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intense cratering into epochs far later than the LHB, and even overprinted
the LHB. 7
It is possible that planetesimals interior to Mercury's orbit never formed
or that they were destroyed (e.g., by their mutual, high-velocity collisions)
before the epoch of the LHB. But if they did survive such early pro-
cesses, it appears that their subsequent depletion by Yarkovsky effect drift
might have lasted for several billion years, perhaps resulting in appreciable
post-LHB cratering of Mercury. 8 (The Yarkovsky effect has been found to
have a potent effect in changing the orbits of small solar system bodies;
it acts on rotating bodies due to the asymmetry between insolation and
re-radiation in the thermal infrared.) The point here is not to assert that
current assumptions that tie Mercury's absolute geological chronology to
the LHB are wrong, but that such a chronology should not be taken as a
strong constraint. There are other geophysical aspects of Mercury that were
surprising to the Mariner 10 researchers and originally seemed dicult to
reconcile. Mercury appears to have an active dynamo-generated magnetic
field, although this has been debated. Mercury's buckled crust (expressed as
a global distribution of lobate scarps) suggests global cooling and shrinking
in post heavy-cratering epochs, but perhaps the cooling and crustal short-
ening has not gone to completion. Recent bistatic radar interferometric
studies 9 suggest the presence of a molten layer within Mercury. Theoreti-
cal modeling combined with hypotheses concerning impurities in the core
may have reconciled Mercury's small size and rapid cooling rate with these
indications of a still molten portion of the planet's core. Nevertheless, the
evaluation of these geophysical issues should not be strongly constrained
by any particular cratering chronology. Mercury's heavily cratered regions
may, in fact, reflect LHB cratering and its internal geological processes may
have shut down soon afterwards (access to molten magma may have been
closed off by crustal compression). Alternatively, both the cratering and the
faulting could have extended billions of years closer to the present time, if
vulcanoid cratering was important.
3. Secondary Cratering
Secondary cratering may be a much more important process than previously
thought. Studies of both the sparsely cratered surface of Europa 10 and of
Mars 11 have recently suggested that the steep branch of the crater size-
frequency relation for craters smaller than a few km (originally identified by
Shoemaker 12 as the “secondary” branch but later attributed to the inherent
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