Ice Loss From the Interior of Small Airless Bodies According to an Idealized Model

Abstract

Ice in main belt asteroids and near-Earth objects is of scientific and resource exploration interest, but small airless bodies gradually lose their ice to space by outward diffusion. Here we quantitatively estimate the time it takes a porous airless body to lose all of its interior ice, based on an analytic solution for the interior temperature field of bodies in stable orbits. Without latent heat, the average surface temperature, which is lower than the classic effective temperature, is representative of the body interior and hence an appropriate temperature to evaluate desiccation timescales. In a spherically averaged model, an explicit analytic solution is obtained for the depth to ice as a function of time and the time to complete desiccation. Half of the ice volume is lost after 11% of this time. A bilobate structure emerges from the strong latitude dependence of desiccation rates. Cold polar regions can harbor subsurface ice, even when the body center does not. Latent heat retards ice loss, and we obtain a succinct expression for the temperature difference between the surface and the ice. In the outer main belt, nearly all bodies 10 km in size or larger should have been able to retain ice in their interiors over the age of the solar system. Each of the following factors favors the presence of ice inside near-Earth objects: a semimajor axis in the outer belt or beyond, a mantle of very low thermal inertia, a young age, or a small and stable axis tilt.