The regolith of Mercury: present knowledge and implications for the Mercury Orbiter mission

Mercury is the largest of all planetary bodies for which the evolution of the surface has been driven primarily by external causes during the last few billion years. The trademark of such an evolution is a surface saturated with small craters and covered by a regolith. The most extensively studied example is the Moon. The crater formation process on Mercury should be very similar to the lunar case. On such large planetary bodies, gravity plays a major role in crater formation, contrary to small bodies such as Phobos, Deimos or asteroids. Furthermore, impact rates of large meteoroids may have been similar on the Moon and Mercury. There are however major differences in the environment of these two bodies which should have an effect on the evolution of the regolith: the ratio of micrometeoroids (mainly originating from comets) to meteoroids (which are transferred in chaotic corridors from the main belt of asteroids) is likely to be much higher than on the Moon; impact energies are larger for cometary material than for asteroidal material; the flux of the solar wind increases by a factor of up to 10 on Mercury although the relative importance of magnetospheric screening and of magnetospheric particles is difficult to evaluate; Solar Cosmic Rays which result in the formation of particle tracks also increase by a factor of up to 10 when compared to the Moon. However, surface temperatures reach 700 K, which can result over millions of years in the annealing of irradiation effects. Overall, the regolith of Mercury is expected to be significantly more mature than the lunar regolith, with smaller grain sizes and larger proportions of glassy particles. A possible exception could be radiation damage due to annealing. This has implications on the interpretation of remote sensing investigations during an orbiter mission, such as the lighter albedo of plains material on Mercury when compared to the maria of the Moon, the search for ray craters or the relationship of near-infrared spectra to the mineralogical composition.