ExoMarsSearching for Life on the Red Planet

ExoMarsSearching for Life on the Red Planet

E stablishing whether life ever existed onMars, or is still active today, is an

outstanding question of our time. It is alsoa prerequisite to prepare for future humanexploration. To address this importantobjective, ESA plans to launch the ExoMarsmission in 2011. ExoMars will also develop anddemonstrate key technologies needed to extendEurope’s capabilities for planetary exploration.

Mission ObjectivesExoMars will deploy two science elementson the Martian surface: a rover and asmall, fixed package. The Rover willsearch for signs of past and present lifeon Mars, and characterise the water andgeochemical environment with depth bycollecting and analysing subsurfacesamples. The fixed package, theGeophysics/Environment Package (GEP),will measure planetary geophysics para-meters important for understandingMars’s evolution and habitability,identify possible surface hazards tofuture human missions, and study theenvironment.

The Rover will carry a comprehensivesuite of instruments dedicated to exo-biology and geology: the Pasteur payload.It will travel several kilometres searchingfor traces of life, collecting andanalysing samples from inside surfacerocks and by drilling down to 2 m. Thevery powerful combination of mobilityand accessing locations where organicmolecules may be well-preserved isunique to this mission.

Jorge Vago, Bruno Gardini, Gerhard Kminek,Pietro Baglioni, Giacinto Gianfiglio,Andrea Santovincenzo, Silvia Bayón& Michel van WinnendaelDirectorate for Human Spaceflight,Microgravity and Exploration Programmes,ESTEC, Noordwijk, The Netherlands

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ExoMars

ExoMars will also pursue importanttechnology objectives aimed at extend-ing Europe’s capabilities in planetaryexploration. It will demonstrate thedescent and landing of a large payload onMars; the navigation and operation of amobile scientific platform; a novel drill toobtain subsurface samples; and meetchallenging planetary protection andcleanliness levels necessary to achieve themission’s ambitious scientific goals.

The Search for LifeExobiology, in its broadest terms, denotesthe study of the origin, evolution anddistribution of life in the Universe. It iswell established that life arose very earlyon the young Earth. Fossil records showthat life had already attained a large degreeof biological sophistication 3500 millionyears ago. Since then, it has provedextremely adaptable, colonising even themost disparate ecological habitats, fromthe very cold to the very hot, and spanninga wide range of pressure and chemicalconditions. For organisms to have emergedand evolved, water must have been readilyavailable on our planet. Life as we know itrelies, above all else, upon liquid water.Without it, the metabolic activities ofliving cells are not possible. In the absenceof water, life either ceases or slips intoquiescence.

Mars today is cold, desolate and dry. Itssurface is highly oxidised and exposed tosterilising and degrading ultraviolet (UV)radiation. Low temperature and pressurepreclude the existence of liquid water;except, perhaps, in localised environments,and then only episodically. Nevertheless,numerous features such as large channels,dendritic valley networks, gullies and

sedimentary rock formations suggest thepast action of surface liquid water on Mars– and lots of it. In fact, the sizes of out-flow channels imply immense discharges,exceeding any floods known on Earth.

Mars’s observable geological recordspans some 4500 million years. From the number of superposed craters, the oldest terrain is believed to be about4000 million years old, and the youngestpossibly less than 100 million years.Most valley networks are ancient(3500–4000 million years), but as manyas 25–35% may be more recent. Today,water on Mars is only stable as ice at thepoles, as permafrost in widespreadunderground deposits, and in traceamounts in the atmosphere. From abiological perspective, past liquid wateritself motivates the question of life onMars. If Mars’ surface was warmer andwetter for the first 500 million years ofits history, perhaps life arose independ-ently there at more or less the same timeas it did on Earth.

An alternative pathway may have beenthe transport of terrestrial organismsembedded in meteoroids, delivered fromEarth. Yet another hypothesis is that lifemay have developed within a warm, wetsubterranean environment. In fact, giventhe discovery of a flourishing biosphere akilometre below Earth’s surface, a similarvast microbial community may be activeon Mars, forced into that ecological nicheby the disappearance of a more benignsurface environment. The possibility thatlife may have evolved on Mars during anearlier period surface water, and thatorganisms may still exist underground,marks the planet as a prime candidate inthe search for life beyond Earth.

Hazards for Manned Operations on MarsBefore we can contemplate sendingastronauts to Mars, we must understandand control any risks that may pose athreat to a mission’s success. We can begin to assess some of these risks withExoMars.

