Tidally Heated Convection and the Occurrence of Melting in Icy Satellites: Application to Europa

Abstract

Observations of icy satellites have revealed widespread marks of cryovolcanism. Because aqueous cryomagmas are negatively buoyant, two processes are required to explain these observations: one mechanism to generate melt close enough to the surface and another one to transport this melt to the surface. Here, we investigate the generation of melting in a systematic way, using a set of 85 numerical simulations where we vary the viscosity contrast, Rayleigh number, and tidal heating rate. Applied to Europa, considering a hydrosphere composed of pure water, our simulations suggest that isolated melt pockets can be generated close to the surface ( ∼5 km) as long as the ice layer thickness ( d*) remains modest ( 15≤d*≤35 km). However, the generation of melting becomes increasingly difficult as the amount of antifreeze compounds in the subsurface ocean increases. Furthermore, the proportion of melting increases very sharply with increasing tidal heating rate. In particular, when the tidal heating rate exceeds a threshold, an asymptotic regime is reached where the surface heat flux remains constant indicating that the tidal heat generated above this threshold is only used for melting the ice shell. In that regime, we found a direct relationship between the surface heat flux and d*. Finally, we provide a new assessment of Europa's thermal state. Based on available constraints, we propose that the ice shell thickness should exceed 15 km. However, d*∼15–35 km implies a tidal power ( >2 TW) much larger than expected. An extrapolation of the trends suggested by our results indicates that a more reasonable tidal power ( ∼1 TW) would involve d*∼50–90 km.

The Growth of Europa's Icy Shell: Convection and Crystallization

“…This will lead to the consumption of additional latent heat within the convecting layer that we cannot quantify due to the model's one-dimensional nature. Vilella et al (2020) investigated the effect of internal shell melting on ice convection, finding that, for ice shell conditions similar to ours, lateral temperature variations may lead to significant portions (up to 30% for a given depth) of the ice shell with temperatures greater than melting. Vilella et al (2020), when comparing cases with internal melting to cases without, find that the basal heat flux of the layer may increase by up to a factor of 3 as tidal heat intensity increases.…”

Entrainment and Dynamics of Ocean‐Derived Impurities Within Europa's Ice Shell

“…Knowing the composition of the ice provides the chance to evaluate the formation, evolution, and longevity of water or brine systems within Europa's ice shell. For example, shallow lenses of liquid water are suggested to form in situ via melting of the ice shell (Schmidt et al, 2011; Vilella et al, 2020) (Figure 1a) or by injection through diking processes (Manga & Michaut, 2017; Michaut & Manga, 2014). Here, we investigate the salinity profile produced when a lens formed via in situ melting within a shell originally derived from a 34‐ppt terrestrial ocean chemistry refreezes.…”

Laboratory exploration of mineral precipitates from Europa's subsurface ocean

“…The two largest multi-ringed craters on Europa similarly point to a thickness of $20 km at their time of formation (Schenk, 2002). More recent analysis of likely tidal heating, dissipation and conductive cooling suggests an average thickness of 15-35 km (Quick & Marsh, 2015;Vilella et al, 2020). Furthermore, investigation of previously unanalysed high-resolution images from Galileo suggests that the local-scale resurfacing has evolved over time, transitioning from distributed deformation (expressed by the formation of the ridged plains) to discrete deformation (typified by the formation of chaos terrain and isolated fractures), and is probably consistent with progressive shell thickening and cooling (Leonard et al, 2018).…”