Mathematics – Logic
Scientific paper
Nov 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001dps....33.3506w&link_type=abstract
American Astronomical Society, DPS Meeting #33, #35.06; Bulletin of the American Astronomical Society, Vol. 33, p.1100
Mathematics
Logic
Scientific paper
Galileo magnetometer data obtained during recent flybys are consistent with the hypothesis that Jupiter's moon Callisto has a subsurface ocean, or at least a layer of semi-molten incompetant material, underneath its icy crust. This hypothesis was first proposed after morphological studies were made of the large Valhalla multiring impact structure using Voyager data. A test of this hypothesis would be a lack of disrupted terrains at Valhalla's antipode. Unusual hummocky terrains observed at the antipodes of large impact basins on the Moon and Mercury have been suggested to be regions of disrupted surface crust resulting from propagation of seismic energy from large basin-forming impacts through planetary interiors. Subsequent computer modeling suggested that these impacts could produce sufficient energy to disrupt surface crust or produce deep crustal fractures (as potential volcanic conduits) at impact antipodes on Mercury, the Earth, the Moon, Mars, and several icy outer satellites. However, model results for the 4000 km diameter Valhalla impact structure on Callisto were more ambiguous. Watts et al. (Icarus 93, 159-168, 1989) found that, depending upon impactor size and the nature of Callisto's interior, the Valhalla impact event probably had insufficient energy to produce disrupted terrain at Valhalla's antipode. They suggested that a liquid water layer in Callisto's mantle should reduce antipodal pressures and inhibit formation of disrupted terrains. However, Voyager imagery of this area was inadequate to test this hypothesis. In May 2001, NASA's Galileo spacecraft performed its final Callisto flyby and obtained regional resolution (340 m/pixel) coverage of a portion of Valhalla's antipodal region. This image shows no sign of disrupted terrains, although dark patches in intercrater plains may be residue from sublimation erosion or some kind of cryovolcanic fill. We suggest that the lack of disrupted features at the Valhalla antipode supports the hypothesis of a liquid water layer in Callisto's interior.
Chuang Frank C.
Greeley Ronald
Klemaszewski James E.
Williams David. A.
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