O2 concentrations in dense primitive atmospheres: commentary.

Now that astronomers are, I understand, on the verge of detecting extrasolar planets, the question of whether such planets might be inhabited is beginning to be discussed in serious scientific circles. Specifically, astronomers such as Rosenqvist and Chassefiére (see the preceding article) are interested in whether spectroscopic measurements of free O2 in a planet's atmosphere might be used as evidence for life. As such, they have attempted to place constraints on the amount of O2 that might be found in the atmosphere of a lifeless planet or, more specifically, on a planet where oxygenic photosynthesis has not yet been invented. This question can be addressed by photochemical modeling, if one is careful about how one goes about it. The calculations presented here suggest an upper limit of approximately 10 mbar on the O2 partial pressure in a dominantly CO2 atmosphere.