EVIDENCE THAT CHAOS TERRAIN ON JUPITER’S MOON EUROPA IS FORMED BY CRUST-PENETRATING IMPACTS

Analysis of Galileo images and experiment results suggest that chaos terrain on Europa’s surface may be formed by high-energy impactors that penetrate thin brittle crust and bury themselves in a ductile or liquid subsurface. Chaos consists of rough-textured frozen matrix, often encasing rafts or blocks of broken but undeformed pre-existing crust. The matrix:block ratio varies from about 0.6-1. Many chaos areas have circular or equant shapes and escarpment boundaries, consistent with penetration. The matrix may be updomed, and some chaos areas show overflow or extrusion of matrix material onto the surrounding topography, suggestive of resurgence. Some large chaos areas, notably Murias Chaos and Thrace Macula, are associated with clusters of small (<1 km diameter) craters, interpreted as secondary craters. Some chaos areas are surrounded by blobs of matrix material and sometimes small blocks, which may represent splash ejecta. We tested whether impacts could produce chaos-like features by launching ice projectiles at high velocity (117-405 m/sec) into layered targets consisting of ice over water and ice over slush. Impacts at lower energy or into thicker ice produced classic craters. Impacts at higher energy or into thicker ice, however, penetrated the ice layer completely and reproduced the features of chaos terrain in the shattered ice. Penetration features ranged from circular holes filled with slushy matrix (matrix:block=1) and surrounded by splash ejecta to complete fragmentation of the ice plate, producing rafts of ice floating level or tilted in a matrix of slush (matrix:block ≈0.6). For constant ice thickness, simple holes formed at lower impact energies than did complete fragmentation features; and for a constant impact energy, circular holes formed in thick ice and complete fragmentation in thin. The destuction-diameter:bolide ratio ranged from 1-5 for simple holes, but exceeded 20 for complete fragmentation. Chaos areas on Europa include analogues for both simple holes and complete fragmentation. The strong resemblance between the mapped features on Europa and those produced during experimental simulations supports our hypothesis that chaos terrain can be formed by bolide impact.