Predictions of depth-to-ice on asteroids based on an asynchronous model of temperature, impact stirring, and ice loss

•Developed asynchronous numerical method to better quantify ice loss from asteroids.•Expected depth to ice at the poles of Ceres is less than one centimeter.•Ice within GRaND sensing depth predicted poleward of 60° latitude on Ceres.•Ground ice can be within impactor depths on Main Belt Comet 133P/Elst-Pizarro.

Water ice near the surface of main belt asteroids is gradually lost to space. A mantle of low thermal conductivity causes large surface temperature amplitudes, and thus increased cooling by thermal re-radiation, lowering temperatures well below the fast-rotator limit. A computational barrier for modeling this ice loss is the multi-scale character of the problem: accurate temperatures require many time steps within a solar day, but ice retreats slowly over billions of years. This barrier is overcome with asynchronous coupling: Models of temperature, ice loss, and impact stirring each use their own time steps and are coupled with one another. The model is applied to 1 Ceres and 7968 Elst-Pizarro. On Ceres, ice can be expected in the top half meter poleward of 60° latitude on both hemispheres, even if excursions of the axis tilt took place, and even in the presence of impact gardening. At the poles, ice can be expected within a centimeter of the surface. The retreating ice crust leads to emission of water from the surface, mainly at the equator; the gradually retreating ice supplies a water exosphere less dense than has been observed by the Herschel telescope. For Main Belt Comet Elst-Pizarro, depths to ice depend on the properties of the surface mantle. For a dust mantle estimated depths are on the order of a decimeter; for a rocky surface the depth at the pole is on the order of one meter. Hence, it could have been activated by a small impact that exposed buried ice.