Other
Scientific paper
Sep 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999dps....31.5702c&link_type=abstract
American Astronomical Society, DPS meeting #31, #57.02
Other
Scientific paper
During its 20th orbit around Jupiter, the Galileo spacecraft returned moderate resolution (100 - 600 m/pix) images of Callisto. Unlike Ganymede and Europa, Callisto shows little unequivocal evidence of endogenic surface activity, with the possible exception of 1 km-scale pits in some locales and a few landslides from crater walls. Impact craters are the dominant landform. Earlier moderate-to-high resolution Galileo images showed that, contrary to expectations, Callisto's spatial density of craters actually decreases below diameters of about 10 km. The new Callisto images are in regions previously imaged by Voyager only at low ( > 1 km/pix) resolution. Our new crater counts of these images reveal that the decrease in crater density also exists in these regions, which are far from the Valhalla and Asgard basins, further demonstrating that this is a global phenomenon. We find that the density in this region is higher at 10 km diameter and lower at 1 km diameter than the global average, i.e. the slope is shallower. There are three probable causes for the loss of small craters: (1) the global production and distribution of Callisto's dark smooth unit, (2) disaggregation of crater walls, and (3) a shallow production function. Because larger craters are relatively immune to mechanisms (1) and (3), we tentatively propose that the higher density of large ( > 10 km) craters in these images indicates that disaggregation acts less efficiently in this region. (Some of the latest images are slightly overexposed, effectively degrading the resolution, resulting in some confusion between the true decrease in small crater densities and fall-off due to resolution.) We do not see evidence for the irregularly shaped 1 km-scale pits present in other regions, although small pits would be near the resolution limit. For the purpose of comparison, we will present measurements of craters from images taken in the 10th orbit with similar resolution, lighting, and compression.
Bierhaus Beau
Chapman Clark R.
Galileo Imaging Team
Greeley Ronald
Klemaszewski James
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