Mathematics – Logic
Scientific paper
Jan 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003icbg.conf...12a&link_type=abstract
Impact Cratering: Bridging the Gap Between Modeling and Observations, February 7-9, 2003. LPI Contribution No. 1155. Houston,
Mathematics
Logic
1
Fractures (Materials), Formation, Impactors, Craters, Comets, Asteroids, Deformation, Jupiter Satellites, Jupiter (Planet), Gravitation, Geological Surveys, Planetary Evolution, Protoplanets, Spheroids, Mars (Planet)
Scientific paper
Impact phenomena shaped our solar system. From the accretion of planetesimals 4.6 billion years ago to the spallation of meteorites from their parent bodies, this process has left no bit of matter untouched. The study of impact craters on small bodies therefore provides a foundation for understanding accretion and the delivery of meteorites - topics central to the origin of planets. Moreover, geologic-scale impact craters forming in low gravity reveal details of the cratering process that are hidden on high-gravity worlds like the Earth and Moon. The detailed study of small body cratering began with efforts by Housen et al. (1979), Veverka and Thomas (1979) and others, together with efforts related to catastrophic disruption of small bodies. But the discovery of Stickney (the approx. 10 km diameter crater on the approx. 20 km diameter Martian satellite Phobos) and comparably huge divots imaged by Voyager on satellites of Jupiter and Saturn made it clear that small bodies can sustain huge wallops despite the conclusion of scaling models, notably that the impactor responsible for Stickney would have catastrophically disrupted Phobos.. While large impact structures on bodies with significant gravity are much better understood today than they were originally, for small bodies this is not the case. We appear almost to be back-pedaling towards an earlier vision of the asteroid impact process, pioneered by Art Clokey (without much guidance from geologists) in his 1957 Gumby claymation adventure "The Small Planets". Although nobody today confesses to expect clear gravity signatures around approx. 10 m craters on approx. 100 m asteroids (we have yet to obtain clear images of anything much smaller than ten kilometers), few expected copious regolith on bodies the size of Eros (33 x 13 km) either. Surprise is the norm. Fifteen years ago, bodies that size were widely believed to be capable of sustaining a few centimeters of regolith at best. Instead, NEAR Shoemaker confirmed what had been hinted during less clearly resolved Galileo flybys of asteroids Gaspra and Ida: that Eros-sized asteroids can be awash in gravitationally bound debris (collisional or original is anybody's guess) ranging in size from approx. 100 m blocks to submicron grains accumulating in "ponds". Global regolith deposits on Eros range from 100's of m to undeterminable depth, and surface geophysics may even be dominated by quasi-aeolean processes such as electrostatic levitation and seismic shaking. Even on the smallest bodies yet observed, there is evidence for gravity dominance. Asteroid Ida's tiny (1.6 km) satellite Dactyl exhibits a spheroidal shape, as one would expected under self-gravitational control, and its major craters display rims and maybe central peaks. But to contrast Dactyl, Phobos, Deimos and other gravity regime Lilliputians, one finds 60 km Mathilde, a body which trashes every established theory of impact cratering, and which is from impact cratering's point of view one of the most astonishing bodies. Here one sees huge craters devoid of any gravity signature, and devoid of any signature of overprinting, on a pitted spheroid lacking visible fractures or other strength-related deformation. Nothing is here but the huge crater bowls themselves. Ejecta has either all entirely escaped or was never ejected at all, evidently in a target sufficiently porous to not communicate each blow globally, yet sufficiently cohesive for its crater rims not to collapse into softer shapes.
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