CRUST-BREACHING IMPACTS AT EUROPA: HYDROCODE MODELS AND GEOMORPHOLOGIC CONSTRAINTS ON ICE THICKNESS

Estimates of Europa’s surface ice thickness range from 1 to 40 km (median estimate is 6 km). For crust thicknesses in this range, some proportion of impactors is likely to fully penetrate through the ice to water beneath. But because inner solar system bodies behave as fully solid targets (having rocky mantles beneath their crusts), our understanding of the dynamics of penetrative impacts is limited. So we have made numerical simulations —using the iSALE hydrocode—of impacts into ice over water, varying impact energy and ice thickness, to examine impact behaviour under Europan conditions.

We find the entire range of proposed Europan crust thickness liable to full impact penetration. For crusts <5 km thick, icy bolides 500 m diameter (impacting at 26.5 km.s^-1) will penetrate. In 10 km crust, 500 m objects will form craters but anything larger will penetrate to water (by melting through at smaller diameters or by impact breaching as diameter increases). In 20 km crust, a 1.5 km object will crater, but larger ones will go through. For 30 km crust, a 2 km bolide will crater, but a 3 km one will penetrate. At 40 km, a 3 km bolide will crater, but a 5 km one will punch straight through to liquid beneath.

We may be able to constrain ice thickness at Europa by comparing depth/diameter (d/D) ratios of modeled craters with those measured from Galileo imagery, because shapes of the modeled craters vary as a function of impactor energy and ice thickness. Our data suggest thicknesses <10 km, because large craters produced in thicker crusts have profiles that do not match those of features measured on Europa. For example, the crater produced by a 1.5 km impactor in 20 km crust is 38.5 km in diameter and 1.3 km deep after 50 hours; but actual impact features in the ≈40 km size range on Europa are much shallower (only 50-100 m). We find d/D ratios consistent with Europan observations only for crusts <10 km thick.