Outcrop-to-outcrop Hydrothermal Circulation under Ocean World Gravity Conditions

Geophysical and geochemical observations indicate that ocean worlds (OWs) such as Europa and Enceladus are likely to sustain active hydrothermal systems resulting from interactions between a sub-ice ocean, silicate core, and heat. Earth's seafloor hydrothermal systems are driven by a variety of heat sources, and result in a range of flow rates and discharge temperatures; these systems may provide useful analogs for processes on OWs. We compare simulations of a well-understood, seafloor hydrothermal system on Earth, driven by lithospheric cooling, to a similar system operating under lower gravity conditions. The Earth system includes recharge and discharge that occur through rock outcrops separated by 50 km, with reaction temperatures near 65°C in a shallow crustal aquifer, and discharge of hydrothermal fluid at ~10 kg/s. Simulations completed to date, with heat input of 180 mW/m2 at the lower boundary, show that the same system operated under low-gravity conditions can have higher reaction and discharge temperatures, because there is a smaller pressure difference between recharging and discharging ends of the hydrothermal syphon. Flow restriction at the discharge end of this kind of system, as would be created by heterogeneity in connectivity between a planetary core and the overlying ocean, helps to increase vent temperatures. As a result, water-rock interaction and venting temperatures 90C, as inferred to occur on Enceladus, can result from relatively shallow circulation that extends to just a few kilometers into the silicate core, provided there is a sufficient thermal gradient. Somewhat deeper circulation should result in commensurately higher reaction temperatures, or similar temperatures with lower heat input. Understanding links between internal heating, buoyancy, and flow restriction is helpful for predicting the hydrothermal processes on OWs, including those that contribute to conditions suitable for creating and sustaining microbial habitat.