Limits to Ice on Asteroids (24) Themis and (65) Cybele

TL;DR

This study uses optical spectra to investigate water ice on asteroids Themis and Cybele, setting limits on sublimation and surface composition, and proposing impact excavation as a potential exposure mechanism for buried ice.

Contribution

It provides new observational constraints on the presence and properties of water ice on main-belt asteroids, and introduces an impact excavation model for ice exposure.

Findings

No emission lines detected, limiting water production rates to <400 kg/s.

Surface ice, if present, must be relatively clean and cover less than 10% of the surface.

Impact excavation could expose buried ice, maintaining a steady-state presence over thousands of years.

Abstract

We present optical spectra of (24) Themis and (65) Cybele, two large main-belt asteroids on which exposed water ice has recently been reported. No emission lines, expected from resonance fluorescence in gas sublimated from the ice, were detected. Derived limits to the production rates of water are < 400 kg/s (5{\sigma}), for each object, assuming a cometary H2O/CN ratio. We rule out models in which a large fraction of the surface is occupied by high albedo ("fresh") water ice because the measured albedos of Themis and Cybele are low (0.05 - 0.07). We also rule out models in which a large fraction of the surface is occupied by low albedo ("dirty") water ice because dirty ice would be warm, and would sublimate strongly enough for gaseous products to have been detected. If ice exists on these bodies it must be relatively clean (albedo >0.3) and confined to a fraction of the Earth-facing surface <10%. By analogy with impacted asteroid (596) Scheila, we propose an impact excavation scenario, in which 10 m scale projectiles have exposed buried ice. If the ice is even more reflective (albedo >0.6) then the timescale for sublimation of an optically thick layer can rival the 10^3 yr interval between impacts with bodies this size. In this sense, exposure by impact may be a quasi steady-state feature of ice-containing asteroids at 3 AU.