Biology
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.b45a..01d&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #B45A-01
Biology
0406 Astrobiology And Extraterrestrial Materials, 0414 Biogeochemical Cycles, Processes, And Modeling (0412, 0793, 1615, 4805, 0424 Biosignatures And Proxies, 0448 Geomicrobiology, 0456 Life In Extreme Environments
Scientific paper
The exploration of Mars will enhance the biogeosciences by deepening our understanding of life, planetary environments, and their long-term evolution. Following a decline in geologic activity on Mars, potentially habitable environments have retreated from near-surface locales in the ancient crust to the deep subsurface today. Geologic activity on Earth has largely obliterated its earliest record of environments and life. Thus we don't know whether life required 700 years or 700 million years to begin. Studies of the meteorite ALH84001 revealed that the Martian crust has probably preserved, at the nm scale, a 4.4 billion year record of near- surface environments and processes. Coordinated studies of Earth and Mars might therefore capture a broader history of life in the inner solar system. To determine how life can begin is to understand how planetary and biological processes interact at the most fundamental level. Exploring another inhabited planet should help to discriminate between universal characteristics of life versus features that represent "merely" local solutions to surviving Earth's unique environmental history. An extant subsurface Martian biosphere would provide another dramatically different example. The search for life begins with a search for environments that might have supported it. A habitable environment provides sources of energy for metabolism, chemical building blocks for cells, and conditions that can sustain liquid water. Mars exploration must therefore "follow the water, follow the energy, and follow the carbon and other nutrients." Missions have made a solid start. For example, the Mars Exploration Rovers (MER) found evidence of ancient habitable environments in late Noachian-early Hesperian rocks. Opportunity found evidence of sulfate-rich playa lakes and persistent saline groundwater that could have sustained certain acidophilic Earthly microorganisms. Spirit documented several types of volcanic rocks that been pervasively altered and oxidized by aqueous fluids, including some that had transported ferric and magnesium sulfates. But it remains unknown whether such brines were sufficiently fresh and pervasive to sustain an ancient Martian biosphere. The OMEGA instrument on ESA's Mars Express mission found hydrated phyllosilicates in Noachian crust that is probably older than the bedrock at the MER sites and that might have preserved a record of more favorable environments. Candidate environments include playa lakes, near-surface aquifers, hydrothermal systems and springs. Astrobiological exploration is founded on the principle that signatures of life (biosignatures) that are encountered elsewhere will be recognizable. Studies of ancient biosignatures on Earth provide a key first step. For example, ancient stromatolites reveal that microbial biofilms were an early, highly successful lifestyle. NASA-supported studies of biofilms have revealed several secrets to their successes and also how they can be fossilized. Such studies of biofilms guide our search for potential biosignatures on Mars. Should ancient biosignatures be found, the excellent preservation of the ancient Martian crust promises to provide a deeper understanding of the origins and early evolution of life.
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