Ionising radiation is probably the singlemost important limiting factor for human interplanetary flight. To evaluateits danger and to define efficient mitigationstrategies, it is desirable to incorporateradiation-monitoring capabilities duringcruise, orbit and surface operations onprecursor robotic missions to Mars.

Another physical hazard may resultfrom the basic mechanical properties ofthe Martian soil. Dust particles will invade the interior of a spacecraft during surface operations, as shownduring Apollo’s operations on the Moon. Dust inhalation can pose a threat to astronauts on Mars, and evenmore so under microgravity during thereturn flight to Earth. Characteristics ofthe soil, including the sizes, shapes andcompositions of individual particles,can be studied with dedicated in situinstrumentation. However, a more in-depth assessment, including a toxicity analysis, requires the return of asuitable Martian sample.

Reactive inorganic substances couldpresent chemical hazards on the surface.Free radicals, salts and oxidants are veryaggressive in humid conditions such as thelungs and eyes. Toxic metals, organics andpathogens are also potential hazards. Aswith dust, chemical hazards in the soil willcontaminate the interior of a spacecraftduring surface operations. They coulddamage the health of astronauts and the

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operation of equipment. Many potentialinorganic and organic chemical hazardsmay be identified with the ExoMarssearch-for-life instruments.

Geophysics MeasurementsThe processes that have determined thelong-term ‘habitability’ of Mars dependon the geodynamics of the planet, and on its geological evolution and activity.Important issues still need to be resolved.

What is Mars’ internal structure? Is thereany volcanic activity on Mars? Theanswers may allow us to extrapolate intothe past, to estimate when and how Mars lost its magnetic field, and theimportance of volcanic outgassing for theearly atmosphere.

ExoMars will also carry the Geophysics/Environment Package, accommodated onthe Descent Module and powered by asmall radioisotope thermal generator.

Searching for Signs of LifeIf life ever arose on the Red Planet, itprobably did so when Mars was warmerand wetter, during its initial 500–1000 million years. Conditions then weresimilar to those on early Earth: activevolcanism and outgassing, meteoriticimpacts, large bodies of liquid water, anda mildly reducing atmosphere. We mayreasonably expect that microbes quicklybecame global. Nevertheless, there isinevitably a large measure of chanceinvolved in finding convincing evidence ofancient, microscopic life forms.

On Earth’s surface, the permanentpresence of running water, solar-UVradiation, atmospheric oxygen and lifeitself quickly erases all traces of anyexposed, dead organisms. The onlyopportunity to detect them is to find theirbiosignatures encased in a protectiveenvironment, as in suitable rocks.However, since high-temperature meta-morphic processes and plate tectonicshave reformed most ancient terrains, it isvery difficult to find rocks on Earth olderthan 3000 million years in good condition.Mars, on the other hand, has not sufferedsuch widespread tectonic activity. Thismeans there may be rock formations fromthe earliest period of Martian history thathave not been exposed to high-temperaturerecycling. Consequently, well-preservedancient biomarkers may still be accessiblefor analysis.

Even on Earth, a major difficulty insearching for primitive life is that, inessence, we are looking for the remnants ofminuscule beings whose fossilised formscan be simple enough to be confused withtiny mineral precipitates. This issue lies atthe heart of a heated debate among

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The ExoMars Rover will be able to drill down to 2 m for samples

A Mars Express image of the Ares Vallis region,showing evidence of ancient, vast water discharges. This immense channel, 1400 km long, empties intoChryse Planitia, where Mars Pathfinder landed in1997. (ESA/DLR/FU Berlin, G. Neukem)

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Mission strategy to achieve ExoMars’s scientific objectives:

1 To land on, or be able to reach, a location with high exobiology interest for past and/orpresent life signatures, i.e. access to the appropriate geological environment.

2 To collect scientific samples from different sites, using a Rover carrying a drill capable ofreaching well into the soil and surface rocks. This requires mobility and access to thesubsurface.

3 At each site, to conduct an integral set of measurements at multiple scales: beginning witha panoramic assessment of the geological environment, progressing to smaller-scaleinvestigations on interesting surface rocks using a suite of contact instruments, andculminating with the collection of well-selected samples to be studied by the Rover’sanalytical laboratory.

4 To characterise geophysics and environment parameters relevant to planetary evolution,life and hazards to humans.

To arrive at a clear and unambiguous conclusion on the existence of past or present life atthe Rover sites, it is essential that the instrumentation can provide mutually reinforcing lines ofevidence, while minimising the opportunities for alternative interpretations.

It is also imperative that all instruments be carefully designed so that none is a weak link inthe chain of observations; performance limitations in an instrument intended to confirm theresults obtained by another should not generate confusion and discredit the wholemeasurement.

