The range of ice thicknesses, 1–40 km, corresponds to the published range of estimates for Europa. The modeled target consisted of a solid ice sheet overlying liquid water that extended to the base of the computational mesh. All impacts were vertical (required by the axisymmetric structure of the 2-D code).
#ISALE TILLOTSON EOS CODE#
We used the iSALE (impact Simplified Arbitrary Lagrangian Eulerian) code v.7.0, which was developed from earlier SALE code specifically for impact simulations. These shock physics codes can predict impactor and target responses from first principles and are therefore a standard mechanism for investigating the dynamics and outcomes of impacts on Earth and elsewhere. Simulating planetary impact dynamics, because of the high shocks and strain rates involved, requires code that can solve equations for conservation of momentum, mass, and energy simultaneously with the equation(s) of state and constitutive relationships (that describe the material response). And thus, it is plausible that impacts may mediate transfer of prebiotic components to the ocean. But we also show that-no matter what ice thickness is preferred for Europa-there is an impactor size with geologically reasonable recurrence interval that is likely to breach it.
By comparing the geometry of numerically simulated craters with measurements from Europa, we test existing estimates for ice thickness and confirm that it is likely to be close to 10 km. Using impactor population statistics, we estimate likely recurrence intervals for crust-breaching impactors and show that breaching must be a recurring event at Europa. In this study we model the limiting conditions for ice penetration and define a set of breaching criteria. Impacts have the potential to excavate through the ice and create surface-to-ocean conduits via which two-way mass transfer might be achieved, but whether this is an important process on Europa-or whether it occurs at all-has not been established. This latter is particularly relevant to exobiological modeling: organic molecules have certainly been delivered by impacting comets, and radiative bombardment within Jupiter's magnetosphere produces oxidants and organic molecules at Europa's surface but transport to the ocean is required to provide an inventory of biologic precursors and energy sources to Europa's putative biotic system. The presence of salts in surface ice indicates transfer of ocean materials to the exterior, but how that happens is not understood, nor do we know how materials might move from the surface into the ocean beneath. How does Europa's sub-ice ocean communicate with the surface? And how thick is the ice barrier? These questions have been at the forefront of Europa studies since Galileo data revealed the probable existence of a liquid water layer. Astrobiological materials could be transported to the ocean via these impact-created conduits.
#ISALE TILLOTSON EOS FULL#
The geomorphic expression of full penetration must exist on the surface chaos terrain is a candidate. The largest known impact sites, Callanish and Tyre, probably represent transition from craters to penetrating impacts. Thus, it seems that Europa's ice has been penetrated often in the past and possibly in geologically recent time. If actual ice thickness is 8–13 km (indicated by comparing Europan and simulated crater geometries), the ocean could be exposed by impactors in the range 0.7–1.5 km, which have recurrence intervals ≈3 to 7 Ma. Thinner ice would be breached more frequently, thicker ice less so, but even the 40 km upper estimates for ice thickness would be penetrated by comets with recurrence intervals less than 250 Ma. Results indicate that a 0.5 km comet would penetrate 5 km ice, and a 5 km comet could breach 40 km ice. Breaching becomes inevitable when transient cavity depth exceeds 90% of ice thickness. Numerical simulations show that impactors can penetrate Europa's ice, creating conduits to the underlying ocean.
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