EUROPA’S ICE CAN BE BREACHED BY IMPACT: HYDROCODE MODELING CONSTRAINS THE CONDITIONS

How thick is Europa’s ice? How and how often is there communication between Europa’s surface and its sealed-in ocean? Can mass and energy be transmitted from the exterior into the liquid beneath? These questions persist in discussions about Europa as a mission target and exobiological prospect. Analysis of impact dynamics provides some answers.

Numerical simulations using iSALE show that impactors in the Jupiter Family Comet size distribution can penetrate through the full range of likely Europan ice thicknesses. The breaching criterion is met when transient cavity depth H_t exceeds 90% of ice thickness T_ice. Results indicate that the limiting penetrator size for 5 km ice is 0.4-0.6 km diameter (depending on density and impact velocity), and that and a 5-7 km comet could breach 40 km ice.

Comparing simulated crater geometry with that of Europan impact features suggests that Callanish and Tyre, the largest known impact sites, represent boundary cases (H_t ≈ 0.9 T_ice) preserving the transition from craters to penetrating impacts, and suggesting that larger, full penetrations (H_t > T_ice) must be represented by some other category of features.

The relationship between Europan crater geometry and simulation data also points to actual ice thickness in the range 8-13 km. This means that the ocean could be exposed by impactors with diameters 0.7-1.5 km, which have recurrence intervals ≈3-7 m.y. Thus it seems that Europa’s ice has been penetrated often in the past, and possibly in geologically recent time. Astrobiological materials could be transported to the ocean via these impact-created openings in the ice, which would serve as conduits for mass and energy exchange with the underlying ocean.