Ganymede's surface is subject to constant bombardment by Jovian magnetospheric and Ganymede's ionospheric ions. These populations sputter the surface and contribute to the replenishment of the moon's exosphere. Thus far, estimates for sputtering on the moon's surface have included only the contribution from Jovian ions. In this work, we have used our recent model of Ganymede's ionosphere Carnielli et al., 2019 to evaluate the contribution of ionospheric ions for the first time. In addition, we have made new estimates for the contribution from Jovian ions, including both thermal and energetic components. For Jovian ions, we find a total sputtering rate of 2.2 x 10(27) s(-1), typically an order of magnitude higher compared to previous estimates. For ionospheric ions, produced through photo- and electron-impact ionization, we find values in the range 2.7 x 10(26)-5.2 x 10(27) s(-1) when the moon is located above the Jovian plasma sheet. Hence, Ganymede's ionospheric ions provide a contribution of at least 10% to the sputtering rate, and under certain conditions they dominate the process. This finding indicates that the ionospheric population is an important source to consider in the context of exospheric models.

Low energy secondary ions ejected by the solar wind are an important component of tenuous exospheres surrounding airless bodies, since these ions carry information on the planetary surface composition. In this work we examine the dependence of secondary-ion abundance, as a function of energy and mass, on surface composition. The surface compositions of two Apollo soils (10084 and 62231) and a synthetic Corning glass lunar simulant were measured with X-ray photoelectron spectroscopy and correlated with the spectra of secondary-ions ejected from the same soils by 4 key He ions. XPS spectra for lunar soils show that the surface compositions are similar to the bulk, but enriched in Fe and 0, while depleted in Mg and Ca. 4 keV He irradiation on the lunar soils and a glass simulant preferentially removes 0 and Si, enriching the surface in Al, Ti, Mg, and Ca. Secondary-ion species ejected from the Apollo soils by 4 keV He include: Na+, Mg+, Al+, Si+, Ca+, Ca++, Ti+, Fe+, and molecular species: NaO+, MgO+ and SiO+. Secondary ion energy distributions for lunar soil 10084 and 62231 rise rapidly, reach a maxima at similar to 5 eV for molecular ions and Na+, similar to 7.5 eV for Fe+, and similar to 10 eV for Mg+, Al+, Si+, Ca+ and Ti+, then decrease slowly with energy. We present species-dependent relative conversion factors for the derivation of atomic surface composition from secondary-ion count rates for 4 keV He irradiation of lunar soils 10084 and 62231, as well as the Corning glass lunar simulant. (c) 2014 Elsevier Inc. All rights reserved.