QUESTION:
If it is true, as I have read recently, that water from space is falling
pretty much continuously to Earth, then what about Mars? Wouldn't the 
same be true there (although the water would be falling as snow or 
ice)? Where does all the water from the comets go when they hit Mars?

ANSWER from Mary Urquhart on July 14, 1997:
You are quite right. If small comets are constantly hitting the atmosphere
of the Earth then they could also be hitting Mars.  Whether or not they
are depends on the source of the comets. I don't think that the scientific
community in general is convinced that small comets are in fact hitting
the atmosphere of the Earth.  The debate is still going on. However if
they are hitting the Earth and Mars these small comets could be a
significant source of water for Mars, water for which we don't have any
evidence.  In that case, the question of what happened to the water is an
unanswered one that will be an interesting area of research. 

It may very well be that these small comets (once again, if they exist)
are only hitting the Earth and not hitting Mars at all.  If they are
hitting Mars, portions of the tiny boulder-sized comets might make it to
the surface of Mars as icy meteorites.  They probably wouldn't make it
snow. 

Mary Urquhart                                    
Laboratory for Atmospheric and Space Physics  
University of Colorado at Boulder

ANSWER from Bruce Jakosky on July 9, 1997:
Icy comets that originate in the outer solar system pass through the inner
solar system occasionally.  And, every now and then, one hits a planet.
There is lots of evidence for impacts onto the Earth, Venus, and Mars. So,
certainly, icy comets are hitting Mars.

When one hits, the water (and other gases, like carbon dioxide) would
immediately evaporate into the atmosphere as a result of all of the heat.
Once in the atmosphere, the water can be carried around by the winds and can
freeze out on the polar caps.  In other words, once the water is on Mars, it
acts just like any other water on Mars.  So, the overall effect is to add
water to the planet.

Just how much water has been added this way over time is uncertain.  In all
likelihood, though, the amount of water added by comets is less than was
present within the solid planet and released to the surface.  While water of
cometary origin may be present on Mars, it probably is not the major amount
of water on the planet.