeen present.

– The Theiikian (named, in Greek, after thesulfate minerals that were formed), last-ing until about 3.5 billion years ago, wasa period of volcanic activity. In addition tolava, gases - and in particular sulfur dioxide- were released, combining with water tocreate sulfates and an acidic environment.The vulcanism era left features indicativeof the interaction between water and lavaand/or magma on Mars.

– The Siderikan, from 3.5 billion years agountil the present. With the end of volcan-ism and the absence of liquid water, themost notable geologic process has beenthe oxidation of the iron-rich rocks by at-mospheric peroxides, leading to the rediron oxides that give the planet its familiarcolor.

The new insight provided by this analysis hasled planners of future missions to Mars to theconclusion that deposits from the Phyllocianera are the best candidates to search for evi-dence of past life on the planet. Although ob-servations from orbit can still contribute to theglobal characterization of the planet, to test themost exciting hypotheses about the presenceof liquid water, and perhaps life, the acquisi-tion of samples for complex and detailed in situanalysis is considered to be essential.

Briefly following Mars Express, NASA’stwin Mars Exploration Rovers Spirit andOpportunity launched toward Mars on June 10and July 7, 2003, landing on Mars January 4and January 25, 2004. While Spirit has notbeen communicating with Earth since March22, 2010, Opportunity is still driving on thesurface of Mars. Primary among the mission’sscientific goals is to search for and character-ize a wide range of rocks and soils that holdclues to past water activity on Mars. The space-craft were targeted to sites on opposite sides ofMars that appear to have been affected by liq-uid water in the past. The landing sites are atGusev Crater, a possible former lake in a giantimpact crater, and Meridiani Planum, wheremineral deposits (hematite) suggest Mars had awet past. It was indeed found that, most likely,Meridiani once had abundant acidic groundwa-ter, arid and oxidizing surface conditions, andoccasional liquid flow on the surface (Squyreset al. 2006), while the granular rocks found inGusev, the so-called “blueberries”, have beeninterpreted to be volcanic ash and/or impactejecta deposits that have been modified byaqueous fluids during and/or after emplace-ment (Arvidson et al. 2006). These findingsadd to those obtained from previous missionsin outlining an image of Mars’ past as a wetenvironment favourable to the evolution of life,

Coradini and Orosei: Exploration of Mars 331

but in the current sterilizing surface conditions(due to intense UV radiation), no evidence ofpast or present life can be preserved. Futuresteps in the search for life on Mars will thusrequire access to the subsurface for in situ anal-ysis.

In the unfolding of NASA’s strategy forthe exploration of Mars, the next mission wasMars Reconnaissance Orbiter (MRO), whichwas launched on August 12, 2005, and attainedMartian orbit on March 10, 2006. In November2006, after five months of aerobraking, it en-tered its final orbit and began its science activ-ity, which still continues today. MRO’s scien-tific goal is to search for evidence that waterpersisted on the surface of Mars for a long pe-riod of time: while other Mars missions haveshown that water flowed across the surface inMars’ history, it is still unclear whether wa-ter was ever around long enough to providea habitat for life. Another important goal forMRO is to provide support to lander missionsin the form of high-resolution observations ofthe surface for the detection of landing hazards.

Very high resolution images obtained byMRO revealed fluvial landforms that havebeen interpreted as the result of sustained pre-cipitation, surface runoff, and fluvial deposi-tion occurring during the Hesperian on theplateaus adjacent to Valles Marineris (Weitz etal. 2010). Thus, liquid water would be presentnon only in the Noachian but also through theHesperian, in the form of rain perhaps thanksto an enhanced greenhouse effect from burstsof volcanic activity releasing vast quantities ofCO2 in the atmosphere.

The SHARAD subsurface sounding radaron MRO, endowed with a tenfold-better res-olution compared to MARSIS, provided evi-dence that Mars’ climate undergoes dramaticperiodic changes and may now be in a warmingtrend (Phillips et al. 2008). This can be seen inFig. 2, showing a radar cross-section of Mars’north polar ice cap acquired by SHARAD: icelayering shows rhythmic cycling between bun-dles of dust-containing layers and interspersedclean ice, reflecting changes in environmentalconditions during deposition. The driving forcefor the climate changes appears to be the largevariations in Mars planetary motions. As with

Earth, Mars’ orbit is eccentric, its rotationalaxis precesses, and especially, its obliquity (ax-ial tilt) oscillates. At low obliquity, less sun-light falls on polar regions, which accumulatesnow. At high obliquity the poles receive moresunlight and the equator less, so snow migratesto equatorial regions. At present the obliquityof Mars is calculated to be roughly 25◦ and de-creasing, indicating that in (geologicly) recenttimes the axial tilt was substantially larger andice would be expected near the equator.

