Self-consistent modelling of Mercury’s exosphere by sputtering, micro-meteorite impact and photon-stimulated desorption

Wurz, P.; Whitby, J.A.; Rohner, U.; Martín-Fernández, J.A.; Lammer, H.; Kolb, C. (2010). Self-consistent modelling of Mercury’s exosphere by sputtering, micro-meteorite impact and photon-stimulated desorption. Planetary and space science, 58(12), pp. 1599-1616. Elsevier 10.1016/j.pss.2010.08.003

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A Monte-Carlo model of exospheres (Wurz and Lammer, 2003) was extended by treating the ion-induced sputtering process, photon-stimulated desorption, and micro-meteorite impact vaporisationquantitatively in a self-consistent way starting with the actual release of particles from the mineralsurface of Mercury. Based on available literature data we established a global model for the surfacemineralogy of Mercury and from that derived the average elemental composition of the surface. Thismodel serves as a tool to estimate densities of species in the exosphere depending on the releasemechanism and the associated physical parameters quantitatively describing the particle release fromthe surface.Our calculation shows that the total contribution to the exospheric density at the Hermean surfaceby solar wind sputtering is about 4 x 107m–3, which is much less than the experimental upper limit ofthe exospheric density of 1012m–3. The total calculated exospheric density from micro-meteoriteimpact vaporisation is about 1.6 x 108m–3, also much less than the observed value. We conclude thatsolar wind sputtering and micro-meteorite impact vaporisation contribute only a small fraction ofMercury’s exosphere, at least close to the surface. Because of the considerably larger scale height ofatoms released via sputtering into the exosphere, sputtered atoms start to dominate the exosphere ataltitudes exceeding around 1000 km, with the exception of some light and abundant species releasedthermally, e.g. H2and He. Because of Mercury’s strong gravitational field not all particles released bysputtering and micro-meteorite impact escape. Over extended time scales this will lead to an alterationof the surface composition.

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