Absorption lines from molecules such as methane should be very easy to detect. Water vapor would come next. So, I would have to say that absorption line features would probably be the first used. Molecules such as ozone which require biogenic activity, are also absorption lines in the ultraviolet, so again I would have to rate absorption lines as the most useful for a first reconisance of a planet's chemistry.
Copyright 1997 Dr. Sten Odenwald
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