Signature of life on exoplanets: Can Darwin produce false positive detections?

Darwin (ESA) and Terrestrial Planet Finder-TPF (NASA) are two projects of space telescopes 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 mum band of O-3 which may be the signature of an O-2-rich atmosphere produced by photosynthetic life forms. In this paper, we point out that O-2, and hence O-3, can also be produced by photochemistry and we investigate the risk of false positive detection of life incurred by these missions. For this purpose, we have developed new photochemical and radiative-convective models of terrestrial planet atmospheres. By modelling the photochemistry of some realistic atmospheres, (including present and past Earth and Mars) we show that O-2-rich atmospheres (up to 5%) and IR absorbing O-3 layers can build up without life from H2O and CO2 photolysis. However, Darwin can still provide a reliable way to detect, through their mid-infrared signatures, ecosystems which have developed oxygenic photosynthesis. Indeed, the two photochemical sources of O-2 are shown to interfere with each other; second, when the CO2 pressure is high enough (>50 mbar) to produce appreciable amounts of O-2 and O-3, it also masks the O-3 feature; and third, the by-products of H2O photolysis destroy O-3. As a result, whereas the unique detection of O-2 remains ambiguous, the simultaneous infrared detection of O-3, CO2 and H2O, provided by Darwin, is established to be a robust way to discriminate photochemical O-2 production from biological photosynthesis: none of the atmospheres modelled exhibits this triple signature, even in the most extreme high risk cases.