Thin Ice Lithospheres and High Heat Flows on Europa From Large Impact Structure Ring-Graben

Craters are probes of planetary surface and interior properties. Here we measure depths, widths, and spacing of circumferential ring-graben surrounding the two largest multiring impact structures on Europa, Tyre and Callanish. We estimate formation conditions including the ice shell structure. The radial extension necessary to form these graben is thought to be caused by asthenospheric drag of warmer, more ductile ice and/or water flowing toward the excavated center of the crater, under a brittle-elastic lithospheric lid. Measurements of graben depths from stereo-photoclinometric digital elevation models result in estimates of displacement, strain, and stress experienced by the ice shell. Graben widths are used to estimate the intersection depth of the bounding normal faults, a quantity related to the brittle-ductile transition depth that approximates elastic shell thickness during crater collapse. Heat flows at the time of crater formation as well as ice lithosphere and total shell thickness are thus also constrained. Average widths and depths tend to decrease with increasing distance from the structure center, while inter-graben spacing generally increases. Varied assumptions yield plausible total conductive ice shell thickness estimates between 4-8 and 2.5-5 km for Tyre and Callanish, respectively, and heat flows of similar to 70-115 (+/- 30) mW m-2 for realistic thermal conductivities, consistent with other geophysical estimates for Europa. Higher heat flows are consistent with thin (less than or similar to 10 km), conductive ice shells and impact breaching, or penetration of the stagnant lid for a convecting ice shell. Callanish, geologically younger, formed in a time or region of greater heat flow than Tyre. Jupiter's moon Europa has an outer icy shell overlying a global subsurface ocean. The thickness of this icy shell controls the appearance of impact craters. The shell thickness is not well known, and may change over time, but geological indicators suggest it is on the order of 10 km thick or more. This is a relatively thin layer for craters to form in, compared the hundred-km or thicker ice shells on most other icy bodies in the solar system. The two largest europan impacts created sets of circumferential ring-faults that take the form of graben/troughs in the outer portions of their structures. We use measurements of the graben widths and depths to derive minimum estimates of the ice lithosphere (upper brittle portion) and total shell thicknesses and heat flows on Europa at the time the structures formed. The measurements are consistent with a minimum ice shell thickness between 2.5 and 8 km and high to exceptionally high heat flows. Greater total ice shell thicknesses are permitted if the shell is convectively overturning at depth. Notably, our heat flow estimates for Callanish (the younger structure) are uniformly higher than for Tyre (the older), contrary to expectations for Europa. Circumferential ring-graben depths, widths, and spacing measured for the two largest known impact structures on Europa: Tyre and CallanishGraben widths give plausible depths to the brittle-ductile transition of similar to 2-3 km at the time of and under the conditions of impactHeat flows are high, dependent on fault structure and thermal conductivity, but consistent with impact breaching of a thin ice shell or lithosphere