Sustaining Hydrothermal Circulation With Gravity Relevant to Ocean Worlds

Abstract

Some ocean worlds may sustain active, seafloor hydrothermal systems, but the characteristics and controls on fluid‐heat transport in these systems are not well understood. We developed three‐dimensional numerical simulations, using a ridge‐flank hydrothermal system on Earth as a reference, to test the influence of ocean world gravity on fluid and heat transport. Simulations represented the upper ∼4–5 km below the seafloor and explored ranges of: heat input at the base, aquifer thickness, depth, and permeability, and gravity values appropriate for Earth, Europa, and Enceladus. We tested when a hydrothermal siphon could be sustained and quantified consequent circulation temperatures, flow rates, and advective heat output. Calculations illustrate a trade‐off in energy between the reduction of buoyancy at lower gravity, which tends to reduce the primary forces driving fluid circulation, and the concomitant reduction in secondary convection, which consumes available energy. When a siphon was sustained under lower gravity, circulation temperatures tended to increase modestly (which should lead to more extensive geochemical reactions), whereas mass flow rates and advective heat output tended to be reduced. Deeper subseafloor circulation resulted in higher temperatures and flow rates, with a deeper, thin aquifer being more efficient in removing heat from the rocky interior. Water‐rock ratios were lower when gravity was lower, as was the efficiency of heat extraction, whereas the time required to circulate the volume of an ocean‐world's ocean through the seafloor increased. This may help to explain how small ocean worlds could sustain hydrothermal circulation for a long time despite limited heat sources.

Plain Language Summary

Ocean worlds are planetary bodies that have a liquid ocean, often under an icy shell or within the rocky interior. In Earth's solar system, several moons of Jupiter and Saturn are ocean worlds. Some ocean worlds are thought to have hydrothermal circulation, where water, rocks, and heat combine to drive fluids in and out of the seafloor. Hydrothermal circulation would impact the chemistry of the water and rock of ocean worlds, and could help life to develop deep below the icy surface. This study shows results from computer simulations of hydrothermal circulation, based on a well‐understood system on Earth, to measure the influence of lower gravity at values appropriate for ocean worlds smaller than Earth. The simulations with ocean world (lower) gravity result in fluid circulation much like that occurring on and below Earth's seafloor, but with several important differences. Lower gravity reduces buoyancy, so fluids don't become as light when heated, and this reduces flow rates. This can raise temperatures in the circulating fluid, which could allow more extensive chemical reactions, perhaps including those that sustain life. Lower flow means less heat transport, and this could help these flows to last longer in an ocean world.

Key Points

• Outcrop‐to‐outcrop hydrothermal circulation, driven by heating from below, can be sustained under ocean‐word gravity

• Lower gravity tends to generate higher circulation temperatures, lower mass fluxes, and reduced heat output

• Lower gravity also results in lower water‐rock ratios and should create more evolved fluids and a longer circulation time