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E.B. Bierhaus (Univ. of Colorado, SwRI), C.R. Chapman, W.J. Merline (SwRI)
We present new measurements of the location and sizes of small craters on Europa from 95 Galileo images (representing 21 regions), taken at high resolution (less than 100 m/pix). Our analysis is two-fold: (1) we present the size-distribution for the combined measurements, providing a reasonable snapshot of Europa's overall small crater population; (2) using a variety of statistical tools, we have divided the overall population (based on degrees of spatial clustering) into primary and secondary craters, and we present separate size-distributions for these two populations. This is the first time that secondary craters have been statistically removed from the crater size-distribution to reveal the true, underlying primary crater population on a planetary surface. As we have earlier demonstrated (Bierhaus et al.\ 2001, {\em Icarus}, 153, pp. 264--276), secondary craters can dominate local cratering on Europa, but this work actually quantifies the fraction of the small crater population due to secondaries, and is based on a much more significant sample size. Not only does this work provide a much more accurate representation of the small primary impactor population in the Jovian system, but it also provides the first statistically significant size-distribution for the far-flung, high-velocity ejecta that makes distant (more than a few 100 km from their parent primary) secondary craters.
The images examined contain the best combination of characteristics suitable for crater identification, namely: low sun angle, low compression, and few data gaps. The number of measurements represents a substantial increase in small-crater statistics over any previously reported results; the remaining, un-measured Galileo Europa images have either high sun-angle, low resolution, or other characteristics unsuitable for geomorphic analysis of small craters. Thus, our data represent the best compilation of Europa's small crater size-distribution.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.