Release of neutral sodium atoms from the surface of Mercury induced by meteoroid impacts

Meteoroid impact has been shown to be a source of sodium, and most likely of other elements, on the Moon. The same process could be also relevant for Mercury. In this work we calculate the vapor and neutral Na production rates on Mercury due to the impacts of meteoroids in the radius range of 10−8-10−1 m. We limit our calculations to this size range, because meteoroids with radius larger than 10−1 m have not to be found important for the daily production of the exosphere. This work is based on a new dynamical model of the meteoroid flux at the heliocentric distance of Mercury, regarding objects in the size range 10−2-10−1 m. This size range, never investigated before, is not affected by nongravitational forces, such as the Poynting-Robertson effect, which is dominant for particles smaller than 10−2 m. In order to evaluate the release of neutral sodium atoms also for smaller meteoroids we have used the distribution reported by M.J. Cintala [1992. Impact-induced thermal effects in the lunar and mercurian regoliths. J. Geophys. Res. 97, 947-973] calculated for particle size range 10−8-10−3 m. We have extrapolated this distribution up to 10−2 m and we have based the impact calculations on a new surface composition assuming 90% plagioclase and 10% pyroxene. The results of our model are that (i) the total mass of vapor produced by the impact of meteoroids in the size range 10−8-10−1 m is 4.752×108g per year, and (ii) the production rate of neutral sodium atoms is 1.5×1022 s−1.