The Escape of Light Gases from Planetary Atmospheres

Abstract

The Jeans-Spitzer treatment of atmospheric escape by evaporation must be supplemented, for a mixture of gases, by a discussion of the mutual diffusion and mixing of the components. A recent description for the atmosphere of Titan is here applied to other objects. A range of conditions is found (corresponding to easy escape) in which the escape flux is determined by diffusion, and a simple expression is given for the flux. The state of hydrodynamic blowoff of the light gas is included, and is found to be orderly, not ?chaotic? normally, the heavier gases are not carried along. For the present Earth, the flux of hydrogen is obtained within a factor of 2 from the H2O mixing ratio in the stratosphere. The Spitzer description applies to helium: the loss rate is determined by the exospheric temperature, not by diffusion. However, diffusion-limited flow could prevail during occasional periods at 2000K; if most of the helium loss is at such times, equal time constants for He4 and He3 would follow automatically. An existing model of Mars is well described, but problems are found with Venus. For a primitive Earth rich in hydrogeneous compounds, the excess hydrogen could readily be lost in a time of 109 years, and a large amount of free oxygen produced. For example, with an atmospheric H2 mixing ratio of 0.4% by volume, the escape rate is such as to liberate the oxygen from 750 m of liquid water, enough to provide the O2 in the atmosphere and the CO2 and Fe2O3 in the crust. Illustrative models are given of such primitive atmospheres, some of which show hydrodynamic blowoff of H2.