Biosignatures of early earths

A major goal of NASA's Origins Program is to find habitable planets around other stars and determine which might harbor life. Determining whether or not an extrasolar planet harbors life requires an understanding of what spectral features (i.e., biosignatures) might result from life's presence. Consideration of potential biosignatures has tended to focus,on spectral features of gases in Earth's modem atmosphere, particularly ozone, the photolytic, product of biogenically produced molecular oxygen. But life existed on Earth for about 11/2 billion years before the-buildup of atmospheric oxygen. Inferred characteristics of Earth's earliest biosphere and studies of modem microbial ecosystems that share some of those characteristics suggest that organosulfur compounds, particularly methanethiol (CH3SH the sulfur analog of methanol), may have been biogenic products on early Earth. Similar production could take place on extrasolar Earth-like planets whose biota share functional chemical characteristics with Earth life. Since methanethiol and related organosulfur compounds (as Well as carbon dioxide) a absorb at wavelengths near or overlapping the 9.6-mum band of ozone, there is potential ambig ity in interpreting a feature around this wavelength in an extrasolar planet spectrum.