Discovery that secondary craters dominate Europa's small crater population

Details Discovery that secondary craters dominate Europa's small crater population Discovery that secondary craters dominate Europa's small crater population

Aug 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt........13b&link_type=abstract

Thesis (PhD). UNIVERSITY OF COLORADO AT BOULDER, Source DAI-B 65/02, p. 857, Aug 2004, 293 pages.

Physics

4

Scientific paper

This thesis presents data and analysis that demonstrate secondary craters (craters formed by material ejected from a primary impactor) dominate the small-crater (<1 km) population on Europa. Of the 17,000+ impact craters I measured in high-resolution images that cover only 0.2% of Europa's surface, 90% are clustered. I applied three spatial analysis techniques, including a novel hybrid of Monte Carlo and hierarchical clustering algorithms, to identify the clustered population. Additional analysis suggests that many unclustered craters are also secondaries; the true percentage of secondary craters is at least 95%. Least-squares, non-linear power-law fits to the differential (dN = kDb dD) size-distributions demonstrate that the secondaries have “steep” exponents, typically b < -4. Because the regions examined are at least hundreds of km away from any large primary crater, this is the first robust study of far- field secondary craters (those formed by material ejected at hundreds of m/s to over 1 km/s). I also measured 7,000+ near-field (only several parent crater radii distant) secondaries around Tyre, a 44 km primary crater on Europa, and measured 1,000+ near-field secondaries in a smaller area around Pwyll, a 26 km primary. The Pwyll data indicate a peak size for the near-field secondaries; the size-distribution at diameters larger than the peak size has very steep exponents, -6.3 to -7.8. The combined measurements of near- and far-field secondaries demonstrate that primary cratering events are extraordinarily efficient in generating ejecta for both populations. This research is the first to demonstrate that, at least on Europa, distant secondary craters overwhelm the small primary craters. Among the many potential implications of my research, two are profound: (1)the population of objects (now known to be ecliptic comets) that hits Europa to form primary craters must have a shallow (b > -2) size- distribution for objects <100 m diameter; and (2)to the degree that cratering in ice and rock are the same, any planetary surface with even modest populations of large primary craters (e.g. the Moon, Mars) must have significant numbers of secondary craters. Indeed, the Moon's crater size-distribution also develops a steep slope (b < -3.5) near 1 km; my results suggest that secondaries may dominate the small-crater population on the Moon.

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