Scientific objectives of the ExoMars Rover
The science objectives of the ExoMars Rover, in order of priority, are:
- To search for signs of past and present life on Mars;
- To characterise the water/geochemical environment as a function of depth in the shallow subsurface.
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| The ExoMars rover. Credit: ESA |
The ExoMars Rover mission will pursue one of the outstanding questions of our time by attempting to establish whether life ever existed, or is still active on Mars today.
The Rover will carry a comprehensive suite of analytical instruments dedicated to exobiology and geochemistry research: the Pasteur payload. The Rover will travel several kilometres searching for signs of past and present life, collecting and analysing samples from within rocky outcrops and from the subsurface, down to a depth of
Searching for evidence of past and present life on Mars
If life ever arose on the Red Planet, it probably did when Mars was warmer and wetter, sometime within the first billion years following planetary formation. Conditions then were similar to those when microbes gained a foothold on the young Earth. This marks Mars as a primary target for the search for signs of life in our So-lar System.
Unfortunately, on our planet, high-temperature metamorphic processes and plate tectonics have resulted in the reformation of most ancient terrains. It is very difficult to find rocks on Earth that are older than 3 billion years and in good condition. Hence, the physico-chemical record of the very early evolution of life on Earth is no longer accessible to us. Mars, on the other hand, has not suffered such widespread tectonic activity. This means that rock formations from the earliest period of Martian history, which have not been exposed to high-temperature recycling, are likely to exist. Consequently, well-preserved, ancient biomarkers may still be accessible for analysis.
Another important reason to study ancient Martian rocks is that they may provide important clues about de-positional processes and habitable conditions on early Earth.
On Earth, microbial life quickly became a global phenomenon. A similar explosive process could have oc-curred on the young Mars. Perhaps, even more interesting would be the discovery and study of life forms that have successfully adapted to modern Mars. However, this presupposes the prior identification of geo-logically suitable, life-friendly locations where it can be demonstrated that liquid water still exists, at least for short periods, throughout the year. Today we do not know of any such place. For these reasons, the first exobiology science team advised ESA in the "Red Book" of 1999 to focus mainly on the detection of extinct life; but also, to build enough flexibility into the mission design to be able to identify signatures of present life.
The Martian environment and the need for subsurface exploration
The ExoMars Rover’s surface mobility and the 2-m vertical reach of the drill are both crucial for the scientific success of the mission.
The ExoMars rover will search for two types of life-related signatures: morphological and chemical. This will be complemented by an accurate determination of the geological context.
Morphological information related to biological processes may be preserved on the surface of rocks. Possi-ble examples include the bio-mediated deposition of sediments, fossilised bacterial mats, stromatolitic mounds, etc. Such studies require mobility and an imaging system capable of covering a range of distances from the metre scale down to sub-millimetre resolution (to discern micro-textural information in rocks).
An effective chemical identification of biomarkers requires access to well-preserved organic molecules. Because the Martian atmosphere is more tenuous than Earth’s, three important physical agents reach the surface of Mars with adverse effects for the long-term preservation of biomarkers:
- The ultraviolet (UV) ra-diation dose is higher than on our planet and will quickly damage potential exposed organisms or bio-molecules.
- UV-induced photochemistry is responsible for the production of reactive oxidant species that,when activated, can also destroy biomarkers; the diffusion of oxidants into the subsurface is not well characterised and constitutes an important measurement that the mission must perform. Finally,
- ionising radiation penetrates into the uppermost metres of the planet’s subsurface. This causes a slow degradation process that, over many millions of years, can alter organic molecules beyond the detection sensitivity of analytical instruments. The ionising radiation effects are depth dependent: the material closer to the surface is exposed to a higher dose than that buried deeper.
