HOW TO EVIDENCE LIFE ON A DISTANT PLANET

Considering the future importance of the search for evidences of primi tive life on a distant planet, we have revisited some points of the O2 and O3 detection criteria. The budget of free oxygen and organic carb on on Earth is studied. If one includes the organic carbon in sediment s, it confirms that O2 is a very reactive gas whose massive presence i n a telluric planet atmosphere implies a continuous production. Its de tection would be a strong indication for photosynthetic activity, prov iding the planet is not in a runaway greenhouse phase. In principle, t he direct detection of O2 could be possible in the visible flux of the planet at 760 nm (oxygen A-band) but it would be extremely difficult, considering the much larger flux from the star. The alternative searc h for the 9.7 mum absorption of O3 may be easier as the contrast with the star is improved by 3 orders of magnitude. A simple atmospheric mo del confirms that the O3 column density is not a linear tracer of the atmospheric O2 content, as was found in the pioneer work by Paetzold ( 1962). However, the detection of a substantial O3 absorption (tau > 30 %) would probably indicate, within the validity of this model, an O2 g round pressure larger than 10 mbar. The question is raised of whether this pressure is sufficient to indicate a photosynthetic origin of the oxygen. If the answer was positive, it would be an even more sensitiv e test of photosynthetic activity than the detection of the oxygen A-b and. Further studies of these points are clearly needed before determi ning an observing strategy.