The Blue Dot Workshop: Spectroscopic Search for Life on Extrasolar Planets

Free Oxygen in the Atmospheres of Mars and Venus Mark Allen, Jet Propulsion Laboratory, Califomia Institute Of Technology Yuk L. Yung, California Institute of Technology 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 abiofic processes. Both Venus and Mars have atmos_P3heres dominated by CO2. In the latter, O2 has been measured to have a fractional abundance of 10and 03 a fractional abundance of 10 -8 (column average). However, in the former, there is only an upper limit to 02 of 3 x 10 "7. The observations of 02 and 03 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 H20, and exospheric escape of atomic H, H2, and atomic O (Nalr, 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 02 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 Oa fractional abundance as high as 4 x 10 -2. In this case, the 02 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 02 level. Other scenarios may lead to high 02 abundances depending on variations in the relative rates of H and O escape. The abundance of 03 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 03/02 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 03, along with measurements of physical conditions and other composition variables, one can infer the abundance of 02. 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 HOx and C10× species, with the relative importance of the different processes still to be