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 CO2 atmosphere of Mars predicts molecular sputtering yields S•, • 0.029 and S,• • 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 02 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 x 10 -4, respectively. The yield at the lower energy can have significant effects on atmospheric equilibrium should keV proton fluxes equal or exceed about 108 cm -2 s -l.

Interaction of the Jovian magnetosphere with Europa: Constraints on the neutral atmosphere

“…Consequently, we compare our method with a different approach [Haft and Watson, 1979] (17)). This independent calculation confirms that our simplified approach for atmospheric sputtering yields satisfactory results.…”

Monte Carlo calculations of plasma ion‐induced sputtering of an atmosphere: SO2 ejected from Io

“…Therefore that model is in fact more appropriate for atmospheric sputtering if the planar surface binding is replaced by gravitational (spherical) binding and if the correct cross sections are used. Such models, first applied to atmospheres by Haft and Watson [1979], have been described in detail [Johnson, 1990[Johnson, , 1992[Johnson, , 1994a Johnson, 1990Johnson, , 1994a]. If the incident ion is fast and the ion-particle collision cross section is small, then it has also become customary to write the energy deposition in the surface as Ft,(o) • [a FlxSn/ (cos0.q) p] [Sigmund, 1981], where Sn is the stopping cross section for a single collision described above and the quantities p and a are determined from a transport equation, which accounts for the anisotropy of the energy transfer events [Sigmund, 1969 The linear dependence of the model yield on tr•..q is shown in Fig.…”

“…Here we describe the multiple-collision loss of SO2 and ignore dissociation. This set of calculations is done to provide a clearer picture of multiple-collision loss, allowing us to test the analytic models which have been developed from transport theory and applied to atmospheric sputtering [Haft and Watson, 1979;Johnson, 1990Johnson, , 1994. A more complete calculation involving a multicomponent atmosphere (SO2 plus fragments) is in progress.…”

Sputter ejection of matter from Io

“…Other ionic species will also be ejected from the atmosphere Our estimates for the atmospheric case are based on the anal-by sputtering, roughly in proportion to their abundance. Igysis developed by Haftand Watson [1979], applied for illustra-noting the effects of diffusive separation, which will lead to tive purposes to a 100% SO2 atmosphere. These authors show enhanced sputter emission of lighter molecules [ Watson et al, that the atmospheric sputtering yield is inversely proportional 1980a, b], we find that if Na and K are present in the form of to U, the gravitational binding energy of the molecule at the the sulfides Na2S and K2S, then YNa2s ~ 0.04, and Y•t2s ~ 0.03 exobase.…”