Hydrogen Escape in the Terrestrial Atmosphere at Low Oxygen Levels: A Photochemical Model

Abstract

It has already been shown that a decrease of the oxygen level in the atmosphere of the earth would lower significantly the temperature of the exosphere, thus limiting the escape of hydrogen. The reduction of the escape flux would have important consequences on the evolution of oxygen in the last two billion years if O2 comes entirely from photodissociated water. The evaluation of the escape flux is possible only if the mixing ratios of atomic and molecular hydrogen are known at the homopause. In this paper a detailed photo-chemical model including transport by eddy and molecular diffusion is developed for an oxygen level of 0.1 PAL (preexisting atmospheric level). Jeans flux is assumed to be the dominant escape mechanism. The exospheric temperatures used in the model are 500 and 1000 K representing the expected range of variability at 0.1 PAL. It is shown that for stratospheric mixing ratios of total hydrogen comparable to those observed in our epoch, the escape flux is reduced to about 5 ? 106 cm?2 s?1 for 5 ppm and 500 K and to 2.3 ? 108 for 10 ppm and 1000 K. For comparison results are presented at 1 PAL for exospheric temperatures of 500 and 1000 K. As a by-product ozone densities resulting from such models are also presented taking into account both odd hydrogen chemistry and other possible sinks. Within the assumptions of low exospheric temperature and stratospheric humidity, it is argued that the role of water photodissociation as one of the major sources of atmospheric oxygen might have been overestimated.