Synthetic spectra of simulated terrestrial atmospheres containing possiblebiomarker gases

NASA's proposed Terrestrial Planet Finder, a space-based interferometer, wi ll eventually allow spectroscopic analyses of the atmospheres of extrasolar planets. Such analyses would provide information about the existence of li fe on these planets. One strategy in the search for life is to look for evi dence of O-3 (and hence O-2) in a planet's atmosphere; another is to look f or gases that might be present in an atmosphere analogous to that of the in habited early Earth. In order to investigate these possibilities, we have c alculated synthetic spectra for several hypothetical terrestrial-type atmos pheres. The model atmospheres represent four different scenarios. The first two, representing inhabited terrestrial planets, are an Earth-like atmosph ere containing variable amounts of oxygen and an early Earth-type atmospher e containing methane. In addition, two cases representing Mars-like and ear ly Venus-like atmospheres were evaluated, to provide possible "false positi ve" spectra. The calculated spectra suggest that ozone could be detected by an instrument like Terrestrial Planet Finder if the O-2 concentration in t he planet's atmosphere is greater than or equal to 200 ppm, or 10(-3) times the present atmospheric level. Methane should be observable on an early-Ea rth type planet if it is present in concentrations of 100 ppm or more. Meth ane has both biogenic and abiogenic sources, but concentrations exceeding 1 000 ppm, or 0.1% by volume, would be difficult to produce from abiogenic so urces alone. High methane concentrations in a planet's atmosphere are there fore another potential indicator for extraterrestrial life. (C) 2000 Academ ic Press.