It has been more than 3 decades since the Mariner 10 mission took a close look at the surface of the innermost planet Mercury. In January 2008 NASA’s MESSENGER spacecraft flew past and the 4 July issue of Science contained a special section on the early observations (Several reports 2008. Messenger Special Section. Science, v. 321, p. 58-94). These involve images, spectral observations, laser altimetry, estimates of chemistry in Mercury’s surrounding space and measurements of the mercurial magnetic field. The data bear on surface mineralogy, geological structures, regolith formation, cratering – especially the giant Caloris Basin, and evidence for volcanism.
Oh dear; water on the Moon…
The accepted wisdom about the Moon is that it is and always has been supremely dry. That notion stems from analyses of every single solid rock brought back by the Apollo astronauts, and the probability that the Moon formed from incandescent vapour blasted into orbit by a giant collision between the original Earth and an errant planet as big as Mars. Water and indeed most volatile elements and compounds ought to have been driven off the orbiting gas and debris that coalesced to form the Moon around 4.5 Ga ago. Most people believe that more or less everything the astronauts dragged back to Houston has been analysed: not so. There are millions of glass beads that constitute a sizeable fraction of the lunar regolith. Some of these turn out to be volatile rich, and may have been blown out by early lunar volcanism (Saal, A.E. et al. 2008. Volatile content of lunar volcanic glasses and the presence of water in the Moon’s interior. Nature, v. 454, p. 192-195). If the glasses are volcanic in origin, that implies there is water in the Moon’s mantle. So, you might ask, how come the Moon is not a vibrant place rather than being as dead as a doorknob? The Earth is so interesting partly because it is a wet planet. The Moon has very little in the way of heat production, so even if its mantle contained hydrous phases, it cannot reach basalt solidus temperatures unless energy is delivered mightily by impacts. That did happen around 4 Ga, when the lunar maria formed and became floored by gigantic floods of basalt. Yet those basalts are extremely dry, thereby posing a bit of a question for Saal and his colleagues.
See also: Chaussidon, M. 2008. The early Moon was rich in water. Nature, v. 454, p. 170-172.