Geoscience Reference
In-Depth Information
REVIEW OF MARINER 10 OBSERVATIONS: MERCURY
SURFACE IMPACT PROCESSES
CLARK R. CHAPMAN
Southwest Research Institute, 1050 Walnut Street
Suite 400, Boulder, CO 80302, USA
cchapman@boulder.swri.edu
The major evidence concerning Mercury's craters remains the imaging of
Mariner 10 from over three decades ago. We are beginning to gain information
about Mercury's unimaged side from Earth-based radar. The MESSENGER
mission will start making major advances in a few years. Issues that have devel-
oped and remain to be resolved include the specific roles of the Late Heavy
Bombardment and of hypothetical “vulcanoids” in cratering the planet, which
affect calibration of the absolute chronology of Mercury's geological and geo-
physical evolution, and the role of secondary cratering, by ejecta from both the
visible craters and from the numerous large basins.
1. Introduction
Mercury's surface was first revealed by Mariner 10 imaging. Generally,
Mercury is heavily cratered, with less cratered regions, superficially sim-
ilar to the Moon. Craters range from nearly saturated small-sized craters
(many show clustering suggestive of secondaries) up to enormous multi-
ringed basins, of which Caloris is the most prominent in Mariner 10 images.
Dozens of basins have been tentatively identified. Morphometric statistics
of Mercury's craters have been compared with those for other terrestrial
bodies and interpreted in terms of differences in surface gravity and other
factors.
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Craters are used for both relative and absolute age-dating of geologi-
cal units on planetary bodies. Early interpretations of Mercury's cratering
record drew analogies from the Moon: it was supposed that most craters
and basins formed about 3.9 Ga, during the same Late Heavy Bombardment
(LHB) that dominated lunar cratering.
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Through superposition relation-
ships with other geological features (e.g., lobate scarps), a tentative chronol-
ogy for Mercury's geological history was derived (tied to the old Tolstoj
basin and the younger Caloris basin), raising potential incompatibilities
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