The science strategy for the Pasteur payload is therefore to provide a self-consistent set ofinstruments to obtain reliable evidence, for or against, the existence of a range ofbiosignatures at each search location.

Spacecraft: Carrier plus Descent Module (including Rover and GEP)Data-relay provided by NASA

Launch: May–June 2011, from Kourou on Soyuz-2b (backup 2013)

Arrival: June 2013 (backup 2015)

Landing: Direct entry, from hyperbolic trajectory, after the dust storm season. Latitudes 15˚S–45˚N, all longitudes, altitude: <0 m, relative to the MGS/MOLA* zero level

Science: Rover with Pasteur payload:mass 120–180 kg, includes: Drill System/SPDS and instruments (8 kg); lifetime 180 sols

Geophysics Environment Package (GEP):mass <20 kg; includes: instruments (~4 kg); lifetime 6 years

Ground Mission control and mission operations: ESOCSegment: Rover operation on Mars surface: Rover Operations Centre

GEP operations: to be decided

*MGS/MOLA: Mars Global Surveyor/Mars Orbiter Laser Altimeter

palaeobiologists. It is therefore doubtfulthat any one signature suggestive of life– whether it is an image implying abiostructure, an interesting organiccompound or a fractionated isotopic ratio– may reliably demonstrate a biogenicorigin. Several independent lines ofevidence are required to construct acompelling case. ExoMars must thereforepursue a holistic search strategy, attackingthe problem from multiple angles,including geological and environmentalinvestigations (to characterise potentialhabitats), visible examination of samples(morphology) and spectrochemicalcomposition analyses.

In 1976, the twin Viking landersconducted the first in situ measurementsfocusing on the detection of organiccompounds and life on Mars. Theirbiology package contained threeexperiments, all looking for signs ofmetabolism in soil samples. One, thelabelled-release experiment, producedprovocative results. If other informationhad not been also available, these datacould have been interpreted as proof ofbiological activity. However, theoreticalmodelling of the atmosphere and regolithchemistry hinted at powerful oxidantsthat could more-or-less account for theresults of the three experiments. Thebiggest blow was the failure of the Vikinggas chromatograph mass spectrometer(GCMS) to find evidence of organicmolecules at the parts-per-billion level.

With few exceptions, the majority of thescientific community has concluded thatthe Viking results do not demonstrate thepresence of life. Numerous attempts havebeen made in the laboratory to simulatethe Viking reactions. While some havereproduced certain aspects, none hassucceeded entirely. Incredibly, 30 yearsafter Viking, the crucial chemical oxidanthypothesis remains untested. ExoMarswill include a powerful instrument tostudy oxidants and their relation toorganics distribution on Mars.

Undoubtedly, the present environmenton Mars is exceedingly harsh for thewidespread proliferation of surface life: itis simply too cold and dry, not to mentionthe large doses of UV. Notwithstanding

Recommended Pasteur Exobiology Instruments1

PPaannoorraammiicc To characterise the Rover’s geological context (surface and subsurface). Typical scales span from panoramic to IInnssttrruummeennttss 10 m, with a resolution of the order of 1 cm for close targets.

Panoramic Camera 2 wide-angle stereo cameras and 1 high-resolution camera; to characterise the Rover’s environment and itsSystem geology. Also very important for target selection.

Infrared (IR) For the remote identification of water-related minerals, and for target selection.Spectrometer

Ground Penetrating) To establish the subsurface soil stratigraphy down to 3 m depth, and to help plan the drilling strategy.Radar (GPR)

CCoonnttaacctt To investigate exposed bedrock, surface rocks and soils. Among the scientific interests at this scale are:IInnssttrruummeennttss macroscopic textures, structures and layering; and bulk mineralogical and elemental characterisation. This

information will be fundamental to collect samples for more detailed analysis. The preferred solution is todeploy the contact instruments using an arm-and-paw arrangement, as in Beagle-2. Alternatively, in case of masslimitations, they could be accommodated at the base of the subsurface drill.

Close-Up Imager To study rock targets visually at close range (cm) with sub-mm resolution.

Mössbauer Spectrometer To study the mineralogy of Fe-bearing rocks and soils.

Raman-LIBS2 external To determine the geochemistry/organic content and atomic composition of observed minerals. These opticalheads are external heads connected to the instruments inside the analytical laboratory.

SSuuppppoorrtt These instruments are devoted to the acquisition and preparation of samples for detailed investigations in theIInnssttrruummeennttss analytical laboratory. They must follow specific acquisition and preparation protocols to guarantee the optimal

survival of any organic molecules in the samples. The mission’s ability to break new scientific ground, particularlyfor signs-of-life investigations, depends on these two instruments.

