RETURN TO TYRE: HEAT FLOW ESTIMATES FROM MULTIRING IMPACT GRABEN ON EUROPA AND ELSEWHERE

The collapse of large transient impact cavities may lead to the creation of one or more exterior rings. The existence and extent of such ring systems depend on the thickness of the mechanical lithosphere at the time and place of impact. Icy satellites offer a valuable laboratory to explore this paradigm, and Ron Greeley took particular interest in these as part of his work on the Galileo imaging team, which we extend here. The two largest impact structures on Europa are Tyre (≈160 km across) and Callanish (≈95 km across). Tyre and Callanish both possess compact systems of circumferential graben-like troughs, essentially miniature versions of the much larger Valhalla and Asgard/Utgard multiringed structures on Callisto, and plausibly related to the hemispherical-scale furrow systems on Ganymede. The compact nature of these structures implies a relatively thin icy lithosphere, consistent with present-day steep temperature gradients due to strong tidal heating. We use measurements of trough width (and depth) to constrain heat flows at the time of impact. Assuming the troughs are graben, and that they originated or nucleated at the brittle-ductile transition (BDT), we find that fault intersection depths are equal to graben width (to within 10%). The depth to the BDT is related to the surface heat flux through the temperature profile in the ice shell and the rheology of water ice. The BDT occurs at a depth and temperature where the differential stress required for ductile flow at a given strain rate is equal to brittle yield stress. For the high strain rates appropriate to post-impact crater collapse, we find anomalously high heat flows, >300 mW m^–2. If, however, the “hourglass” model of graben formation is more correct than the classic “keystone” model (R.A. Schultz et al., in The Geology of Mars, 2007), then our BDT depths are underestimates. Furthermore, preexisting fractures imply lower thermal conductivity for lithospheric ice, and the combination implies true heat flows closer to 100 mW m^–2, similar to other literature estimates. Conductivity remains a principal uncertainty, but one amenable to future observational constraints, especially through an active remote sensing technique such as ice-penetrating radar.