Hydrothermal Systems in Small Ocean Planets

Kurzfassung

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 time scales. 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 — e.g. high or low temperature, or chemically driven. 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 hydrothermally significant heat through viscous interaction with a 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 Earthlike material properties and cooling rates, we find depths of circulation are more than an order of magnitude greater than in Earth. In Europa, Enceladus, Titania, Oberon, and Triton, 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 10⁹ and 10¹⁰ molecules cm^-2 in the absence of crustal rejuvenation and under our assumed ideal conditions. We examine how different kinetic considerations affect the longevity of such systems. These values lie at the limiting extreme capable of sustaining life on Earth. The absence of macronutrient delivery, specifically phosphorus and electron acceptors (CO2, NO3-, etc.), may further inhibit the potential for biology under these conditions. Serpentinization accompanying the initial onset of ocean/seafloor interaction, on the other hand, enjoys much shorter lifetimes on the order of 10⁶ to 10⁸ years depending on temperature and fluid accessibility. The concomitant heat and hydrogen production is on the order of that encountered in hydrothermal systems on Earth today, where such conditions sustain biology in the absence of sunlight.