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C.R. Chapman (Southwest Research Inst., Boulder CO 80302)
With the MESSENGER spacecraft successfully launched and scheduled for its first Mercury encounter in a little over three years, it is time to reconsider issues concerning the cratering history of Mercury. Since Mariner 10's encounters three decades ago, several issues remain unresolved. First, there is the question of vulcanoids, whose potential existence (now or during past epochs) near and within Mercury's orbit remains unresolved, despite theoretical studies and ongoing observational attempts to constrain the present population. If sufficient vulcanoids have existed, Mercury's cratering chronology could extend much closer to the modern epoch (e.g. thrust faulting could be continuing today) than if the temporal history of Mercury's cratering has been similar to the Moon's. Even if vulcanoids have not existed, the similarity of Mercury's cratering history to the Moon's depends, to a degree, on the specific cause of the lunar Late Heavy Bombardment and on the relative proportions of comets and Near Earth Asteroids that have more recently continued to impact Mercury. Another set of issues concerns the role of secondary cratering. In the context of the dozens of basins that have already been identified in Mariner images of just 40% of Mercury's surface, it is worth noting that it is still not resolved what fraction of moderate-sized (tens of km) lunar craters are basin secondaries. Moreover, recent research on smaller secondary craters on Europa (Bierhaus) and on Mars (McEwen), if applicable to Mercury, suggests that most craters smaller than several km diameter may be far-field secondary craters, complicating prospects for relative age-dating of small geological units on Mercury.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.