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 th e last few billion years. The trademark of such an evolution is a surf ace saturated with small craters and covered by a regolith. The most e xtensively studied example is the Moon. The crater formation process o n Mercury should be very similar to the lunar case. On such large plan etary bodies, gravity plays a major role in crater formation, contrary to small bodies such as Phobos, Deimos or asteroids. Furthermore impa ct rates of large meteoroids may have been similar on the Moon and Mer cury. There are however major differences in the environment of these two bodies which should have an effect on the evolution of the regolit h: the ratio of micrometeoroids (mainly originating from comets) to me teoroids (which are transferred in chaotic corridors from the main bel t of asteroids) is likely to be much higher than on the Moon; impact e nergies are larger for cometary material than for asteroidal material; the flux of the solar wind increases by a factor of up to 10 on Mercu ry, although the relative importance of magnetospheric screening and o f magnetospheric particles is difficult to evaluate, Solar Cosmic Rays which result in the formation of particle tracks also increase by a f actor of up to 10 when compared to the Moon. However, surface temperat ures reach 700 K, which can result over millions of years in the annea ling of irradiation effects. Overall, the regolith of Mercury is expec ted to be significantly more mature than the lunar regolith, with smal ler grain sizes and larger proportions of glassy particles. A possible exception could be radiation damage due to annealing. This has implic ations on the interpretation of remote sensing investigations during a n orbiter mission, such as the lighter albedo of plains material on Me rcury when compared to the maria of the Moon, the search for ray crate rs or the relationship of near-infrared spectra to the mineralogical c omposition. (C) 1997 Published by Elsevier Science Ltd.