Habitability on Early Mars and the Search for Biosignatures with the ExoMars Rover

Abstract

The second ExoMars mission will be launched in 2020 to target an ancient location interpreted to have strong potential for past habitability and for preserving physical and chemical biosignatures (as well as abiotic/prebiotic organics). The mission will deliver a lander with instruments for atmospheric and geophysical investigations and a rover tasked with searching for signs of extinct life. The ExoMars rover will be equipped with a drill to collect material from outcrops and at depth down to 2 m. This subsurface sampling capability will provide the best chance yet to gain access to chemical biosignatures. Using the powerful Pasteur payload instruments, the ExoMars science team will conduct a holistic search for traces of life and seek corroborating geological context information. Key Words: Biosignatures-ExoMars-Landing sites-Mars rover-Search for life. Astrobiology 17, 471-510.

FIG. 1.

Sketch of terrestrial planet evolution applied to early Mars and Earth. (A) Very high temperatures developed during accretion. (B) As they cooled down, rocky planets outgassed supercritical atmospheres. (C) Global oceans formed once atmospheric water could condense. (D) Each planet followed a separate path; Mars maintained some surface liquid water through most of the Noachian. A possible window of opportunity for life opened once water temperatures dropped <80°C (indicated by the blue bar on top). We show with a tapering orange bar the onset of (gradual) change toward less habitable conditions. (E) Modern Mars is a very cold, desert-like planet. Subtle white shading represents the relative intensity of meteoritic delivery to the inner Solar System. To maximize our chances of finding signs of past life, we must target the “sweet spot” in Mars' geological history—the early Noachian. (F) The approximate age of the deposits (orange bars) and main targets of interest (superimposed green bars) at the two ExoMars candidate landing sites.

FIG. 2.

Diagram showing plausible Mars habitable environments during the Early- to Middle-Noachian. Some of these settings may have been active long enough to witness the appearance of life (especially in the case of long-term hydrothermal activity); others could have hosted already flourishing microorganisms.

FIG. 3.

ExoMars Biosignature Score: A possible system to assign a confidence value (the score) to a group of robust observations aiming at establishing whether a location hosted life. We have indicated with a gray background the biosignatures that the ExoMars rover payload is not equipped to assess.

FIG. 4.

(Top) Front and rear views of the ExoMars rover with general dimensions (in mm). (Middle) The drill can acquire samples at depths ranging between 0 and 2 m. The drill box lies horizontally across the rover's front face when traveling (A). It is raised (B), rotated counterclockwise (C), and lowered vertically to commence drilling operations (D). Once a sample has been acquired, the drill is elevated (E), turned clockwise (F), and further inclined to deliver the sample (G). The inlet port to the analytical laboratory can be seen on the rover's front, above the drill box, to the left. (Bottom) The rover's locomotion configuration is based on a triple-bogie concept and has flexible wheels to improve tractive performance.

FIG. 5.

Sketch of ExoMars rover showing the location of the drill and the nine Pasteur payload instruments.

FIG. 6.

(Top) Front and rear depictions of the ExoMars rover ALD housing MicrOmega, RLS, MOMA, and the SPDS. (Middle) The UCZ envelops the entire sample-handling path and is sealed at positive pressure until open on Mars. (Bottom) SPDS mechanisms: The sample is deposited in the CSTM and, after being imaged with CLUPI and PanCam, is retracted into the ALD. The rock CS crushes the sample and discharges the resulting particulate matter into a DS. The DS pays out the necessary amount of sample material onto the refillable container or into a MOMA oven, as necessary. ALD, analytical laboratory drawer; CLUPI, close-up imager; CS, crushing station; CSTM, core sample transport mechanism; DS, dosing station; MOMA, Mars organic molecule analyzer; SPDS, sample preparation and distribution system; UCZ, ultra clean zone.

FIG. 7.

Major activities in the rover RSM include six ECs and two VSs. The VSs will be conducted at particularly interesting locations identified during the course of the mission. ECs, experiment cycles; RSM, reference surface mission; VSs, vertical surveys.