Impact-driven supply of sodium and potassium to the atmosphere of Mercury

Abstract

Vaporization due to meteoritic impact on Mercury supplies sodium atoms to the atmosphere from both the impacting meteoroids and the regolith. The supply rate of sodium depends on the spatial mass density, velocity distribution, and sodium abundance for the meteoritic material, as well as the elemental abundance of sodium in the regolith. In the most extreme case (which includes no sodium in the regolith), the average supply rate of sodium must be at least 0.15 × 10 23 atoms sec −1, and the supply rate could easily approach 1.4 × 10 24 atoms sec −1 (moderate amounts of sodium in the regolith). The production of vaporized sodium due to meteoritic impact varies with the instantaneous distance of Mercury from the Sun, and the form of this dependence differs from that characteristic of source and sink processes, which are driven by solar radiation. The spatial distribution of sodium production across the planet due to impact-driven vaporization should also be different from the spatial distribution of sodium production due to solar radiation-driven processes, and these differences may provide an observational test of the relative importance of the meteoritic impact-driven processes to the supply of sodium to the atmosphere of Mercury. Impact-driven vaporization will produce the Na/K ratio observed in the atmosphere only if both the meteoroids and the regolith of the planet are deficient in potassium relative to other sampled objects in the Solar System, with the exception of comets.