Impact Crater Morphology and the Structure of Europa's Ice Shell

Abstract

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 the 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 that a conductive‐convective shell (thick shell) is more likely. Although our study does not provide a 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.

The surface temperature of Europa

“…Europa has a deep (∼100 km) ocean that underlies an icy shell, more than several kilometers deep (e.g., Cassen et al, 1979 , Carr et al, 1998 , Pappalardo et al, 1998 , Kivelson et al, 2000 , Hussmann et al, 2002 , O'Brien et al, 2002 , Tobie et al, 2003 , Schenk and Pappalardo, 2004 , Zhu et al, 2017 ), where chemical interactions at the rocky bottom of the ocean may enable the existence of a habitable environment (e.g., Chyba and Phillips, 2001 , Chyba and Phillips, 2002 , Greenberg, 2010 , Mann, 2017 ). The Voyager and Galileo (and to a lesser extent, the Cassini-Huygens and New Horizons) spacecrafts/missions discovered many interesting features of Europa including chaos terrains ( Schmidt et al, 2011 , Walker and Schmidt, 2015 ) and craters ( Lucchitta and Soderblom, 1982 , Moore et al, 1998 , Greeley et al, 2000 , Silber and Johnson, 2017 ). More recently, based on the Hubble telescope observations, scientists raised the possibility of water vapor plumes at Europa's south pole ( Roth et al, 2014 , Sparks et al, 2016 ).…”

A nonlinear and time‐dependent visco‐elasto‐plastic rheology model for studying shock‐physics phenomena

“…In the meantime, the algorithm described in this paper has been incorporated into the developer version of iSALE and is used in the form described here by Johnson et al 32 and Lyons et al 64 A version modified for ice rheology appeared in Johnson et al, 65 Silber et al, 66 and Bowling et al 67 We anticipate that future publications will use this rheology as the developer version of iSALE is migrated to the user version.…”

Martian Ice Revealed by Modeling of Simple Terraced Crater Formation

“…The uncertainty of the crater radius is the difference between the values of the crater radius measured at the rim and at the preimpact surface, divided by four (cf. Figure 3 and Figure 5 in Silber & Johnson, 2017). The crater diameter error is then the double of this value.…”

“…Crater and pit diameters of the observed and modeled impact structures are measured from rim to rim. Differently to other works of literature (i.e., Silber & Johnson, 2017; Watters et al, 2015), depths of both observed and modeled craters are measured from the zero‐level surface, since we would like to make a direct relation between crater features and target stratigraphy. In the case of the modeled crater, it is given by the preimpact surface setup in the numerical grid.…”

“…The error in the measurements was computed following the approach of Silber and Johnson (2017). The uncertainty of the crater radius is the difference between the values of the crater radius measured at the rim and at the preimpact surface, divided by four (cf.…”