Cratering on the Galilean Satellites: Implications for the Size Distribution of Cratering Impacts in the Solar System
Clark R. Chapman (Southwest Research Inst., 1050 Walnut St. #426, Boulder CO 80302)
Instead, small craters are well below saturation on all three satellites. The situation is least clear for Ganymede, for which we so far have limited high- resolution images. But in four regions of Callisto, 100 m craters are undersaturated by a factor of about 20 and undersaturated in various parts of Europa by 1 to 2 orders of magnitude .
Certainly, ongoing processes of crater erosion and/or resurfacing contribute to their fewer numbers. On Callisto, there seems to be ongoing disaggregation of craters as well as infilling of small craters by widespread deposits. And Europa exhibits ongoing resurfacing processes, especially ridge formation and localized upwelling and collapse, which erase small craters.
A straightforward interpretation is that the retention ages for 100 m craters on these bodies is of order 1 to 100 million years, based on Shoemaker's  recent estimate of the cometary cratering rate, extrapolated to smaller sizes using the same size distribution (with "steep" differential exponent, -4 to -4.5) observed on the Moon, Gaspra, and -- apparently -- on the younger units of Uruk Sulcus on Ganymede.
However, there is little to no evidence concerning the actual size distribution of comets that would form craters <10 km diameter on the Galilean satellites. In spite of arguments  that comets may be collisional fragments like the asteroids, with a steep size distribution, several lines of evidence suggest that small comets may be underabundant.
If the production population of small comets follows a shallower power-law, or even curves over, then the impact rate would be lower and inferred resulting ages older. There is a further complication. Secondary craters are thought to contribute only modestly to the numbers of observed craters on the Moon and other terrestrial bodies, away from the evident chains and rays of major primary craters. However, if the production population of small comets is very low on the Galilean satellites, it may be that secondary craters from observed, recent large craters >10 km in diameter actually dominate the numbers of craters 100 m in size. That would certainly complicate the relative age-dating of geological units, because one would have to consider proximity of primary craters before assessing crater densities in terms of relative age. This possibility would make the absolute ages younger than if all craters were interpreted as primaries.
Obviously, if the few-small-comet alternative is correct, it would have implications for bodies other than the Galilean satellites (e.g. other outer solar system bodies where comets, rather than asteroids or circum- planetary objects, dominate the impactor population).
There is the opportunity, especially on Europa, to assess the secondary crater populations (in the absence of confusing primaries) from the several large craters and impact-maculae that have been imaged by Galileo. The results of these studies could clarify the nature of secondary cratering and perhaps, even, help us reassess the importance of secondary cratering on terrestrial bodies. Also, studies of catenae (crater chains due to Shoemaker-Levy 9-like impacts on Ganymede and Callisto) being imaged by Galileo may provide independent evidence concerning the numbers of cometary impactors too small to be estimated from Earthbased astronomical surveys.
Acknowledgments: I thank W. Merline, E. Asphaug, J. Moore, G. Neukum, R. Wagner, M. Carr, A. McEwen, P. Geissler, B. Bierhaus, J. Keller, S. Brooks and (other) members of the Galileo Imaging Team for discussions.
References:  C.R. Chapman et al. (1997), LPSC 28th.  E.M. Shoemaker (1996), Abstracts of Europa Ocean Conf. (San Juan Capistrano, CA, 12-14 Nov. 1996), 65.  P. Farinella and D.R. Davis (1996), Science 273, 938.
Abstract ID Number: 5261
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