Relevance of oxygen and ozone detections in the search for primitive life in extra solar planets

Abstract

Considering the future importance of the search for evidences of primitive life on a distant planet, we have revisited some points of the O 2 and O 3 detection criteria. The budget of free oxygen and organic carbon on Earth is studied. If one includes the organic carbon in sediments, it confirms that O 2 is a very reactive gas whose massive presence in a telluric planet atmosphere implies a continuous production. Its detection would be a strong indication for photosynthetic activity, provides the planet is not in a runaway green-house phase. In principle, the direct detection of O 2 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 search for the 9.7 μm absorption of O 3 may be easier as the contrast with the star is increased by 3 orders of magnitude. A simple atmospheric model indicates that the O 3 column density is not a linear tracer of the atmospheric O 2 content. However, the detection of a substantial O 3 absorption ( τ > 25%) would indicate, within the validity of this model, a O 2 ground 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 sensitive test of photosynthetic activity than the detection of the oxygen A-band. Further studies of these points are clearly needed before determining an observation strategy.