Micrometeorite impact annealing of ice in the outer Solar System

Abstract

The spectra of water ice on the surfaces of icy satellites and Kuiper Belt Objects (KBOs) indicate that the surface ice on these bodies is in a crystalline state. This conflicts with theoretical models, which predict that radiation (galactic cosmic rays and solar ultraviolet) should damage the crystalline structure of ice on geologically short timescales. Temperatures are too low in the outer Solar System for the ice to anneal, and reflectance spectra of these bodies should match those of amorphous solid water (ASW). We assess whether the kinetic energy deposited as heat by micrometeorite impacts on outer Solar System bodies is sufficient to anneal their surface ice down to a near-infrared optical depth ( 350 μ m ) . We calculate the kinetic energy flux from interplanetary micrometeorite impacts, including gravitational focusing. We also calculate the thermal diffusion of impact heat in various surfaces and the rate of annealing of ice. We conclude that the rate of annealing from micrometeorite impacts is sufficient to explain the crystallinity of ice on nearly all the surfaces of the saturnian, uranian and neptunian satellites. We discuss how the model can be used in conjunction with spectra of KBOs to probe dust fluxes in the Kuiper Belt.