The effects of thermal gradient of the ice crust on the morphology of Titan’s craters

Titan, the largest moon of Saturn, is the target of NASA’s Dragonfly mission, a rotorcraft lander. It is the only planetary body in our solar system, besides Earth, that has stable liquids (e.g. methane) on its surface and a thick, nitrogen-rich atmosphere. NASA’s Cassini mission detected an unusually low number of impact craters on Titan’s surface, likely because of degradation and burial by fluvial erosion and aeolian infilling. Because the rates of these crater degradation processes and the ages of Titan’s craters are unknown, the ‘uneroded’ morphology of Titan’s impact craters is not well understood. Knowing the morphologies of fresh craters would allow us to break this degeneracy and constrain the amount of erosion that has occurred on Titan. A comparison of Titan and Ganymede crater depths shows that craters on Titan are hundreds of metres shallower. However, Titan and Ganymede have different surface compositions (methane clathrate vs. water ice) and interior thermal structures that might influence the cratering process. Numerical investigation would allow for an improved understanding of fresh crater morphologies on Titan. We use the impact-Simplified Arbitrary Lagrangian Eulerian (iSALE) shock physics code to simulate crater formation on Titan. The simulations explore the effect of thermal gradient in the ice crust on crater depths over a range of diameters. We fix the impact velocity at 10.5 km/s and vary the impactor diameters from 2 to 10 km. The lower thermal conductivity of methane clathrate results in a higher thermal gradient; we thus consider thermal gradients from 3 K/km (pure water ice case) to 10 K/km (methane clathrate layer case). Our simulations also investigate the accepted range of thicknesses for Titan’s ice crust (50–150 km). We then compare the depths of fresh Titan impact craters inferred from the model outputs to observed crater depths on Titan, to determine the extent of erosion that has occurred since their emplacement.