The study of the terrestrial planets has provided an opportunity to understand the factors controlling the abundance of free oxygen-primarily O₂ and O₃-in abiotic atmospheres. This experience should be able to guide future assessments of detections of free oxygen in extrasolar planetary atmospheres with regard to the relative contributions of biotic and abiotic processes.

Both Venus and Mars have atmospheres dominated by CO₂. In the latter, O₂ has been measured to have a fractional abundance of 10^-3 and O₃ a fractional abundance of 10^-8 (column average). However, in the former, there is only an upper limit to O₂ of 3 x 10^-7.

The observations of O₂ and O₃ in the Martian atmosphere may be reproduced by an atmospheric model with only gas-phase chemistry, surface/atmosphere exchange buffering the level of CO₂ and H₂O, and exospheric escape of atomic H, H₂, and atomic O (Nair et al., 1994). The good agreement between model and measurements was achieved by varying literature-recommendations for rate coefficients within their uncertainties and adopting a high oxygen loss from the atmosphere (to space or to the surface). This study highlights the need for a detailed understanding of kinetic rate coefficients, surface/atmosphere interactions, and planetary escape rates before one can simulate quantitatively atmospheric composition. Accepting that we have a correct model for the processes controlling Martian atmospheric species abundances, we then can use this model to explore the abundance of O₂ in a Mars-like atmosphere, but under different physical conditions. One test run with this model-for a completely dry atmosphere at current pressure and temperatures-yields an O₂ fractional abundance as high as 4 x 10^-2. In this case, the O₂ level is controlled by the slow recombination of O and CO to reform CO₂. Since this reaction has a temperature dependence of exp(-2184 K/T), we would expect a colder atmosphere to have an even higher O₂ level. Other scenarios may lead to high O₂ abundances depending on variations in the relative rates of H and O escape.

The abundance of O₃ in the Martian atmosphere is directly related to the level of O₂. In the current epoch the abundances of free oxygen are regulated by catalytic cycles involving HOx species. Nair et al. (1994) show the derivation of an algebraic expression that describes the O₃/O₂ relationship in terms of kinetic and photolytic rate coefficients, abundances of major and minor species, and the escape flux of O. Consequently, from observations of O₃, along with measurements of physical conditions and other composition variables, one can infer the abundance of O₂.

The Venus atmosphere presents a contrasting situation of a highly oxidized atmosphere with little free oxygen. In current models (Yung and DeMore, 1982; Krasnopolsky and Parshev, 1983), the CO₂ atmosphere is stable and the O₂ level is a balance between CO₂ photolysis and catalytic cycles involving HO_x and ClO_x species, with the relative importance of the different processes still to be confirmed. However, the O₂ abundance in these models is ten times the observational upper limit for this species. Further progress in understanding the level of free oxygen in Venus will require more observations and more laboratory measurements of rate coefficients for potentially important reactions.

From the combined experience of modeling free oxygen in the atmospheres of Mars and Venus, we suggest that the abundance of free oxygen in an abiotic atmosphere may be simulated with reasonable accuracy if key physical and chemical observations and laboratory measurements are available. In addition simple relationships between O₂ and O₃ can be devised. In conclusion, if appropriate observations of an extrasolar planet were to be available, it should be possible to assess whether the detected free oxygen levels were solely controlled by abiotic processes or, in the absence of a good simulation on that basis, whether biotic processes might be actively influencing atmospheric composition.

References:

KRASNOPOLSKY, V. A., AND V. A. PARSHEV 1983. Photochemistry of the Venus atmosphere. In Venus (D. M. Hunten, L. Colin, T. M. Donohue, and V. I. Moroz, Eds.), pp. 431-458, University of Arizona Press, Tucson.

NAIR, H., M. ALLEN, A. D. ANBAR, Y. L. YUNG, AND R. T. CLANCY 1994. A photochemical model of the Martian atmos-phere. Icarus 111, 124-150.

YUNG, Y. L., AND W. B. DEMORE 1982. Photochemistry of the stratosphere of Venus: Implications for atmospheric evolu-tion. Icarus 51, 199-247.