QUESTION:
You say that most clouds on Mars are made of water ice. Why just water ice?
Can you define the composition of these clouds? I think it is important for
any future human settlement of Mars. It would be relatively easy to obtain
water from fog by filtering. But if the fog is made of carbon dioxide, such
methods would be impossible.

ANSWER from Mike Mellon on September 13, 1997:
There are two types of condensation clouds on Mars; water ice and carbon
dioxide ice. Which type forms will depend on the temperature, pressure and
humidity of the atmosphere at a given altitude. The Martian atmosphere
typically contains very little moisture so water ice clouds will generally
be thin, like cirrus clouds on Earth.

Water as a resource for human settlement, I agree, is very important.  Some
of the suggested sources of Martian water have been atmospheric water vapor
(extracted by a condenser of some sort), ground ice (melted and filtered
from the soil), polar ice (from surface ice deposits) or ground water
(obtained from wells that would be many kilometers deep). For a variety of
reasons atmospheric water vapor or ground ice may be the most accessible
sources of water. Water ice clouds are probably at too high an altitude to
be useful.

Also keep in mind that whatever the source of water, it needs to be reliable
and steady. Waiting for a fog may leave the colonists very thirsty.