Subsurface Drill Capable of obtaining samples from 0 m to 2 m depths, where organic molecules might be well-preserved. It also integrates temperature sensors and an IR spectrometer for borehole mineralogy studies.

Sample Preparation Receives a sample from the drill system, prepares it for scientific analysis, and presents it to all analytical and Distribution laboratory instruments. A very important function is to produce particulate material while preserving the System (SPDS) organic and water content.

AAnnaallyyttiiccaall To conduct a detailed analysis of each sample. The first step is a visual and spectroscopic inspection. If theLLaabboorraattoorryy sample is deemed interesting, it is ground up and the resulting particulate material is used to search for organic

molecules and to perform more accurate mineralogical investigations.

Microscope IR To examine the collected samples to characterise their structure and composition at grain-size level. These measurements will also be used to select sample locations for further detailed analyses by the Raman-LIBS spectrometers.

Raman-LIBS To determine the geochemistry/organic content and elemental composition of minerals in the collected samples.

X-ray Diffractometer (XRD) To determine the true mineralogical composition of a sample’s crystalline phases.

Urey (Mars Organics Mars Organics Detector (MOD): extremely high-sensitivity detector (ppt) to search for amino acids, nucleotide and Oxidants Detector) bases and PAHs in the collected samples. Can also function as front-end to the GCMS. Mars Oxidants

Instrument (MOI): determines the chemical reactivity of oxidants and free radicals in the soil and atmosphere.

GCMS Gas chromatograph mass spectrometer to conduct a broad-range, very-high sensitivity search for organic molecules in the collected samples; also for atmospheric analyses.

Life-Marker Chip Antibody-based instrument with very high specificity to detect present life reliably.

1Mass (without drill and SPDS): 12.5 kg. 2LIBS: Laser-Induced Breakdown Spectroscopy.

Recommended Pasteur Environment Instruments3

EEnnvviirroonnmmeenntt To characterise possible hazards to future human missions and to increase our knowledge of the Martian IInnssttrruummeennttss environment.

Dust Suite Determines the dust grain size distribution and deposition rate. It also measures water vapour with high precision.

UV Spectrometer Measures the UV radiation spectrum.

Ionising Radiation Measures the ionising radiation dose reaching the surface from cosmic rays and solar particle events.

Meteorological Package Measures pressure, temperature, wind speed and direction, and sound.

3Mass: 1.9 kg. The Pasteur environment instruments are presently planned to be accommodated in the GEP.

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these hazards, basic organisms could stillflourish in protected places: deepunderground, at shallow depths inespecially benign environments, or withinrock cracks and inclusions.

The strategy to find traces of pastbiological activity rests on the assumptionthat any surviving signatures of interestwill be preserved in the geological record,in the form of buried/encased remains,organic material and microfossils.Similarly, because current surfaceconditions are hostile to most knownorganisms, as when looking for signs ofextant life, the search methodology shouldfocus on investigations in protectedniches: underground, in permafrost orwithin surface rocks. This means thatthere is a good possibility that the samesampling device and instrumentationmay adequately serve both types ofstudies. The biggest difference is due tolocation requirements. In one case, theinterest lies in areas occupied by ancientbodies of water over many thousands of years. In the other, the emphasis is onwater-rich environments close to thesurface and accessible to our sensorstoday. For the latter, the presence ofpermafrost alone may not be enough.Permafrost in combination with asustained heat source, probably of volcanicor hydrothermal origin, may be necessary.Such warm oases can only be identified byan orbital survey of the planet. In the next

few years, a number of remote-sensingsatellites, like ESA’s Mars Express andNASA’s Mars Reconnaissance Orbiter(MRO), will determine the water/iceboundary across Mars and may help todiscover such warm spots. If they do exist,they would be prime targets for missionslike ExoMars.

On Earth, microbial life quickly becamea global phenomenon. If the sameexplosive process occurred on the youngMars, the chances of finding evidence of itare good. Even more interesting would bethe discovery and study of life forms thathave successfully adapted to the modernMars. However, this presupposes the prioridentification of geologically suitable, life-friendly locations where it can bedemonstrated that liquid water still exists,at least episodically throughout the year.For these reasons, the ‘Red Book’ scienceteam advised ESA to focus on thedetection of extinct life, but to buildenough flexibility into the mission to beable to target sites with the potential forpresent life.

