  We performed numerical simulations of impact crater formation on Europa to
infer the thickness and structure of its ice shell. The simulations were
performed using iSALE to test both the conductive ice shell over ocean and the
conductive lid over warm convective ice scenarios for a variety of conditions.
The modeled crater depth-diameter is strongly dependent on thermal gradient and
temperature of the warm convective ice. Our results indicate that both a fully
conductive (thin) shell and a conductive-convective (thick) shell can reproduce
the observed crater depth-diameter and morphologies. For the conductive ice
shell over ocean, the best fit is an approximately 8 km thick conductive ice
shell. Depending on the temperature (255 - 265 K) and therefore strength of
warm convective ice, the thickness of the conductive ice lid is estimated at 5
- 7 km. If central features within the crater, such as pits and domes, form
during crater collapse, our simulations are in better agreement with the fully
conductive shell (thin shell). If central features form well after the impact,
however, our simulations suggest a conductive-convective shell (thick shell) is
more likely. Although our study does not provide firm conclusion regarding the
thickness of Europa's ice shell, our work indicates that Valhalla-class
multiring basins on Europa may provide robust constraints on the thickness of
Europa's ice shell.
