QUESTION:
Do you know where the frozen water is on Mars? Is it drinkable?

ANSWER from Mike Mellon on February 16, 1997:
It is possible that drinkable water exists in abundance deep beneath 
the martian surface. The geologic evidence suggests that a large amount 
of water once existed at the surface. Since that time the water may 
have all seeped into the ground and may now reside deep in the 
planet's crust.

The next question is how to get at it... How deep must we drill and 
where? The surface of Mars is cold, rather like Antarctica, and is 
globally covered by permafrost (permanently frozen ground). Any water 
present near the surface would be frozen solid. To reach liquid ground 
water we would have to drill to a depth beneath this frozen layer, which 
could be several kilometers thick, even at its thinnest point.  
That's some difficult drilling. Ice-rich permafrost is rather like concrete. 

Where to drill is, perhaps an equally difficult problem. Certainly we 
could drill where the frozen part of the crust is thinnest, but we may 
find no water. We need to know much more about the geology of the martian 
crust and the nature of its permafrost to best locate water. 
But I wouldn't rule it out as a future resource.

ANSWER from Mike Mellon on April 8, 1997:
The place on Mars where scientists know there is most water is the 
martian polar ice caps. They are very similar to our own polar ice 
caps here on Earth. The martian north ice cap is about 4 kilometers 
thick. If all that water were spead evenly over the surface of Mars it
would make a layer about 15 meters thick. The next likely place to 
find alot of frozen water is ice-rich permafrost. The Martian 
permafrost is several kilometers thick all over the globe and could be
as much as 50% ice, but scientists don't know for sure how much ice is 
in the permafrost.

ANSWER from Mike Mellon on April 21, 1997:
There are several place where drinkable water can be found on Mars; 
ground water deep beneath the permafrost (permanently frozen soil), melted
ice from the polar caps, melted ice from ice-rich permafrost, or water
condensed from atmospheric vapor. Properly prepared, these are all 
sources of drinkable water. It is possible, however, that salt might 
be present in the ground water or ground ice in quantities that would
make it undrinkable. A thirsty person on Mars would have to purify 
this water by distilling it to remove the salts. Polar ice is probably
free of salt but may contain alot of dust and dirt, requiring filtering
or some sort. Condensing water from the atmosphere (such as on a cold 
surface) would be drinkable without filtering and may be the easiest way
to get water for human consumption.

ANSWER from Bruce Jakosky on April 21, 1997:
There is clear evidence in the geology on Mars for liquid water.  Of
course, we only see where the water has flowed over the surface, but
in some cases it is clear that the source for the water must have been
beneath the surface. The best estimates are that there is a lot of
subsurface water, perhaps enough to make a global layer of water a
kilometer thick if it all were to be released to the surface.

And, although water at the surface would freeze, temperatures are
warmer deep beneath the surface, and water can be liquid.  At the
equator, one would have to dig only a few kilometers beneath the
surface to find a depth where water could exist as a liquid.  At the
poles, where surface temperatures are lower, one would have to dig
deeper, perhaps to 10 kilometers.

ANSWER from Mike Mellon on April 22, 1997:
There is abundant evidence that liquid water eroded the martian surface 
causing a variety of channels and valley networks. There is still
debate, though, about how much water and where it came from. It may
have been subsurface water leached from the ground, rain, or snowmelt.
Of course water flows downhill and probably ponded somewhere. But
where is that water today? We don't know. It partly depends on the type
of climate at the time. For example if the climate is cold and the 
ground is frozen the water can't soak in. 

Based on comparison with Earth and basic physics and geology,
scientists theorize that if Mars has alot of water, then there should
be large undeground aquifers like on Earth and the permafrost (a zone
of permanently frozen ground extending several kilometers in depth) 
would be saturated with ice. On the other hand, if Mars has a small
about of water, then all the ground water is probably locked up as
ground ice and little-to-no water exists in a liquid state beneath the
permafrost. (Water will migrate toward the coldest regions undergound)
I'm being somewhat vague about what a small or large amount of water
is, beacuse it depends on unknown factors.

So it's possible that liquid ground water exists on Mars. "Likely"? I
don't think we have enough information to say. And to know for sure 
will require deep drilling beneath the permafrost (not easy).
- Mike Mellon