Ice loss from the interior of small airless bodies according to an idealized model

TL;DR

This paper presents an analytic model estimating the timescale for interior ice loss in small airless bodies, revealing conditions under which ice can be retained over solar system timescales.

Contribution

It introduces a novel analytic solution for interior temperature and ice desiccation timescales in small airless bodies, accounting for latitude dependence and latent heat effects.

Findings

Half of the ice is lost after 11% of the total desiccation time.

Cold polar regions can preserve subsurface ice even when the core is dry.

Bodies larger than 10 km in the outer main belt likely retain interior ice over the solar system's age.

Abstract

Ice in main belt asteroids and Near Earth Objects (NEOs) 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 time scales. 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 NEOs: a semi-major axis in the outer belt or beyond, a mantle of very low thermal inertia, a young age, or a small and stable axis tilt.