Detection of Past Life Elsewhere in the Solar System

There is no compelling evidence that life, as we know it, presently exists elsewhere in our solar system or beyond. Even Mars, the most obvious extraterrestrial planetary candidate for life, is unlikely to have had surface (or near surface) conditions conducive for life during the past 3 to 4 billion years (e.g. Shuster and Weiss, 2005). Fossil and geochemical data indicate that life may have existed on Earth for as far back in time as the rock record extends (approximately 3.5 Ga). If this is true, then it will not be possible to search the Earth's rock record for evidence of life's origin unless older rocks of appropriate type and thermal history are discovered. Alternatively, this evidence may be sought from meteorites and planetary materials that were formed during the solar system's first 500 million years. Criteria are presented that can be used to determine if life's building blocks (e.g. amino acids) in these ancient materials are relicts of early life or if they were formed by non-biological processes. Additional criteria are presented for distinguishing indigenous organic compounds in ancient rocks from more recent contaminants.

D.L. Shuster and B.P. Weiss (2005) Martian surface paleotemperature from thermochronology of meteorites, Science 309, 594-600.