Tushar Mittal (Penn State) – Regimes of hydrothermal plumes on icy ocean worlds​

The icy ocean worlds (e.g., Enceladus, Europa) are promising astrobiological targets since they potentially have regions with active water-rock interaction and hydrothermal activity at present-day. However, these habitable environments are typically overlain by a thick (>10 km) ocean and ice shell. Thus, to interpret surface observations, we need to understand the efficiency and the timescale over which fluids and particles get transported from the ocean-- core to the surface. In this study, we use high-resolution (~ 40-80m grid resolution) fluid dynamical simulations to analyze hydrothermal plume dynamics in an icy ocean world context. Our results significantly expand upon previous work by Goodman et al. (2004, 2012) by considering a larger range of: (i) hydrothermal heat fluxes (in particular lower heat fluxes < 100 W/m2 - consistent with estimates from tidal dissipation models), (ii) planetary rotation rates, and (iii) plume latitudes (polar to equatorial). We also consider the possibility of rapid vertical transport by bubble rich plumes.

We find that, in contrast to typical terrestrial hydrothermal plumes, baroclinic eddies play  a critical role in the rotational plume dynamics in a deep icy ocean worlds in presence of minimal ocean stratification. The eddies efficiently transport heat laterally away from the vent location on a timescale faster than plume rise timescale. Consequently, a buoyant rotating plume rises much more slowly compared to a non-rotating plume. Using scaling results calibrated with the simulations, we find that the transit time across Enceladus's ocean for highest 1% of hydrothermal plume particles is at least ~ 100 yrs, if not significantly longer. This timescale significantly exceeds the months-to-a-few-years estimate based on a core hydrothermal activity model for silica nanoparticles observed in Enceladus’s plume. Thus, alternative models for silica nanoparticle formation need to be considered given the physical implausibility of fast transit times. Although bubbles can provide additional plume buoyancy, we find that unrealistically large bubbles are needed for rapid across-ocean transit. Our results also have significant implications for interpreting the measured geyser fluid compositions (e.g., methane, hydrogen, CO2) in the context of seafloor habitability.