2009 Annual Science Report
Massachusetts Institute of Technology Reporting | JUL 2008 – AUG 2009
Astrobiological Exploration of Mars
NASA spacecraft have discovered both chemical and physical evidence that liquid water once flowed on the martian surface. Close examination of the images and spectroscopic data from these spacecraft, and understanding what they tell us, are critical to selecting the best sites for future rover missions. This project aims to maximise the knowledge gained from orbiting and landed spacecraft and apply it effectively in future planning and execution of new missions.
The key questions for astrobiology are not so much “was water present?” as “what were its properties?” and “How long did it persist?” Using thermodynamic calculations, one can approach both questions, using mineral identifications made by the MER rovers and CRISM. We find that waters at Gusev and Meridiani planum grew extremely salty as evaporation proceeded, reaching conditions that would challenge known life on Earth. We also learn that in a number of places on the martian surface, minerals deposited billions of years ago as a result of water-rock interactions have seen little or no water since that time.
Our group has substantial involvement with on-going and future Mars missions. John Grotzinger is the Project Scientist for Mars Science Laboratory mission (MSL), a Participating Scientist for the Mars Exploration Rover mission (MER), and a Participating Scientist for the Mars Reconnaissance Orbiter (MRO) mission (HiRISE instrument). Andrew Knoll is a co-Investigator on the MER mission. Roger Summons has served as a Review Board member for development of the SAM instrument on MSL. Grotzinger and Knoll are both members of the Jet Propulsion Laboratory Advisory Council.
Our collective involvement in these ongoing and future missions insures cross-fertilization between NAI-directed research and mission planning/development, and strategic/tactical landed surface operations. The MSL mission is particularly important in the context of our NAI objectives, and the lessons learned from our NAI research will no doubt feed forward into the planning of this exciting Astrobiology mission. As an example of this, Knoll and Summons currently participate in the MSL Biosignature/Carbon Compound Preservation Working group (Summons is Chair) that was chartered by the MSL Project and NASA HQ. The core objective of the MSL mission is the elucidation of both early Martian surface environments, particularly those which may have been habitable by microorganisms, and early Martian surface processes, particularly those which may have been favorable for the preservation of microbial biosignatures and/or abiotic organic compounds. The pursuit of this goal is what will distinguish MSL from all previous missions, and it provides a well-defined pathway for the focus of landed surface operations. The objective of the Biosignature/Carbon Compound Preservation Working group is to assess the potential diversity of “taphonomic windows” that may be recorded in the ancient terrains that MSL will be exploring. The synergy between the search for taphonomic windows on the MSL mission and our NAI research which in part explores how organisms and other biosignatures become preserved in Earth’s early rock record is clear.
Minerals precipitated from solution preserve a record of the fluid’s water activity at the time of deposition, if we know the ionic composition of the solution. The chemical composition of brines present on the ancient mars surface can be estimated from experimental and theoretical studies of basalt dissolution, as well as by empirical data from the MER rovers. Using these estimates, thermodynamic calculations suggest that however dilute Meridiani and Gusev brines may have been at formation, they matured to become extremely salty. Indeed, calculated water activities for slats at these locations would challenge essentially all known life forms on Earth.
We have also been able to use thermodynamic calculations to investigate the duration of water at the martian surface. Chemical sediments and the aqueous alteration products of volcanic rocks clearly indicate the presence of water, at least episodically, at the Martian surface. Compared to similar materials formed on the early Earth, however, Martian deposits are juvenile, or diagenetically under-developed. Using kinetic formulations based on terrestrial sedimentary geology, we quantified the integrated effects of time and temperature for a range of possible burial and thermal histories of precipitated minerals on Mars. From this, we estimated thresholds beyond which these precipitates should have been converted to the point of non-detection in the presence of water. Surface water has been shown to be at least episodically present in recent times. Nonetheless, the integrated duration of aqueous activity recorded over geologically long intervals by hydrated amorphous silica, smectite clays and Fe-sulfate minerals suggests that where these minerals occur water did not persist much beyond their initial deposition. This geochemical conclusion converges with geomorphologic studies that suggest water limitation during the late Noachian–Hesperian peak of valley formation and a still more limited footprint of water since that time.
PROJECT INVESTIGATORS:John Grotzinger
Project InvestigatorAndrew Knoll
Project InvestigatorRoger Summons
PROJECT MEMBERS:Nicholas Tosca
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.