Water on Mars

The atmospheric pressure on Mars is so low that it is impossible for water to exist as a liquid.

Therefore, water on Mars goes through a cycle in which it moves directly from vapor to solid and back again, without ever reaching a liquid state. Water vapor solidifies in the soil in the form of ice at a temperature of -80 º C.

When the temperature rises back above this limit the ice sublimates, turning directly into vapor.

The amount of water vapor in the Martian atmosphere is very small, making up only 0.01% of the atmosphere. If this vapor were to condense, as a liquid, all over the surface of Mars, it would form a film only about one-hundredth of a millimeter thick.

The Polar Ice Caps

Large masses of ice, which remain perpetually frozen, make up the North and South Polar Ice Caps. In the winter, the ice caps become blanketed with a coating of dry ice, or frozen carbon dioxide

In the spring, the dry ice sublimates, becoming vaporous carbon dioxide, and the polar ice caps retreat. The South Polar Ice Cap has disappeared twice in the past hundred years.

Did Mars Ever Have Liquid Water?

If Mars ever had an abundant amount of liquid water, it would have had a much denser atmosphere. The increased density of the atmosphere would have caused the temperature to be higher as well.

There is some evidence that suggests that water once existed as a liquid on Mars.

Formations around some Martian craters can be explained by the previous existence of permafrost, or frozen soil.

In the high latitudes of the southern hemisphere, there are gullies that seem to have been formed by torrents of water and the soil and rocks that the water carried.
The Mars Odyssey probe has identified what seem to be the coastlines of two ancient oceans.