Icy grains as a source of CO in comet 29P/Schwassmann-Wachmann 1

Abstract

A model of micron-sized icy grains is used to simulate the behaviour of solid material ejected from comet 29P/Schwassmann-Wachmann 1, which resides at ~6 AU from the Sun. It is demonstrated that a population of grains composed primarily of water ice can release their ice content on timescales of order 106 s. This justifies the idea that icy grains can serve as an extended source of more volatile species, as was assumed when evidence for a coma source of CO was found in radio observations of this comet. The model also predicts short lifetimes for the icy grains inside 3 AU from the Sun, removing the extended appearance of the CO production at smaller heliocentric distances, as seen in comet Hale-Bopp. A grain population sublimating in a time interval similar to that derived for the coma source of CO in comet 29P/Schwassmann-Wachmann 1 must have most of its mass concentrated to small (with radii <5 μm) or porous grains. Those grains must carry a significant ice content, with an refractory to ice ratio of less than ~30%. By introducing continuous grain fragmentation the extended source activity can be reproduced for a grain population that includes larger particles. This would call for grains to split into smaller pieces on a timescale of ~ s.