<<What about her other question? Is ice really that reflective? If not, what's likely to cause the glow? And BTW, is it safe to assume it would be water ice?>>

My own limited understanding of the matter is that what we're seeing from Holmes is dust, not vaporized ice. The bright yellow color is characteristic of dust envelopes (gas envelopes tend to be fainter but more colorful, with blue or green hues depending on the compounds involved). In fact, the yellow color is from reflected sunlight, though cometary dust itself is very dark. However, if you put a *lot* of something poorly reflective together, you can end up with something very bright.

In any event, if water ice is involved, it's probably a secondary component only.

__________________
"Call me old-fashioned, but I think fire is magic. And it scares me a lot."

--The State