CRATERING ON MERCURY

Mercury is a heavily cratered world, in some ways resembling the Moon. Until MESSENGER's first flyby early in 2008, Mercury's craters were studied chiefly from images taken by Mariner 10 in the 1970s. At that time, it was recognized that the size-frequency relationship for craters resembles that for the Moon, suggesting that the impacting objects were probably the same population, but differs in that there are diminished numbers of craters some tens of kilometers in diameter on Mercury, attributed to a greater prevalence of possibly volcanic intercrater plains on Mercury.

MESSENGER images, obtained during its pre-orbit-insertion flybys of the planet, have shown a more complex variety of size distributions in different regions. It is plausible that especially widespread volcanism, aided sometimes by Mercury's unique tectonic activity, has competed with crater-forming processes to shape the variety of crater populations (and the diverse crater morphologies) in different regions of the planet. While several specific, relatively recently formed impact basins have been revealed in detail by MESSENGER’s images, large basins may be rarer on Mercury than on the Moon. We interpret Mercury's craters in the context of the model developed by Strom and others that the basins and more heavily cratered terrains were formed during the Late Heavy Bombardment, which ended on the Moon about 3.8 Ga, and that more recent terrains have been impacted by planet-crossing asteroids (and some comets) since then. The possibility that the cratering record has been partly formed by more recent impacts by hypothesized vulcanoids (“asteroids” orbiting the Sun inside Mercury’s orbit) has not been foreclosed, but specific evidence for such impacts is lacking and early search images have detected no vulcanoids.

Mercury's crater population <8 km in diameter appears dominated by secondary craters, which appear to be more prominent and abundant than on the Moon and are chiefly associated with recent impact basins. Abundance of basin secondaries may be regionally variable, and whether the abundance represents an important Mercury-specific attribute of cratering (due, for instance, to impactor velocity, target material, or differences in gravity) remains for study once MESSENGER goes into orbit.