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).

Polymerization of Building Blocks of Life on Europa and Other Icy Moons

“…The occurrence of high temperatures and alkaline solutions has been observed in serpentine-hosted hydrothermal systems (Kelley et al, 2001(Kelley et al, , 2005Suda et al, 2014). Similar hydrothermal systems may prevail at the seafloor of icy moons (Vance et al, 2007). However, high temperatures also favor decomposition of organic compounds (e.g., Larralde et al, 1995;Levy and Miller, 1998).…”

“…Moreover, amino acids can be produced through a process of shock synthesis from cometary material impacting the icy surface (Martins et al, 2013). If Europa's seafloor has hydrothermal systems (Vance et al, 2007), they could be candidate sites that supply amino acids to the surface through tectonic deformation of the ice crust. Furthermore, synthetic schemes for the production of amino acids in Europa's subsurface ocean have been proposed (Abbas and Schulze-Makuch, 2008).…”

10th Applied Isotope Geochemistry Conference

“…Recent evidence indicates that liquid water was once abundant at surface of Mars, and also that serpentinization was and may continue to be an important process there (e.g., Michalski et al, 2013). Additional evidence points to the contemporary presence of liquid water in icy satellites, with Europa and Enceladus emerging as places that may host serpentinization-derived life to the present day (Vance et al, 2007). Assuming the possibility that the observed methane and ethane degassing from the Cabeço de Vide serpentinized ultramafic geological environment may also occur in the serpentinized rocks on Mars, we might speculate on the significance of Cabeço de Vide as a Mars analog site.…”

Subsurface Water Oceans on Icy Satellites: Chemical Composition and Exchange Processes

“…However, on large satellites (Titan, Ganymede, Triton), hydrothermal circulation was unlikely because of impermeable rocks and separation of oceans from silicate mantles by high-pressure ices. On Europa, hydrothermal circulation could have affected only suboceanic rocks, and on smaller bodies (Enceladus) circulation may have affected the entire silicate mantle (Vance et al 2007;Glein et al 2008, and Sect. 4 of this paper).…”

“…The depth to which microfractures form may serve as an upper limit for the depth of water-rock interaction. Vance et al (2007) adopt a model for the thermally-induced formation of microfractures controlled by the development of residual stresses at grain boundaries. In a cooling rock, anisotropic thermal stresses will increase along grain boundaries.…”