The Effects of Impacts on Europa’s Ice Shell Dynamics

Jupiter’s moon Europa is one of the prime targets for planetary exploration due to its high astrobiological potential. Its young surface age, on average between ∼40 – 90 Myr old (Bierhaus et al., 2009) suggests that some form of resurfacing has occurred in the past, with impacts being one of several possible triggering mechanisms. Moreover, impacts onto the ice shell of Europa likely have affected the ice shell dynamics leading to a convective state. 

 

Europa’s ice shell thickness is poorly known with literature values ranging from <1 km (Billings et al., 2005) to 90 km (Villela et al., 2020), with recent studies favoring a range of 23 - 47 km (Howell, 2021). Basin and crater shapes provide important information about the ice shell’s thermal state, thickness, and dynamics (conductive vs. convective). A transition in crater morphology for diameters larger than ∼8 km indicates a weak layer at ~7–8 km depth, as inferred from numerical modelling and observational crater-depth studies (Bray et al., 2014; Schenk, 2002). This layer could potentially represent warm convecting ice or the presence of the liquid ocean (e.g., Silber and Johnson, 2017). A recent study about multiring basins on Europa suggests an ice shell thickness larger than 20 km consisting of a 6-8 km conductive layer overlying a warm convecting region (Wakita et al., 2024).

 

Here, we investigate how impacts affect the dynamics of Europa’s ice shell using the geodynamic code GAIA (Hüttig et al., 2013). Impact thermal-induced and compositional anomalies are parameterized using scaling laws (Melosh, 1989). We assume that the water produced as a consequence of the impact process rapidly recrystallizes, but leaves behind a chemical and thermal anomaly in the shallow layers of the ice shell. Our models include a composite rheology (Goldsby & Kohlstedt, 2001), pressure- and temperature-dependent thermal expansivity and thermal conductivity (Feistel & Wagner; Wolfenbarger et al., 2021), and the effects of tidal heating (Tobie et al., 2003). We test scenarios with different impactor sizes (0.5 km - 1.8 km), thermal states at the time of the impact (i.e. cold conductive or warm convective ice shell), and ice shell rheology (via changing the grain size). We vary the chemical density anomalies due to impactor material assuming mixtures of ice, salts, and dust. To this end, we consider the presence of salts in concentrations ranging between that of the Earth's ocean and twice as high. 

 

Our models show that impacts can initiate thermal convection in an otherwise conductive ice shell. The material introduced by impacts may remain trapped in the cold conductive upper layer if no surface mobilization occurs. For large impacts, the impactor material can reach the convective ice layer and become mixed into the ice shell, reaching the ice-ocean boundary.

 

In a future step, we will consider the impact-induced thermal anomalies based on shock physics models instead of scaling laws. We will use the modelled density anomalies associated with thermal and compositional anomalies introduced by impacts to determine their gravity signature that could be potentially detected by Europa Clipper and JUICE.