Expert: Tom Whiting - 2/3/2009

Question
Hi,
What state does water appear in the form of on Mercury, Venus, and Mars.
My guesses:
Mercury- Liquid and gas
Venus- Gas or not at all
Mars- I'm not really sure

Also, how would Earth's hydrological cycle be different if its orbit was outside the orbit of Mars?
My guess:
Water would not be able to exist in all 3 forms as it does now.  It may only be able to exist in either an ice form or a gas form.

Thank you!
Tracie

Answer
Hi Tracie,
First right off the top, planet Earth is the only body in
the Solar System that can sustain liquid water on the surface for
any length of time. Why? It takes both atmospheric pressure and
the correct temperature range (32-212 F) to maintain liquid water on the surface. So any body without a substantial atmosphere is
eliminated for liquid water ON the surface....Mercury, the Moon, and even Mars, which only has an atmosphere equal to about 100,000 feet on the Earth...not enough air pressure at the surface for liquid water ON the surface.  (However, in the distant past, it's thought that Mars had a thicker atmosphere, thus the talk and research goes on about Mars' distant past.)  But this doesn't preclude underground water which CAN exist under ground pressure, but I'm going to assume
that you mean just...surface water.

So that only leaves water vapor (gaseous state) and water ice
(solid state).  We also know that the surfaces of Mercury, Venus
and even the sunlit side of the moon are well above 212 degrees,
(but there are craters at the poles on Mercury and the Moon that
never see sunshine, so there could be water comet ice buried in
polar craters on both the Moon and even Mercury....we just don't
know as yet, but occasionally you hear and read about possible
water ice on the moon, and even on Mercury.
So that leads to the following answers:
Mercury - water ice only (no atmosphere precludes water vapor)
Venus - too hot (950 degrees F) to have even polar crater ice
so water vapor in the atmosphere is the only possible answer,
with Venus hot, thick atmosphere.
Moon - water ice in polar craters (again, no atmosphere eliminates
water vapor).
Mars -both water vapor in the thin atmosphere, and polar ice caps containing some water ice, although it's thought that most of the polar ice caps are frozen carbon dioxide (dry ice).
And perhaps there is liquid underground water, but we just don't
know as yet about that one....we're working on it. That's why we
are exploring Mars right now.  We also know, thanks to the Phoenix
mission, at the higher latitudes there exists a layer of permafrost...solid water ice...just below the surface.


Yes, if the Earth (in it's present condition) were suddenly moved
to beyond Mars' orbit, then everything would freeze up, so the
Earth would be in severe Ice Age condition, so water ice and water vapor in the atmosphere would be the only answer.
Of course, who's to say that the Earth, if originally formed beyond
Mars' orbit, who says that it would have evolved to it's present
state?  It probably wouldn't have, that's why I hate those weird,
impossible, hypothetical questions because if you change one
condition, who is to say other factors might be different too?
They probably would be.
Hope this helps,
Clear skies,
Tom Whiting
Erie, PA

PS...Why is water in any form so important?  Because water is heavy;
we can't be hauling water out to the Moon and Mars for several astronauts for several years.  Any permanent base has to have water
there to begin with, or else we can't go there....we can't haul all
that needed water, it has to be at that location, typically in
the form of water ice, otherwise we can't go there. We can haul dry
food stuffs and oxygen, but not water too. That's why the discovery
of water ice is so important for a permanent lunar base, and an
eventual Mars trip.  So now you know the rest of the story.