Where and How to Search for Life

The distant past of Mars will shape the exploration of the red planet in the immediate future. The most recent discoveries of the European Mars Express and the American Spirit and Opportunity rovers will reorientate research into past Life towards Mars' oldest surfaces. Meteoritic bombardments and wind processes as liquid flows, probably violent and transitory, have contributed to unmasking hidden layers that can testify to events of ancient times, when liquid water could have remained on Mars. Currently, the scientific community is determining the location of the planned MSL rover landing site, taking into account the astrobiological vocation of the mission. Among the candidate sites, the clays identified by the spectro-imager OMEGA are the favourites. These surfaces could have preserved fossilized traces of a form of elementary life by protecting them over billions of years from a Mars environment that became very hostile.

In 2009, NASA will send this MSL (Mars Science Laboratory) rover to Mars; the aim of the scientific payload of this mission will be, in particular, to search for the presence of extinct or extant traces of life, or prebiotic chemistry that could have existed. It will use a drilling system able to attain depths of some decimeters and consequently able to sample zones where inorganic material might bear witness to epochs when Mars' atmosphere was capable of nurturing the development of life and where organic remnants might have been preserved from destruction.100,101

The SAM experiment onboard the MSL rover is intended for analysis of the martian atmosphere, ground and underground:

a. To determine the elementary composition of recovered samples of ground or underground, in order to compare with the results of other experiments on the Lander and to provide points ofcomparison for objectives (b), (f) and (g) below.

b. To seek, in the ground and underground, organic traces that could bear witness to a past prebiotic chemistry and, in the best cases, an organized life which could have developed at Mars' surface or underground. The chirality of organic molecules will give clues regarding their origin. In parallel with the search for organic molecules, the study of isotopic ratios will bring additional information.

c. To obtain information on atmospheric noble gases (Ar, Xe, Kr): isotopic abundances and ratios, as well as isotopic ratios of carbon (13/12) in CO2, ofnitrogen (15/14) in N2 and of D/H in H2O. This will make it possible to correlate with other observations (on the lander and from the Earth) and to better understand crustal degassing and atmospheric escape in Mars' history.

d. To measure isotopic ratios of carbon, nitrogen, sulphur (34/32), oxygen (16/17/18) and D/H in samples of ground (or underground) after laser ablation or heating, for correlation with measurements performed on Earth and comparison with atmospheric measurements.

e. To search for atmospheric gases (CH4, H2S, etc.) whose origin could be organic and correlate with (b).

f. To obtain information on the presence of hydrates, carbonates, sulphates, clays in samples from ground (or underground); the presence of mineralogical indices of a dense and wet atmosphere that will support the search for organic traces in the ground (item b). One can also differentiate minerals from biominerals by simplified differential thermal analysis.

g. To obtain, ifthe drilling system allows sampling at different depths, a stratigraphy not only for items (a), (b), (d) and (f) above, but also of gases adsorbed on the solid phase (H2O, CO2, H2O2, etc).

Figure 4.10. Image of the first meteorite identified at the surface of Mars, i.e., on the surface of a planet other than the Earth. This meteorite was located by the panoramic camera of the Opportunity rover close to its heat shield. It was named for this reason "Heat Shield Rock". Its chemical composition of iron and nickel is very close to the iron meteorites found on the Earth. (Credit: NASA/ JPL/Cornell).

Figure 4.10. Image of the first meteorite identified at the surface of Mars, i.e., on the surface of a planet other than the Earth. This meteorite was located by the panoramic camera of the Opportunity rover close to its heat shield. It was named for this reason "Heat Shield Rock". Its chemical composition of iron and nickel is very close to the iron meteorites found on the Earth. (Credit: NASA/ JPL/Cornell).

Figure 4.11. View of MSL 2009 (Credit: NASA MSL web site).
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