What is comet snow?

[halfeye]

I mean, we know water snow is tiny six sided spikey flat things, but what are other substances' snows like? comets are primarily powders or ices from substances that are gases or liquids on Earth. The ices are presumably more or less solid, but what are the snows like, are they like hail, little balls of ice, or do they have crystal structures like water snow?

[DavidSh]

Snow isn't just the six-sided spikey flat things that you see in drawings. There are also needles, columns, simple flat plates, and combinations of these things. Take a look at the pictures at https://eprints.lib.hokudai.ac.jp/dspace/handle/2115/8672. The differences seem to depend on the temperature and humidity of the atmosphere where they were formed. They all tend towards the hexagonal symmetry that ice forms under normal atmospheric pressures.

There have been probes to comets, and they seem to show that surface is mostly dust. Probably surface ices are sublimated off as they approach the Sun. It's not clear that there is anything actually snow-like there. Possibly the "dirty snowball" is very dirty on the outside, and crushed to an iceball on the inside.

I suppose one could look up the crystalline structure of the ices that typically make up comets. At low pressures, for example, carbon dioxide has cubic symmetry, leading to cubes, octahedra, and cuboctahedra. (Like diamonds.)