Brine Migration and Impact‐Induced Cryovolcanism on Europa

Abstract

Despite evidence for plumes on Jupiter's moon Europa, no surface features have been definitively identified as the source of the plumes to date. Furthermore, it remains unknown whether the activity originates from near-surface water reservoirs within the ice shell or if it is sourced from the underlying global ocean. Here we investigate brine pocket migration, studied previously in the context of sea ice on Earth, as a process for transporting brine along thermal gradients. We show that the fracture system located in the center of Europa's Manannán crater is consistent with the formation of a subsurface brine reservoir. After the initial impact, residual aqueous melt concentrated via brine pocket migration as the target material cooled. Freezing and overpressurization then resulted in a cryovolcanic eruption. The volume of the emptied reservoir and the critical composition at the end of migration provide further constraints on the average salinity of Europa's ice shell. Plain Language Summary Jupiter's satellite Europa has a subsurface ocean covered by an icy shell. We show how small pockets of brine can migrate within the ice from colder areas to warmer areas. This can happen even at very low temperatures, below the point where pure water would freeze, because the water becomes saltier and saltier as it migrates. By looking at an impact crater on Europa, which was initially warm in the center and cooled inward from its colder surroundings, we can study how the water migrated toward the center and formed a central water reservoir. As the final water pocket at the center of the crater started to freeze, the increasing pressure lead to a cryovolcanic eruption that emplaced brine onto the surface to form a prominent "spider" feature before the ice collapsed into the cavity below. Using a digital terrain model of the crater and collapse feature, we estimate how much water erupted and how salty Europa's ice shell is.

Radar‐Sounding Characterization of the Subglacial Groundwater Table Beneath Hiawatha Glacier, Greenland

“…The underlying thawed layer might also be trapped by frozen layers above it, a feature observed in firn hydrology (Koenig et al., 2014; Chu et al., 2018). This layer could be liquid because it is confined and pressurized (Steinbrügge et al., 2020), due to refreezing, heat advected by subglacial water flow or higher salinity (Rutishauser et al., 2018).…”

The Salty Secrets of Icy Ocean Worlds

“…Materials retained in the ice could provide clues to the ocean's thermal and chemical evolution, its current composition, and the possible presence of life. Demonstrating mechanisms for extensive material entrainment into the ice would also support the existence of brine reservoirs near Europa's surface, hypothesized to explain the chaotic terrains covering much of Europa's surface (Collins et al., 2000; Kattenhorn & Prockter, 2014; Schmidt et al., 2011; Steinbrügge et al., 2020). Such near‐surface brine reservoirs could themselves be abodes for life, and might be accessible by a future landed spacecraft (Blanc et al., 2020; Hand et al., 2017; Pappalardo et al., 2013).…”

“…The ready percolation of brines along ice grain boundaries (Figure 1; De La Chapelle et al., 1999; McCarthy et al., 2013), should allow near‐surface brines to drain into the ocean on 10,000‐year timescales, if a permable pathway exists through the entire ice shell (Gaidos & Nimmo, 2000; Kalousova et al., 2014; Sotin et al., 2002). In the presence of thermal gradients, brines can also migrate laterally in pockets that then connect to larger reservoirs (Schmidt et al., 2011; Steinbrügge et al., 2020). Through such processes, the salinity of the ice shell may shape the dynamics of the ice and geologic features on the surface.…”

“…The freeze-in also does not account for melting due to tidal heating or solid state convection. However, the high predicted concentrations may serve as a benchmark for regions of the ice where transport of fluids occurs rapidly and is limited in spatial extent (e.g., Schmidt et al 2011;Steinbrügge et al 2020).…”

Delay Doppler SAR Focusing and Quantitative Quality Control of the Radar for Europa Assessment and Sounding: Ocean to Near-Surface (REASON) Sounding Data Product

“…3 and 4. Features that may generate high Doppler frequency reflections within the upper three kilometers of the ice shell are likely to be related to corner reflectors such as those at the edge of eutectics [35] or the bottom of cracks [36,37]. However, artefacts introduced through the focusing of these features with large apertures, if they happen to be imaged near closest approach, can be used as an additional line of evidence that they are indeed point targets in the subsurface and not surface clutter.…”