A major goal of ExoMars is to study ancient (older than 3 billion years) sedimentary rock formations and evaporitic deposits. However, it is only trapped in the subsurface for long periods that the record of early Martian life, if it ever existed, is likely to escape radiation and chemical damage. Studies show that a sub-surface penetration in the range of 2 m is necessary to recover well-preserved organics from the very early history of Mars.
Additionally, it is essential to avoid loose dust deposits distributed by aeolian transport. While driven by the wind, this material has been processed by UV radiation, ionising radiation, and potential oxidants in the at-mosphere and on the surface of Mars. Any organic biomarkers would be highly degraded in these samples.
For all the above reasons, the ExoMars drill will be able to penetrate and obtain samples from well-consolidated (hard) formations, at various depths, down to 2 m.
Building on results from early Mars landers
The successful NASA Mars Exploration Rovers (MER) have demonstrated the past existence of wet envi-ronments on Mars using a geologically oriented instrument package. Their results have persuaded the scientific community that mobility is a must-have requirement for future missions. Recent discoveries from ESA’s Mars Express spacecraft have revealed multiple deposits containing salt and clay minerals that can only form in the presence of liquid water. This reinforces the hypothesis that ancient Mars may have been wetter, and possibly warmer, than it is today. NASA’s Mars Science Laboratory (MSL), planned for a 2011 launch, will study surface geology and organics, with the goal of identifying habitable environments. The ExoMars Rover constitutes the next logical step in Mars exploration. It will have instruments to investigate whether life ever arose on the Red Planet. It will also be the first mission combining mobility and access to subsurface locations where organic molecules may be well-preserved; thus allowing, for the first time, to investigate Mars’s third dimension: depth. This alone is a guarantee that ExoMars will break new scientific ground.
The ExoMars Rover will also provide crucial science results to prepare for an international Mars Sample Re-turn (MSR) mission, as it will establish whether or not it is important that the MSR samples be collected from the subsurface.
The surface mission of the ExoMars Rover
The scientific success of the ExoMars Rover depends on being able to land safely on a scientifically interest-ing location. An appropriate landing site can only be identified on the basis of remote sensing information. Since ExoMars is a ‘search for life’ mission, candidate sites must contain evidence suggestive of a past or present habitable environment, supported by both morphology and mineralogical composition information. A good landing ellipse would include multiple instances of outcrops whose composition is considered suitable for the long-term preservation of biomarkers (e.g. clays, sulphates, etc.) in association with long-lasting flu-vial, lacustrine, or hydrothermal signatures. However, it is the buried deposits that constitute the primary science target.
The mission strategy to achieve the ExoMars rover’s scientific objectives is:
- To land, or to be able to reach, a location possessing high exobiology interest for past or present life signatures, in other words the Rover must have access to the appropriate geological environment.
- To collect scientific samples from different sites, using a rover carrying a drill capable of reaching well into the subsurface and into surface rocky outcrops.
- At each site, to conduct an integral set of measurements at multiple scales: beginning with a pano-ramic assessment of the geological environment, progressing to smaller-scale investigations on sur-face outcrops, and culminating with the collection of well-selected subsurface (or surface) samples to be studied in the Rover's analytical laboratory.
If any organic compounds are detected on Mars, it will be important to show that they were not brought from Earth. The only way to reliably demonstrate that the Rover is free from contaminants is to perform an initial measurement run using a blank sample. For this reason, the ExoMars rover will carry a number of blank calibration samples. Upon landing, one of the first science actions will be for the drill to pass a blank sam-ple to the analytical laboratory. After performing a full investigation, the results should indicate 'no life' and 'no organics'. Failure to obtain this first negative reading could invalidate any search-for-life findings.
Throughout its nominal 180-sol mission, the ExoMars Rover will be able to perform 6 Experiment Cycles (each involving the acquisition and analysis of a surface and a subsurface sample at an individual site); and 2 Vertical Surveys (each vertical survey consists of obtaining and analysing five subsurface samples at 50 cm depth increments, from 0 to 200 cm, to study variations with depth at one location).