Geoscience Reference
In-Depth Information
with geophysical inferences about interior cooling rates and processes that
generate Mercury's magnetic field.
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,
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Craters on Mercury (and other bodies) are studied in several ways.
Beyond the Mariner 10 imaging, and expected imaging from MESSENGER
and BepiColombo missions to Mercury, radar delay-doppler mapping, some
at better than 2 km resolution, from Arecibo
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and other radar telescopes is
starting to rival the resolution of the global-scale Mariner 10 imaging. Stud-
ies comparing Mercury's crater population with those on the Moon, Mars,
and other bodies provides useful understanding. Also relevant is modeling
of impactor populations and simulations of their impact history on Mer-
cury. Theoretical studies of cratering mechanics, ejecta distributions, and
regolith evolution have sometimes been applied to Mercury.
In detail, Mercury's craters have morphological differences from those
on the Moon and Mars, partly due to differences in gravity and impactor
environment (e.g., higher velocity impacts on Mercury), but most of the
differences are probably due to the different geological processes that erode
and degrade craters after they have formed on the various planets. For
example, while many craters on Mars extend back to the Late Heavy Bom-
bardment epoch that may be contemporaneous with the formation of many
of Mercury's craters, the Martian surface has undergone glaciation, rain-
fall/runoff, dust storms, sedimentation, exhumation, and many other pro-
cesses not thought likely to have been relevant on Mercury.
2. Origins of Mercury's Craters
Potential sources for the impactors that formed Mercury's craters are
numerous. In principle, the size distributions and the impact rates could
have varied with time and in ways not necessarily correlated with the crater-
ing histories of other bodies. Sources include: the near-Earth asteroids and
their cousins (of which only three have yet been found) that orbit entirely
interior to Earth's orbit (termed Apoheles); short- and long-period comets,
including sun-grazers; vulcanoids, an as-yet-undiscovered hypothetical pop-
ulation of remnant planetesimals from accretionary epochs, orbiting mainly
inside Mercury's orbit; and secondary cratering by ejecta from basins and
large primary craters. Endogenic crater-forming processes (e.g., volcanism)
are also possible. Differences in asteroid/comet cratering rates (perhaps
including the LHB) are not expected to vary by large factors for Mer-
cury compared with the Earth-Moon system or Mars. But if vulcanoids
were/are important, they could have extended the duration of Mercury's
