Searching for Biogeochemical Cycles on Mars

David J. Des Marais
NASA-Ames Research Center
Moffett Field, CA 94035

The search for life on Mars clearly benefits from a rigorous, yet broad, definition of life that compels us to consider all possible lines of evidence for a Martian biosphere. Recent studies in microbial ecology illustrate that the classic definition of life should be expanded beyond the traditional definition of a living cell. The traditional defining characteristics of life are threefold. First, life is capable of metabolism, that is, it performs chemical reactions which utilize energy and also synthesize its cellular constituents. Second, life is capable of self replication. Third, life can evolve in order to adapt to environmental changes. An expanded, ecological definition of life also recognizes that life is a community of organisms which must interact with their nonliving environment through processes called biogeochemical cycles. This regenerative processing maintains, in an aqueous conditions, a dependable supply of nutrients and energy for growth. In turn, life can significantly affect those processes which control the exchange of materials between the atmosphere, ocean and upper crust. Because metabolic processes interact directly with the environment, they can alter their surroundings and thus leave behind evidence of life. For example, organic matter is produced from single carbon atom precursors for the biosynthesis of cellular constituents. This leads to a reservoir of reduced carbon in sediments which, in turn, can affect the oxidation state of the atmosphere. The harvesting of chemical energy for metabolism often employs oxidation-reduction reactions which can alter the chemistry and oxidation state of the redox-sensitive elements carbon, sulfur, nitrogen, iron and manganese.

For example, rates of interconversion between sulfate and sulfide are greatly accelerated by life's energy harvesting processes. Sulfate and sulfide are widely distributed between earth's atmosphere, ocean, crust and mantle. At temperatures exceeding 250° C, sulfides and sulfates exchange readily via thermal processes. However, abiotic exchange processes are very slow below 200° C. Biological processes catalyze this exchange at lower temperatures, thus life dominates the exchange of sulfide and sulfate under the conditions prevailing at earth's surface. At equilibrium, sulfate is Sulphur 34-enriched, relative to sulide. This isotopic difference increases at lower temperatures, therefore biologically-mediated isotopic exchange between sulfate and sulfide is characterized by a large scatter of S34/ S32 values in crustal reservoirs of sulfide and, to a lesser extent, sulfate. Therefore this scatter of S34/ S32 values observed in ancient sedimentary rocks is a legacy of life and constitutes solid evidence for its existence.

The budget of carbon in crustal sedimentary rocks also has been substantially influenced by the biosphere. Approximately 20 % of crustal carbon is stored as organic matter. The size and chemical composition of this organic reservoir has been fixed by the oxidation state of earth's mantle, tectonic processes affecting the crust, and life. If the synthesis and burial of organic carbon had not occurred, substantially less organic matter would have been stored in the crust.

Our concepts as to how evidence of life might be found on Mars is influence strongly by our understanding of the environmental limits for life on Earth. Temperature, liquid water, and the availability of chemical or light energy appear to be crucial parameters. Our understanding of these limits has been extended recently. For example, life in hydrothermal systems extends up to at least 117° C. The discovery of life in ancient aquifers in the Columbia River plain illustrates that ecosystems can thrive on the basaltic weathering reactions in complete isolation from the surface environment. Both of these findings indicate that life could have persisted in the Martian subsurface for perhaps millions to billions of years.

Our understanding, both of life's ultimate capacity for survival and of its impact upon crustal composition, makes our search for a past or present martian biosphere much more effective. Our definition of a "fossil" must be expanded beyond its traditional limits. Microbial fossils can be preserved cellular structures, macroscopic mineral structures built by communities (e.g., "microbial reefs"), organic molecules, minerals whose deposition was biologically controlled, and stable isotopic patterns in elements such as carbon, nitrogen and sulfur. The most convincing proof of fossils consists of multiple lines of evidence derived from several of these fossil types.

Have there ever been biogeochemical cycles on Mars? Certain key planetary processes can offer clues. Active volcanism provides reduced chemical species which biota can use for organic synthesis. Volcanic carbon dioxide and methane can serve as greenhouse gases. Thus the persistence of volcanism on Mars may well have influenced the persistence of a martian biosphere. The geologic processing of the crust can affect the availability of nutrients and also control the deposition of minerals which could have served as a medium for the preservation of fossil information. Finally, the activity of liquid water is crucial to life. Was there ever an earth-like hydrologic cycle with rainfall? Has aqueous activity instead been restricted principally to hydrothermal activity below the surface? To what extent did the inorganic chemistry driven by sunlight and hydrothermal activity influence organic chemistry (prebiotic chemical evolution)? Our efforts to address these and other key questions will benefit greatly from the first samples returned from Mars.

Vugraphs:

VG 1: Biogeochemical cycles on Mars?

VG 2: Biogeochemical cycles

VG 3: Characteristics of Life

VG 4: Metabolism

VG 5: Redox reactions accelerated by life

VG 6: Biochemical sulfur cycle

VG 7: Earth's carbon budget

VG 8: Biogeochemical carbon cycle

VG 9: Range of conditions that sustains life

VG 10: Recent examples extending life's known limits

VG 11: Records of the early biosphere

VG 12: Martian Biogeochemical Cycles?

VG 13: Conditions which could sustain life on Mars

VG 14: Martian Biogeochemical Samples

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