Abstract
All the planets may have been surrounded originally by atmospheres dominated by hydrogen or one of its compounds. While only the major planets have retained much of their initial atmospheres, the terrestrial planets appear to have lost these gases. Calculations of exospheric temperatures for these smaller planets preclude loss of hydrogen by the selective process of thermal escape, since exceedingly high values are found for these planets. Instead, a general outflow or “planetary wind” is expected to deplete rapidly the thermosphere of all constituents. Low values are obtained for the major planets and their retention of hydrogen is understandable.
Expressions are given for the exospheric temperature of a two-component diffusive equilibrium model which consists of a lighter molecule dominating the thermosphere and absorbing solar EUV, and a heavier constituent providing radiative cooling in the vicinity of the mesopause. The model approximates well the assumed atmospheres, but the resulting expressions may have wider application.
The dynamics of an expanding atmosphere are discussed. A three-dimensional outflow against the back pressure of the solar wind may be configured like a comet's tail.
