QUESTION:
Due to the colder temperatures on Neptune, are the clouds that Hubble
sees made of ice or are they more like the clouds on earth?

ANSWER from Heidi Hammel on April 23, 1996:
Actually, this is not an "or" question.  The clouds we see on Earth
are often composed of water ice - especially the wispy cirrus clouds.
But to answer the question directly - there are two different types
of clouds of Neptune.

We think that the white clouds on Neptune are indeed made of ice, but
not water ice like on Earth.  They are most probably made of methane ice!
In contrast, the dark clouds, like the Great Dark Spot seen best by
Voyager in 1989, is more like a hole in the regular hazy cloud layer. You
might think of it as the "eye" of a very large hurricane, or rotating
storm.  The "clear" part is actually the part that is free of clouds, and
it is therefore darker than its surroundings.  So bright spots are clouds
made of ices, and dark spots are regions clear of ices.