QUESTION:
Is there any hypothesis related to the temperature change on Mars from the
past warm ones (enough to have liquid water) to the current cold
temperatures? Where did all the water go?

ANSWER from Jim Murphy on July 15, 1997:
Yes, there are several hypotheses relating to how Mars may have been warm
and wet in the past but is now colder and drier.  Some theories for a warmer
Mars in the past invoke a much more massive atmosphere of carbon dioxide
which generated a significant "greenhouse" effect which raised surface
temperatures above the water melting point.  Additionally, other plausible
gases could have been present and contributed to greenhouse warming.  If
this CO2 greenghouse effect were the case, then the question is: where did
all that CO2 go?  Some could have dissolved in oceans (if they were present)
and formed carbonate rocks, but the evidence for such rocks is not too
terribly convincing (at least, that is waht I undes`rstand from listening to
my geologist colleagues).  Some researchers believe that Mars may not have
been too much warmer in the past, but that impacts and tectonic avtivity
might have melted and released vast reservoirs of sub-surface ice which then
"ran" down hill forming the large outflow channels like the one the Mars
Pathfinder lander is presently located at the mouth of.

Jim Murphy
Mars Pathfinder ASI/MET Science Team

ANSWER from Peter Thomas on July 14, 1997:
The fate of the Martian water is one of the most important questions in Mars
science today.  There really are only a couple of options (all of which
probably contributed):

1. Water is currently stored below the surface as ground water. The most
obvious place would be in the lowlands of the northern hemisphere although
it may be spread around the planet.

2. Water was lost to space.  The water molecule was broken apart in the
atmosphere into hydrogen and oxygen and at least the hydrogen was lost to
space.

3. Water is bound up in minerals on the surface.

ANSWER from Jack Farmer on July 17, 1997:
We have many solid observations for Mars which indicate that it once had
abundant water flowing over its surface. In particular, the many valley
networks and large outflood channels like Ares Vallis where Pathfinder has
landed. But beyond this sketchy outline, the story of Martian water is far
from complete. We have yet to determine with any certainly how much water is
still present in the Martian crust today as ice, or subsurface water.
Neither have we agreed on how much water was present earlier in Martian
history. Most people think there was always a lot less water on Mars than on
Earth, but the planet still had to be quite wet at certain times in its
history, to account for all the channel features we see.

So where did all the water go? Well some of it (how much is still uncertain)
is still present in the Martian crust and on the pole caps as water ice.
Some is likely to be present at depth as subsurface liquid water
(groundwater). There is a tiny bit in the atmosphere as vapor, and some is
probably tied up in hydrated minerals phases, like clays. But there is
evidence that a lot of the original water was lost to space. The atmosphere
of Mars is enriched in "heavy" hydrogen (called deuterium) which is thought
have been concentrated by the photolysis (process where light energy drives
the breakdown of compounds, in this case water) in the upper atmosphere. The
hydrogen of the H2O molecule is liberated by this porcess and being very
light (atomic mass of one), it is easily lost to space from the top of the
atmosphere. The heavier version of hydrogen (deuterium) tends to hang around
longer and becomes more concentrated as a result. And the oxygen in the H2O
is left behind, eventually combining with the surface rocks and minerals to
form iron-oxides or "rust". This is what imparts a red color to the Martian
regolith. - Jack Farmer