Mission DescriptionThe baseline mission scenario consists ofa spacecraft composite with a Carrier anda Descent Module, launched by aSoyuz-2b from Kourou. It will follow a2-year ‘delayed trajectory’ in order toreach Mars after the dust-storm season.The Descent Module will be released from

the hyperbolic arrival path, and landusing either bouncing (non-vented, as inNASA’s rovers) or non-bouncing (vented)airbags, and deploy the Rover and GEP.In the baseline mission, data-relay forthe Rover will be provided by a NASAorbiter.

An alternative configuration, based onan Ariane-5 ECA launcher, may beimplemented depending on programme,technical and financial considerations.In this option, the Carrier is replacedwith an Orbiter that provides end-to-enddata relay for the surface elements. TheOrbiter will also carry a science payloadto complement the results from theRover and GEP, and provide continuityto the great scientific discoveries flowingfrom Mars Express.

ExoMars is a search-for-life missiontargeting regions with high life potential.It has therefore been classified asPlanetary Protection category IVc. This,coupled with the mission’s ambitiousscientific goals, imposes challengingsterilisation and organic cleanlinessrequirements.

The ExoMars RoverThe Rover will have a nominal lifetime of180 sols (about 6 months). This periodprovides a regional mobility of severalkilometres, relying on solar array electricalpower. The Pasteur model payloadincludes panoramic instruments (cameras,ground-penetrating radar and IR spectro-meter; contact instruments for studyingsurface rocks (close-up imager andMössbauer spectrometer), a subsurfacedrill to reach depths of 2 m and tocollect specimens from exposed bedrock,a sample preparation and distributionunit, and the analytical laboratory. Thelatter includes a microscope, anoxidation sensor and a variety of

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The ExoMars surface science exploration scenario. The Rover willconduct measurements of multiple scales, starting with apanoramic assessment of the geological environment, progressingto more detailed investigations on surface rocks using a suite ofcontact instruments, and culminating with the collection of well-selected samples to be analysed in its laboratory

instruments for characterising theorganic substances and geochemistry inthe collected samples.

A key element is the drill. The reason forthe 2 m requirement is the need to obtainpristine sample material for analysis.Whereas the estimated extinction horizonfor oxidants in the subsurface is severalcentimetres, damaging ionising radiationcan penetrate to depths of around 1 m.Additionally, it is unlikely that loose dustmay hold interesting biosignatures,because it has been moved around bywind and processed by UV radiation. Inthe end, organic substances may best bepreserved within low-porosity material.Hence, the ExoMars drill must be ableto penetrate and obtain samples fromwell-consolidated (hard) formations,such as sedimentary rocks and evaporiticdeposits. Additionally, it must monitorand control torque, thrust, penetrationdepth and temperature at the drill bit.Grain-to-grain friction in a rotary drillcan generate a heat wave in the sample,

destroying the organic molecules thatExoMars seeks to detect. The drill musttherefore have a variable cutting protocol,to dissipate heat in a science-safemanner. Finally, the drill’s IR spectro-meter will conduct mineralogy studiesinside the borehole.

ConclusionNASA’s highly successful 2004 roverswere conceived as robotic geologists. Theyhave demonstrated the past existence oflong-lasting, wet environments on Mars.Their results have persuaded the scientificcommunity that mobility is a must-haverequirement for all future surface missions.Recent results from Mars Express haverevealed multiple, ancient depositscontaining clay minerals that form only in the presence of liquid water. Thisreinforces the hypothesis that ancientMars may have been wetter, and possiblywarmer, than it is today. NASA’s 2009Mars Science Laboratory will studysurface geology and organics, with the

goal of identifying habitable environ-ments. ExoMars is the next logical step.It will have instruments to investigatewhether life ever arose on the RedPlanet. It will also be the first missionwith the mobility to access locationswhere organic molecules may be well-preserved, thus allowing, for the firsttime, investigation of Mars’ thirddimension: depth. This alone is aguarantee that the mission will breaknew scientific ground. Finally, the manytechnologies developed for this projectwill allow ESA to prepare forinternational collaboration on futuremissions, such as Mars Sample Return.

Following the recent accomplishmentsof Huygens and Mars Express,ExoMars provides Europe with a newchallenge, and a new opportunity todemonstrate its capacity to performworld-class planetary science.

ExoMars is now in Phase-B1 and isexpected to begin Phase-B2 in mid-2007and Phase-C/D in early 2008. e

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The analysis sequence within Pasteur’s analytical laboratory

The Pasteur payload’s drill-bit design concept. The drill’s full 2 mextension is achieved by assembling four sections (one drill toolrod, with an internal shutter and sample-collection capability, plusthree extension rods). The drill will also be equipped with an IRspectrometer for mineralogy studies inside the borehole. (GalileoAvionica)