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J. Kieken, F. Selsis, D. Despois, F. Billebaud, M. Dobrijevic, J.P. Parisot (Bordeaux Observatory), Bordeaux Observatory Team
Darwin (ESA) and Terrestrial Planet Finder-TPF (NASA) are two projects of space interferometers aiming at the detection of extra-solar terrestrial planets and some of their atmospheric components. In particular, they will be sensitive to the 9.6 microns band of O3 which may be the signature of an O2-rich atmosphere produced by photosynthetic life forms. We point out that O2, and hence O3, can also be produced by photochemistry and investigate the risk of "false positive" detection of life incurred by these missions. We have developed a numerical model for the simulation of chemical and thermal evolution of terrestrial planet atmospheres, which also computes the thermal emission of the planet. Using this code for a large range of realistic atmospheres (including present and past Earth and Mars), we show that O2-rich atmospheres (up to 5 %) and IR absorbing O3 layers can build up without life from H2O and CO2 photolysis. However the two photochemical sources of O2 interfere with each other, and even when appreciable amounts of abiotic O2 are reached, the O3 feature is masked at CO2 pressure higher than 50 mbar, and the by-products of H2O photolysis destroy O3. As a result, whereas the unique detection of O2 remains ambiguous, the simultaneous infrared detection of O2, CO2 and H2O, provided by TPF and Darwin, is established to be a robust way to discriminate photochemical O2 production from biological photosynthesis: none of the atmospheres modelled exhibits this "triple signature" feature, even in the most extreme "high risk" cases.
The author(s) of this abstract have provided an email address for comments about the abstract: kieken@observ.u-bordeaux.fr