The erosion of planetary and satellite atmospheres by energetic atomic particles

Abstract

Analytic expressions are presented which may be used to compute the sputter‐erosion yield from any unimolecular gravitationally bound gas by any atomic charged particle of any energy. A calculation of solar wind proton and alpha particle induced erosion of the CO 2 atmosphere of Mars predicts molecular sputtering yields S p ≈ 0.029 and S α ≈ 0.21. These numbers are comparable to the results of earlier Monte Carlo simulations. An expression for the emission yield of energetic molecular fragments produced in primary knock‐on events is also given in closed form. Such fragment emission is of secondary importance for mass loss compared to the molecular yield itself. Erosion by radiation belt protons of a hypothetical thin O 2 atmosphere associated with the Jovian satellite Ganymede is considered. Molecular sputtering yields for proton energies at 1 keV and 1 MeV are 0.115 and 6.4×10 −4 , respectively. The yield at the lower energy can have significant effects on atmospheric equilibrium should keV proton fluxes equal or exceed about 10 8 cm −2 s −1 .