Cryolava Dome growth resulting from active eruptions on Jupiter's moon Europa

Europan domes are positive relief features that are typically circular to elliptical in planform shape, and have characteristic diameters <16 km. Although it cannot be ruled out that many of these domes may have been formed from the intrusion of diapirs into Europa's crust, a subset of domes have relatively smooth surfaces that do not mimic the surrounding terrain. These domes appear to obscure the preexisting terrain and have distinct margins which may be lobate or rounded. If all domes on Europa's surface represented structures where the icy crust had simply been punched up by diapiric intrusions, uplifts with these distinct morphologies would not be expected to exist. In this study, we revisit the hypothesis that a subset of europan domes formed in a manner similar to lava domes on Earth and Venus. Previously, we modeled dome formation as a consequence of the extrusion of viscous cryolava. However, that approach only allowed for the investigation of late-stage eruptive processes far from the vent and provided little insight into how cryovolcanic fluids may have arrived at the surface. Consideration of cryolava dome emplacement as fluids erupt onto Europa's surface is therefore pertinent. A volume flux approach, in which dome formation is modeled as fluid extrudes onto the surface at a constant rate, has been successfully applied to the formation of lava domes on Venus. That study showed that neglecting to consider changes in fluid rheology while a constant flux of lava is actively extruded onto the surface may result in overestimates, by several orders of magnitude, of initial lava viscosities at the time of eruption. Obtaining accurate viscosity estimates for Europa's cryovolcanic fluids is a critical step in understanding the properties of near-surface fluids that have participated in subsurface-surface exchange in the geologically recent past. To place improved constraints on the rheology and composition of europan cryolavas, and to better gauge the potential for dome formation on Europa via effusive eruptions, we apply this new volume flux approach to the formation of putative europan cryolava domes. We present a perturbation solution to the generalized form of the Boussinesq equation for fluid flow in a cylindrical geometry and explore dome formation while fluid is continuously extruding onto the surface. We find that at the time of eruption, dome-forming cryolavas may have had viscosities of 10(1)-10(3) Pa s. These viscosity values suggest that cryolavas may be briny slurries composed of a mixture of water, salts, and ice crystals, rather than pure water (viscosity similar to 10(-3 )Pa s) or simple brines (viscosities between 10(-3) and 10(-1) Pa s). Nevertheless, the derived bulk viscosities indicate that dome-forming cryolavas have a rheology more similar to basalt than typical higher-viscosity andesite to rhyolitic domeforming lavas on Earth. Several of the domes in our study may be connected to liquid reservoirs in Europa's crust, and subsurface-surface exchange may be ongoing today. As such, these domes represent compelling targets for multispectral imaging, radar sounding, and surface sampling by future missions to Europa.

Automated summary

By modeling the formation of cryolava domes on Europa, this study suggests that these domes may have been formed by viscous fluids composed of a mixture of water, salts, and ice crystals. The rheology of these cryolavas is more similar to basalt than to typical dome-forming lavas on Earth. Some of these domes may be connected to liquid reservoirs in Europa's crust, indicating ongoing subsurface-surface exchange.