In the wake of MRO, the Phoenix landerwas launched on August 4, 2007, descendingon Mars on May 25, 2008. The mission objec-tive was to search for environments suitable formicrobial life on Mars, and to research the his-tory of water there. The lander completed itsmission in August 2008, and made a last briefcommunication with Earth on November 2 asavailable solar power dropped with the Martianwinter. The mission was declared concluded onNovember 10, 2008, after engineers were un-able to re-contact the craft. Phoenix excavatedinto the upper centimeters of soil to reveal wa-ter ice, confirming predictions made by in 2002by the Mars Odyssey orbiter. It was able to per-form accurate chemical analysis of soil sam-ples thanks to its wet chemistry laboratory, re-vealing the presence of a number of salts, es-pecially perchlorate (Hecht et al. 2009). Thissalt greatly lowers the freezing point of water,and thus may be allowing small amounts of liq-uid water to form on Mars today. Furthermore,perchlorate is used as food by some bacteria onEarth through anaerobic reduction.

4. The future

Discoveries described in the previous sectionhave changed the paradigm of Martian geo-logic evolution that consolidated at the end ofthe Viking era, reviving the possibility of thepresence of life and spurring the developmentof future missions aimed at finding it. As oftoday, NASA and ESA are planning a total offour missions to Mars over the next decade,while Russia and Japan have announced theirinterest in participating. It is becoming increas-ingly clear that, in sharp contrast to the con-quest of the Moon, the future of Mars explo-

332 Coradini and Orosei: Exploration of Mars

Fig. 2. (Top) Radar cross-section from SHARAD orbit 5192 above Planum Boreum, Mars. The North PolarLayered Deposit geologic unit (NPLD) and the Basal Unit (BU) beneath the NPLD are labeled. Internalradar reflections arise from boundaries between layers differing in their fractions of ice, dust, and sand. Theradar reflections in Planum Boreum are clustered into distinct packets of reflectors (numbered). (Bottom)Ground track of orbit 5192 shown on a topographic map of the north polar region of Mars (adapted fromPhillips et al. (2008)).

ration will be based on international collabora-tions involving all space-faring nations in theworld.

The next mission to Mars will be NASA’sMars Science Laboratory (MSL), which isscheduled to be launched in November 2011and land on Mars in August 2012. The MSLrover will be over five times as heavy as andcarry over ten times the weight of scientific in-struments as the rovers Spirit or Opportunity,and its goal will be to determine whether Marsever was, or is still today, an environmentable to support microbial life (Cabane & SAMTeam 2010).

NASA’s Mars Atmosphere and VolatileEvolution (MAVEN) is set to launch in the pe-riod between November 18 and December 7,2013, and will explore the planet’s upper at-mosphere, ionosphere and interactions with thesun and solar wind, to determine the role thatloss of volatile compounds - such as carbondioxide, nitrogen dioxide, and water - from theMars atmosphere to space has played throughtime, giving insight into the history of Mars at-mosphere and climate, liquid water, and plane-tary habitability (Jakosky 2009).

ExoMars Trace Gas Orbiter will be thefirst joint ESA and NASA mission to Mars,a flexible collaborative proposal within NASA

and ESA to send a new orbiter-carrier to Marsin 2016 as part of the European-led ExoMarsmission. This orbiter will deliver the ExoMarsstatic lander and then proceed to map thesources of methane on Mars and other gases,and in doing so, help select the landing sitefor the ExoMars rover to be launched on 2018(ESA 2010a). The ExoMars Rover will, inturn, characterize the biological environmenton Mars in preparation for robotic missionsand then human exploration (ESA 2010b).

This is thus a very exciting time for Marsexploration, and the current decade will prob-ably revolutionize our understanding of Marseven more than the previous one. Still moreimportant, we will gain precious insight on theway life develops in the Universe, and on thelikelihood of the occurrence of life on planetsother than the Earth. Ironically, we may wellconclude that Schiaparelli’s ideas on the pres-ence of life on Mars were not so far off themark. Certainly, one has to concede that thereare channels on Mars, after all.

Acknowledgements. This research has made use ofNASA’s Astrophysics Data System.

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