Photochemistry of CO2 in the atmosphere of Mars

TLDR: In this article, the photochemistry of a carbon dioxide atmosphere is discussed in detail, and the analysis is applied to Mars, where a recombination scheme involving CO3 is proposed and is consistent with the O2 upper limit and abundance.

Abstract: The photochemistry of a carbon dioxide atmosphere is discussed in detail, and the analysis is applied to Mars. Recombination of O(³P) and CO is not sufficiently rapid to prevent the buildup of an amount of O2 in excess of the spectroscopic upper limit derived by Belton and Hunten (1968). A recombination scheme involving CO3 is proposed and is shown to be consistent with the O2 upper limit and abundance. CO3 is formed by a 2-body reaction of O(¹D) with CO2, and recombination occurs in a 2-body reaction of CO3 with CO. Ozone formation in a 3-body reaction of O and O2 is important and serves as an important mechanism for conversion of O(³P) into O(¹D), since O(¹D) is a product of O3 photolysis. Attention is directed toward the importance of CO2 absorption in the wavelength range 1670 to about 2000 A. This process is a dominant source of O below about 40 km on Mars. The abundance of O2 in the CO3 scheme is set by the requirement that the total dissociation rate of O2, mainly by radiation in the Herzberg continuum, should balance the formation rate of O2 in the upper atmosphere. This condition provides a simple explanation for the similarity between total abundances of CO observed on Venus and Mars. Some aspects of CO2 photochemistry with traces of water vapor are briefly discussed. The presence of species such as OH and HO2, produced in H2O photochemistry, can lead to very efficient catalytic action and consequent rapid recombination of O and CO. The precise role of hydrogen cannot be quantitatively assessed at present. Observations of resonance emissions from O and H should be valuable.