SEARCH FOR PRIMITIVE LIFE ON A DISTANT PLANET - RELEVANCE OF O2-DETECTION AND O3-DETECTION

Considering the future importance of the search for evidence of primit ive life on a distant planet, we have revisited some points of the O2 and O3 detection criteria. The budget of free oxygen and organic carbo n on Earth is studied. If one includes the organic carbon in sediments , it confirms that O2 is a very reactive gas whose massive presence in a telluric planet atmosphere implies continuous production. Its detec tion would be a strong indication for photosynthetic activity, provide d the planet is not in a runaway greenhouse phase. In principle, the d irect detection of O2 could be possible in the visible flux of the pla net at 760 nm (oxygen A-band) but it would be extremely difficult, con sidering the much larger flux from the star. The alternative search fo r the 9.7 mum absorption of O3 may be easier as the contrast with the star is improved by three orders of magnitude. An atmospheric model co nfirms that the O3 column density is not a linear tracer of the atmosp heric O2 content as was found in the pioneer work by Paetzold (1962). However, the detection of a substantial O3 absorption (tau > 25%) woul d indicate, within the validity of this model, a O2 ground pressure la rger than 10 mbar. The question is raised of whether this pressure is sufficient to indicate a photosynthetic origin of the oxygen. If the a nswer was positive, it would be an even more sensitive test of photosy nthetic activity than the detection of the oxygen A-band. Further stud ies of these points are clearly needed before determining an observing strategy.