HYDROCODE MODELING OF ICE-PENETRATING IMPACTS ON JUPITER'S MOON, EUROPA

Geological, gravitational, and magnetic-field evidence indicates a subsurface water ocean at relatively shallow depth beneath Europa’s ice crust. If ice thickness is ≤20 km (inferred from crater depth:diameter ratios and lithostatic stress constraints), impacts of sufficient energy could break through to the underlying water. Using the iSALE hydrocode (developed by G. Collins, K. Wünnemann, B. Ivanov, J. Melosh and D. Elbeshausen, whose co-operation we gratefully acknowledge), we have produced 2-D numerical simulations of impacts at Europa to assess the likelihood of impact penetration of the ice crust.

We modeled bolides 0.02-2 km in diameter, and ice crust thicknesses 3-20 km. As most impactors at Europa are likely to be comets, our projectiles were ice spheres (density 910 kg/m³). We used velocities of 15 and 20 km/s (lower than the 26 km/s average at Europa) to permit comparison with work by Bray (thesis 2009; http://tiny.cc/y3gaq). The simulations use Bray's (2009) 3rd-order Tillotson equation of state for the ice layer and an ANEOS for the underlying water. We applied a thermal gradient across the crust (from 100 K at surface to 273 K at base) to progressively weaken the ice with depth, lessening the effects of a sharp material boundary at the ice-water contact. We also implemented acoustic fluidisation for the ice layer.

Preliminary results indicate that a 620 m diameter comet at 15 km/s penetrates crust up to 6 km thick, and a 2.9 km bolide at 15 km/s penetrates 20 km thick crust. These impacts are energetically equivalent to 26 km/s impacts of bolides 430 m and 2 km, respectively. Penetrating impacts produce a water-filled breach extending fully through the ice, with the water comprising both impact-melted crust and exposed water under-layer. In some cases unmelted blocks of ice—ejecta that have fallen back into the opening cavity—persist as rafts in the water. Calculated recurrence intervals at Europa (from equations of Zahnle et al. 2003; Icarus, v. 163) are ≈2 m.y. for a 420 m and ≈10 m.y. for a 2 km comet. So even if ice thicknesses are substantial (≈ 20 km), our simulations suggest that impact penetration at Europa is likely, and may be a regular occurrence. Chaos terrain—where crust has been destroyed and underlying water appears to have been exposed—is unique to Europa, and may be the geomorphologic expression of such events.