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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 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.
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Submitted on : Friday, June 5, 2020 - 2:09:15 PM
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A. Léger, Michel Pirre, F.J. Marceau. Relevance of oxygen and ozone detections in the search for primitive life in extra solar planets. Symposium E4 of the COSPAR 29th Plenary Meeting on Astronomy and Space Science from the Moon, Aug 1992, Washington, United States. pp.117-122, ⟨10.1016/0273-1177(94)90016-7⟩. ⟨insu-02796859⟩

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