In the search for life on Earth-like planets around other stars, the first (and likely only) information will come from the spectroscopic characterization of the planet's atmosphere. Of the countless number of chemical species terrestrial life produces, only a few have the distinct spectral features and the necessary atmospheric abundance to be detectable. The easiest of these species to observe in Earth's atmosphere is O2 (and its photochemical byproduct, O3). However, O2 can also be produced abiotically by photolysis of CO2, followed by recombination of O atoms with each other. CO is produced in stoichiometric proportions. Whether O2 and CO can accumulate to appreciable concentrations depends on the ratio of far-ultraviolet (FUV) to near-ultraviolet (NUV) radiation coming from the planet's parent star and on what happens to these gases when they dissolve in a planet's oceans. Using a one-dimensional photochemical model, we demonstrate that O2 derived from CO2 photolysis should not accumulate to measurable concentrations on planets around F- and G-type stars. K-star, and especially M-star planets, however, may build up O2 because of the low NUV flux from their parent stars, in agreement with some previous studies. On such planets, a "false positive" for life is possible if recombination of dissolved CO and O2 in the oceans is slow and if other O2 sinks (e.g., reduced volcanic gases or dissolved ferrous iron) are small. O3, on the other hand, could be detectable at UV wavelengths (λ < 300 nm) for a much broader range of boundary conditions and stellar types.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science