Dayside pickup oxygen ion precipitation at Venus and Mars: Spatial distributions, energy deposition and consequences

TLDR

The authors modeled dayside pickup O⁺ ion fluxes and energy spectra for Venus and Mars by launching cold ions from hemispherical grids, tracing their trajectories through magnetosheath fields, and weighting impacts by exosphere density and photoion production rates. The resulting maps show an asymmetric dayside energy deposition pattern, negligible heating, but production of high‑energy neutral oxygen atoms that can escape, implying that pickup ion precipitation may contribute significantly to long‑term atmospheric loss.

Abstract

The fluxes and energy spectra of picked‐up planetary O+ ions incident on the dayside atmospheres of Venus and Mars are calculated using the neutral exosphere models of Nagy and Cravens (1988) and the Spreiter and Stahara (1980) gasdynamic model of the magnetosheath electric and magnetic field. Cold (∼10 eV) O + ions are launched from hemispherical grids of starting points covering the daysides of the planets and their trajectories are followed until they either impact the dayside “obstacle” or cross the terminator plane. The impacting, or precipitating, ion fluxes are weighted according to the altitude of the hemispherical starting point grid in a manner consistent with the exosphere density models and the local photoion production rate. Maps of precipitating ion number flux and energy flux show the asymmetrical distribution of dayside energy deposition expected from this source which is unique to the weakly magnetized planets. Although the associated heating of the atmosphere and ionosphere is found to be negligible compared to that from the usual sources, backscattered or sputtered neutral oxygen atoms are produced at energies exceeding that needed for escape from the gravitational fields of both planets. These neutral “winds,” driven by pickup ion precipitation, represent a possibly significant loss of atmospheric constituents over the age of the solar system.