How much liquid there is exactly is unknown.
Much of the photographic evidence we have
obtained so far from orbiting probes shows a
surface which has had a large quantity of flowing
liquids (water) in the distant geologic past.
There is also some evidence that there has been
some smaller quantities of liquid water flowing in
the very near past (within the last million years)
but in smaller volumes. The theory is that at
least some liquid water can be found underground -
much like the aquifers on Earth.
I read an interesting article on the Internet.
Here is an excerpt from the article:
"Because the atmospheric pressure at the
surface of Mars is about 100 times less than it is
at sea level on Earth, liquid water would
immediately begin to boil when exposed at the
Martian surface. Investigators believe that this
boiling would be violent and explosive. There are
gullies that lace the surface of Mars and they
look similar to those carved out on Earth, so
how can these gullies form? Malin explained
that the process must involve repeated outbursts
of water and debris, similar to flash floods on
Earth. "We've come up with a model to explain
these features and why the water would flow down
the gullies instead of just boiling off the
surface. When water evaporates it cools the ground
-- that would cause the water behind the initial
seepage site to freeze. This would result in
pressure building up behind an 'ice dam.'
Ultimately, the dam would break and send a flood
down the gully," said Edgett.
"The occurrence of gullies is quite rare: only
a few hundred locations have been seen in the many
tens of thousands of places surveyed by the
orbiter camera. Most are in the Martian southern
hemisphere, but a few are in the north."
"The water supply is believed to be about 100
to 400 meters (300 to 1300 feet) below the
surface, and limited to specific regions across
the planet. >Each flow that came down each gully
may have had a volume of water of, roughly, 2500
cubic meters (about 90,000 cubic feet) -- about
enough water to sustain 100 average households for
a month or fill seven community-sized swimming
pools. The process that starts the water flowing
remains a mystery, but the team believes it is not
the result of volcanic heating."
If their theory is right, it indicates that
there is around 2500 cubic meters of liquid water
in several hundred locations. Adding this up, we
get something like 500,000 to maybe 1 million
cubic meters of liquid water under the surface.
All-in-all not a lot of water by Earth standards.
So to recap: The prevailing opinion seems to be
that most of the water remaining on the planet is
trapped in solid form (ice), some of it found on
the surface in the polar ice caps and surface
frost and that much of the frozen water is
underground. Some liquid water may exist
underground, accounting for the geologically
recent erosion seen by the Mars Surveyor, but it
does not appear to be large quantities. It will
likely take people on the surface to try to
determine the truth.
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