Ice melting and downward transport of meltwater by two‐phase flow in Europa's ice shell

Abstract

With its young surface, very few impact craters, and the abundance of tectonic and cryovolcanic features, Europa has likely been subjected to relatively recent endogenic activity. Morphological analyses of chaos terrains and double ridges suggest the presence of liquid water within the ice shell a few kilometers below the surface, which may result from enhanced tidal heating. A major issue concerns the thermal/gravitational stability of these water reservoirs. Here we investigate the conditions under which water can be generated and transported through Europa's ice shell. We address particularly the downward two‐phase flow by solving the equations for a two‐phase mixture of water ice and liquid water in one‐dimensional geometry. In the case of purely temperate ice, we show that water is transported downward very efficiently in the form of successive porosity waves. The time needed to transport the water from the subsurface region to the underlying ocean varies between ∼1 and 100 kyr, depending mostly on the ice permeability. We further show that water produced in the head of tidally heated hot plumes never accumulates at shallow depths and is rapidly extracted from the ice shell (within less than a few hundred kiloyears). Our calculations indicate that liquid water will be largely absent in the near subsurface, with the possible exception of cold conductive regions subjected to strong tidal friction. Recently active double ridges subjected to large tidally driven strike‐slip motions are perhaps the most likely candidates for the detection of transient water lenses at shallow depths on Europa.

Water generation and transport below Europa's strike-slip faults

“…Our results suggest that it is mainly the thermal state and the viscosity structure of the ice layer underneath the fault that control the development of the R‐T instabilities, the threshold value of viscosity being approximately 5 × 10 13 Pa s. Once viscosity in the zone underneath the fault drops below this value, the R‐T instabilities form and drain meltwater efficiently to the base of the shell. In this regard, our findings are complementary to those of Kalousová et al [], where the importance of the thermal state is stressed for an alternative drainage mechanism—the two‐phase flow. In Kalousová et al [] this mechanism is reported to allow for very effective meltwater drainage once the temperature in the ice shell reaches the pressure‐melting point in the whole zone underneath the reservoir, and the melt fraction exceeds the percolation threshold.…”

“…However, the methodology developed for terrestrial glaciers cannot be directly applied to planetary ice shells due to number of important differences in the physical setting. These include the absence of subglacial drainage system, closure of the hydrological network from above, limited crack initiation, insufficient meltwater supply, and 1 order of magnitude difference in the thicknesses of ice layers [ Kalousová et al , ]. Although hydrofracturing cannot be completely ruled out as one of the drainage mechanisms operating on Europa, we do not consider it in this study, mainly due to the lack of observational constraints on the drainage system within the ice shell.…”

“…In our earlier work [ Kalousová et al , ], we have advocated an alternative mechanism—meltwater transport by two‐phase flow—in a close analogy with terrestrial magma generation and transport [e.g., McKenzie , ; Scott and Stevenson , ; Šrámek et al , ]. However, the melt transport by microporous (two‐phase) flow can only occur if there is enough melt to form an interconnected network of veins within the ice shell which requires that both phases, ice and liquid water, are at melting temperature.…”

“…Here we focus on the role of this particular mechanism in the strike‐slip faults context assuming a two‐dimensional setting. The limited efficiency of two‐phase flow drainage in this context [ Kalousová et al , ] allows us to simplify the two‐phase model by adopting an impermeable limit, i.e., by neglecting melt percolation and assuming that all meltwater remains locked within the ice matrix and is only advected by the ice flow.…”

Radar attenuation in Europa's ice shell: Obstacles and opportunities for constraining the shell thickness and its thermal structure

“…Morphological analyses of chaos terrains [ Schmidt et al , ] and double ridges [ Dombard et al , ] suggest the presence of shallow lenses of water or brines, perched only a few kilometers below Europa's cold brittle surface. However, it is not clear if liquid water can accumulate in the upper part of Europa's ice shell for long enough [e.g., Tobie et al , ; Kalousová et al , , ]. Nevertheless, recent detections of water vapor over Europa's south pole [ Roth et al , ; Sparks et al , ] suggest the existence of an ongoing plume activity similar to that observed at Enceladus [e.g., Hansen et al , ], and detection of salts [ McCord et al , ; Brown and Hand , ] and hydrated sulfuric acid [ Carlson et al , ] on Europa's surface indicates the presence of chemical elements essential for biology.…”