Extreme Thermophiles, Early Biosphere Evolution and the Exploration for Extraterrestrial Life
Jack D. FARMER*
NASA Ames Research Center
Present evidence for the origin of life suggests that it developed quickly, sometime between 3.8 and 4.4 Ga. The higher heat flow, widespread volcanism, and likelihood of giant impacts during the early Archean supports the view that hydrothermal systems were probably widespread over the Earth during the time the biosphere emerged. In addition, RNA-based molecular phylogenies suggest that the last common ancestor of life was a hyperthermophile that utilized sulfur. This indicates the possibility that the earliest life forms on Earth were extremophiles that lived at temperatures in excess of 80℃. And because hydrothermal environments have been shown to be thermodynamically favorable places for the synthesis of prebiotic organic chemicals, we must also entertain the possibility that life may have actually originated at high temperatures.
There are several planetary bodies and/or their moons in our solar system that may have passed through a hydrothermal phase at sometime during their history. Therefore, we cannot rule out the possibility that life may have developed elsewhere in conjunction with such hydrothermal systems, and may yet persist in the deep subsurface environments of planets and/or moons where liquid water could be stable. This simple concept has forced us to broaden our definition of the habitable zone for life to include not only planetary surface environments within the inner solar system where the conditions for liquid water are maintained by solar heating of the atmosphere, but also planetary interiors where zones of subsurface liquid water are maintained by gravitational heating and/or geothermal energy. This simple concept lies at the very heart of our present roadmap for NASA's exobiological exploration of the solar system.
From a paleontological standpoint, hydrothermal systems are important targets for studies of early biosphere evolution because they are usually rapidly mineralizing environments where many microorganisms are entombed by precipitating minerals and fossilized. Around vents and along spring outflows, microbiological information is captured within deposits in a variety of ways, including cellular fossils, biofabrics and biosedimentary structures (e.g. stromatolites). The rapid changes in temperature and pH observed along thermal spring outflows, and the corresponding changes in the microbiotas of these environments, make thermal springs excellent natural laboratories for studying the processes of stromatolite morphogenesis and microbial fossilization over a broad range of conditions. The results obtained from comparative studies of modern and ancient thermal spring deposits provides a useful context when exploring the paleontology of ancient hydrothermal systems on Earth. But such observations also provide a basis for defining strategies to explore for a fossil record in ancient hydrothermal environments on other planets, like Mars. For example, we recently began our search for ancient hydrothermal environments on Mars using Viking data, and clues from the class of meteorites known as the SNC's which have come to us from the Red Planet. Numerous outflood channels, some associated with potential heat sources, suggest many potentially important targets for exopaleontological exploration during the decade-long Mars Global Surveyor Program.
