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How much liquid is on Mars?
Answer 1:

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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