Relevance of oxygen and ozone detections in the search for primitive life in extra solar planets
Résumé
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 mu m 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 (tau > 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.