Selective Escape of Gases

Abstract

Formulae for gas‐kinetic escape of neutral and ionized gas from celestial bodies are set up, free from usual simplifications and applicable to the highest rates. It is shown that the isothermal model with a constant escape lifetime becomes physically meaningless when the atmospheric mass exceeds a certain, quite modest, limit and the exospheric base disappears; this defines an overall upper limit to the rate of gas‐kinetic escape from a body of given mass; the limit is rather low, by cosmic standards. Absolute upper limits to selective escape are calculated. A solar nebula could not have lost by escape more than 10–13–10–17 of Jupiter's mass in hydrogen, nor could Jupiter's protoplanet have lost very much more. Escape to space cannot account for the deficiency of hydrogen in the atmospheres of the outer planets; other possible processes are considered; snowing‐out of hydrogen in a rotating flattened solar nebula may account for the separation of hydrogen from helium. For the Earth, the upper limit of loss of hydrogen equals one‐third of the water equivalent of the oceans, but the actual loss as determined by the oxidation of the crust, may equal only 14 per cent of the oceans. The escape from Earth of an equivalent amount of hydrogen would require conditions very different from those prevailing now; a suggestion is made to this effect, regarding possible intense volcanic and plutonic activity on Earth during the first undated 1.5 × 10 9 years of the Earth's history. For Venus, a probable exospheric temperature of about 7 000 °K is estimated; without a cold trap, hydrogen may have escaped freely. However, for a crust similar to the terrestrial, the amount of oxygen bound in oxidation can hardly be more than corresponding to 400–800 m of water; the disappearance of water from Venus would thus indicate a several times smaller initial store than on Earth. Residual oxygen in the atmosphere could have disappeared through ionic escape. At 7 000 °K the escape of O+ during 4.5 × 109 years could account for the disappearance of ten times the terrestrial amount of free O 2 even when inhibited by a magnetic field. Copyright © 1963, Wiley Blackwell. All rights reserved