On the dust zoning of rapidly rotating cometary nuclei

The effects of nuclear rotation on the surface of a cometary nucleus (a comet at 1 AU that is H2O dominated and has a radius of 1 km) are considered. It is shown that this dust does not accumulate uniformly on the surface, which here is considered spherical. While dust particles in the two polar cap regions and an equatorial belt remain at rest on the surface, those in two midlatitude bands migrate toward the equator, stopping at the two low latitudes to form dust ridges. As the nucleus spins up, both the polar caps and the equatorial belt shrink in size, and the dust ridges move toward the equator, eventually spinning off the dust from the nucleus when the nuclear rotation period is less than about 3.3 hr. For larger particles for which the gas buoyancy is negligible, migration takes place only if the rotation period is not significantly larger than the critical value of 3.3 hr or if the surface friction is abnormally small.