Liquid water on cold exo-Earths via basal melting of ice sheets

Liquid water is a critical component of habitability. However, the production and stability of surficial liquid water can be challenging on planets outside the Habitable Zone and devoid of adequate greenhouse warming. On such cold, icy exo-Earths, basal melting of regional/global ice sheets by geothermal heat provides an alternative means of forming liquid water. Here, we model the thermophysical evolution of ice sheets to ascertain the geophysical conditions that allow liquid water to be produced and maintained at temperatures above the pressure-controlled freezing point of water ice on exo-Earths. We show that even with a modest, Moon-like geothermal heat flow, subglacial oceans of liquid water can form at the base of and within the ice sheets on exo-Earths. Furthermore, subglacial oceans may persist on exo-Earths for a prolonged period due to the billion-year half-lives of heat-producing elements responsible for geothermal heat. These subglacial oceans, often in contact with the planet's crust and shielded from the high energy radiation of their parent star by thick ice layers, may provide habitable conditions for an extended period.

A Positive Feedback Between Crustal Thickness and Melt Extraction for the Origin of the Martian Dichotomy

A North/South difference in crustal thickness is likely at the origin of the Martian dichotomy in topography. Recent crustal thickness maps were obtained by inversion of topography and gravity data seismically anchored at the InSight station. On average, the Martian crust is 51-71 km thick with a southern crust thicker by 18-28 km than the northern one. The origin of this crustal dichotomy is still debated although the hypothesis of a large impact is at present very popular. Here, we propose a new mechanism for the formation of this dichotomy that involves a positive feedback between crustal growth and mantle melting. As the crust is enriched in heat-producing elements, the lid of a one-plate planet is hotter and thinner where the crust is thicker, inducing a larger amount of partial melt below the lid and hence a larger rate of melt extraction and crustal growth. We first demonstrate analytically that larger wavelength perturbations, that is, hemispherical perturbations, grow faster because smaller wavelengths are more attenuated by thermal diffusion. We then use a parameterized thermal evolution model with a well-mixed mantle topped by two different lids characterized by their thermal structures and thicknesses to study the growth of the crust in the two hemispheres. Our results demonstrate that this positive feedback can generate a significant crustal dichotomy.