DPS 2001 meeting, November 2001
Session 37. Galilean and Other Outer Planet Satellite Posters
Displayed, 9:00am Tuesday - 3:00pm Saturday, Highlighted, Friday, November 30, 2001, 9:00-10:30am, French Market Exhibit Hall

## [37.05] Application of Clustering Methods to Europa's Small Craters

E.B. Bierhaus (Univ. of Colorado, SwRI), C.R. Chapman, W.J. Merline (SwRI)

We provide evidence that many to most of Europa's small (less than 1~km diameter) craters are secondary craters. There is a strong tendency for small craters to appear in clusters. In one well-imaged case, the small craters are associated with crater rays from one of the few large impact craters. Previously (Bierhaus et al. 2001, {\em LPSC}, abstract # 1967) we have investigated the gross characteristics of the small (less than 1~km diameter) crater distribution on Europa; e.g., in most places the spatial distribution of small craters is more, sometimes much more, clustered than would be expected if these craters were made by random, primary impacts.

Here we describe quantitatively how many small craters actually appear in clusters, and thus what percentage of small craters are actually secondaries. We use several different clustering-analysis techniques, some of which were used to successfully identify asteroid families (Zappala et al. 1995, {\em Icarus}, 116, p.~291). We compare the results of the methods to determine whether the clusters are robust. We note which clusters are identified by each of the methods, and how strongly defined the clusters are in each. We also track the spatial distribution of the craters not associated with clusters to determine when, if ever, their spatial distribution approaches the randomness expected for primary impact craters.