Compositional convection in Europa’s ice shell: a scale-coupled approach

The presence of hydrated minerals at the surface of Europa has been suggested by the measurements of the Galileo Near-Infrared Mapping Spectrometer (NIMS, McCord et al., 2001). Laboratory measurements that best match the observed spectra indicate the presence of magnesium sulfates, sodium carbonates and combinations thereof. The source of these impurities has been suggested to lie beneath the crust in a reservoir rich in dissolved salts.

A recent study using spectral data from the Hubble Space Telescope identified the presence of irradiated sodium chloride at the surface of Europa (Trumbo et al., 2019). This detection correlates with the location of the disrupted chaos terrain, a geologically young region on the leading hemisphere that has been suggested to be formed due to subsurface melting and freezing of the ice shell (Schmidt et al., 2001).

The above listed evidence suggests that the ice shell of Europa may contain a significant amount of contaminants (Kargel et al., 2000; McCord et al., 2001; Zolotov & Kargel, 2009), but their exact distribution is poorly constrained. Previous studies have investigated to which extent the presence of salty components within the ice shell offers an explanation of tectonic surface features of Jupiter’s moon (e.g., Han & Showman, 2005). Yet, how these salt impurities were incorporated into the icy shell and how they evolved over time are still subject of a scientific debate. A rigorous analysis of the transient evolution of salt impurities in the ice shell as well as a better quantification of the amount of salts that can be trapped in the near surface ice layers is necessary.

In this work we investigate the spatial distribution of salts in Europa’s ice shell following a two-fold approach:

First, we consider potential salt in-take processes at the ice-ocean boundary. In a transient regime, hence when the ice-ocean boundary propagates towards greater depths, we can expect a mushy region, composed of an ice matrix bathed in a salt rich liquid (brine). The brine may either be rejected back into the ocean, or it will be trapped in the ice leading to salt inclusions. This process could be efficient during an initial fast freezing phase of the ice shell and might have led to a non-homogeneous distribution of salts within the early ice shell.

Our approach is to explicitly model phase-change and mass flux processes at the ice-ocean boundary on the meso-scale. Meso-scale refers to the fact that the interface model has to be set up on a spatial scale that is much smaller than for a typical geodynamical model. Yet, the scale is still large enough to justify a continuum assumption. Hence, temperature, effective salt concentration and ice volume fraction are modelled as fields. This is in contrast to micro-scale models that might consider individual crystals. Similar approaches are known from the sea ice community (e.g., Buffo, 2018).

A fully self-consistent model that quantifies the influx of salts into the ice shell and applies to both the ice’s initial growth and a well-established, thick ice mantle remains to be developed. Yet, it is possible to investigate the gradual refreezing of a salty ocean, hence the ice-ocean interface propagation speed, in a simplified setting, namely formulated as an extended Stefan type problem that accounts for salt in the water, (e.g., Worster 2000). Our analysis allows us to constrain the refreezing rate as a function of depth and salt content in the ocean. This yields first insights into potentially realistic initial salt distributions in the ice shell as needed for macro-scale geodynamical models. 

In a second part, we employ a macro-scale geodynamical ice shell model to investigate how a certain potentially heterogeneous initial salt distribution in the ice would evolve and re-distribute with time (Fig. 1). In the latter we employ ice relevant rheological parameters (e.g., Durham and Stern, 2001).

We test under which conditions salts may remain stable close to the surface or are entirely mixed in the ice shell. The presence of salts may affect locally the melting temperature by lowering the melting point by several tens of Kelvin, the buoyancy force due to the higher density of salt rich inclusions, and possibly the viscosity within the ice shell. The mixing efficiency of compositional heterogeneities depends on the relative importance of chemical to thermal buoyancy. If the density contrast between salt rich inclusions and the surrounding ice is dominant such chemical anomalies will sink to the base of the ice shell. If on the other hand the thermal gradient can overcome the compositional gradient, material would mix and convection would redistribute chemical anomalies in the ice shell.

The distribution of salts in the ice shell of Europa is ultimately important for determining whether subsurface brine pockets or mushy regions may be present that if stable over geological time scales, may provide niches for ice shell habitability. Additionally, such regions may be detectable by upcoming measurements from REASON and RIME radars on board the Europa Clipper and JUICE spacecraft (Schroeder et al., 2016; Hussmann et al., 2017).

 

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