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C.F. Chyba (Center for the Study of Life in the Universe, SETI Institute; and Dept. of Geological and Environmental Sciences, Stanford University)
There is no broadly accepted definition of life, although spacecraft searches for extraterrestrial biology implicitly assume some operational definition. Most search strategies rapidly fall back on the idea of "life as we know it" based on a liquid water solvent, a suite of biogenic elements (most famously carbon) and a useful source of free energy. Searches for life within our solar system naturally focus on those objects that hold the prospect of liquid water, but we must also then consider the availability of biogenic elements and free energy sources on these worlds. Mars and Europa represent the most interesting venues, because of strong evidence for liquid water at the present time, but offer special challenges to biology as well. More exotic possibilities, for example life based on a liquid hydrocarbon solvent, will in effect begin to be explored with the Cassini-Huygens mission to Titan. Decades of radio astronomical detection of interstellar molecules have provided an empirical probe into speculations about silicon-based life, as a rich interstellar carbon chemistry has been revealed without comparable silicon analogues. Beyond the solar system, searches for signs of life can be pursued in two ways: detection and spectroscopic observations of extrasolar planets, and targeted or all-sky surveys for artificial electromagnetic signals. The current SETI Institute search, examining the thousand nearest Sun-like stars from 1 to 3 GHz, should soon (in 2004) be expanded through the completion of the Allen Telescope Array (ATA). The ATA will demonstrate great reductions in the cost of constructing large radio telescopes (the ATA will have 1 hectare of collecting area, but be composed of over 350 6-meter dishes), and will permit the one hundred thousand to one million nearest Sun-like stars to be searched from 1 to 10 GHz over a period of about a decade.
This research is supported in part by a Presidential Early Career Award for Scientists and Engineers and the NASA exobiology program.
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