Impact penetration of Europa's ice crust as a mechanism for formation of chaos terrain

Abstract

Ice thickness estimates and impactor dynamics indicate that some impacts must breach Europas ice crust; and outcomes of impact experiments using ice-over-water targets range from simple craters to chaos-like destroyed zones, depending on impact energy and ice competence. Firstorder impacts--into thick ice or at low impact energy--produce craters. Second-order impacts punch through the ice, making holes that resemble raft-free chaos areas. Third-order impacts--into thinnest ice or at highest energy--produce large irregular raft-filled zones similar to platy chaos. Other evidence for an impact origin for chaos areas comes from the size-frequency distribution of chaos+craters on Europa, which matches the impact production functions of Ganymede and Callisto; and from small craters around the large chaos area Thera Macula, which decrease in average size and density per unit area as a function of distance from Theras center. There are no tiny chaos areas and no craters 50 km diameter. This suggests that small impactors never penetrate, whereas large ones (berPenetrators: 2.5 km diameter at average impact velocity) always do. Existence of both craters and chaos areas in the size range 2-40 km diameter points to spatial/temporal variation in crust thickness. But in this size range, craters are progressively outnumbered by chaos areas at larger diameters, suggesting that probability of penetration increases with increasing scale of impact. If chaos areas do represent impact sites, then Europas surface is older than previously thought. The recalculated resurfacing age is 480 (-302/+960) Ma: greater than prior estimates, but still very young by solar system standards.