title: Hydrothermal systems in small ocean planets.
abstract: We examine means for driving hydrothermal activity in extraterrestrial oceans on planets and satellites of less than one Earth mass, with implications for sustaining a low level of biological activity over geological timescales. Assuming ocean planets have olivine-dominated lithospheres, a model for cooling-induced thermal cracking shows how variation in planet size and internal thermal energy may drive variation in the dominant type of hydrothermal system-for example, high or low temperature system or chemically driven system. As radiogenic heating diminishes over time, progressive exposure of new rock continues to the current epoch. Where fluid-rock interactions propagate slowly into a deep brittle layer, thermal energy from serpentinization may be the primary cause of hydrothermal activity in small ocean planets. We show that the time-varying hydrostatic head of a tidally forced ice shell may drive hydrothermal fluid flow through the seafloor, which can generate moderate but potentially important heat through viscous interaction with the matrix of porous seafloor rock. Considering all presently known potential ocean planets-Mars, a number of icy satellites, Pluto, and other trans-neptunian objects-and applying Earth-like material properties and cooling rates, we find depths of circulation are more than an order of magnitude greater than in Earth. In Europa and Enceladus, tidal flexing may drive hydrothermal circulation and, in Europa, may generate heat on the same order as present-day radiogenic heat flux at Earth's surface. In all objects, progressive serpentinization generates heat on a globally averaged basis at a fraction of a percent of present-day radiogenic heating and hydrogen is produced at rates between 10(9) and 10(10) molecules cm(2) s(1).
authors.affiliation: Astrobiology Program and Department of Earth & Space Sciences, University of Washington, Seattle, Washington, USA. svance@ess.washington.edu
authors.forename: Steve
authors.initials: S
authors.lastname: Vance
authors.forename: Jelte
authors.initials: J
authors.lastname: Harnmeijer
authors.forename: Jun
authors.initials: J
authors.lastname: Kimura
authors.forename: Hauke
authors.initials: H
authors.lastname: Hussmann
authors.forename: Brian
authors.initials: B
authors.lastname: Demartin
authors.forename: J Michael
authors.initials: JM
authors.lastname: Brown
mesh_terms: D018533:Biomass; D018534:Earth, Planet; D004736:Energy-Generating Resources; D018559:Exobiology; D005118:Extraterrestrial Environment; D006358:Hot Temperature; D006859:Hydrogen / Q000737:chemistry; D009792:Oceans and Seas; D000069497:Origin of Life; D016083:Planets; D016082:Solar System
chemical_list: D006859:Hydrogen