This image shows evidence for running water on the surface of Mars.
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Image from: NASA
The presence of water near the surface of Mars, or lack of water, is a big factor in determining the climate of Mars, and the suitability of Mars to support life. Finding out what has happened to the water of Mars is a big question scientists would like to answer.
Evidence for running water, including river channels such as those shown here can be seen on the surface of Mars, and there are frozen, icy polar caps, but there is no running water present today. The atmosphere of Mars seems to contain water. There are clouds and fog. These features suggest that there was water present near the surface at some time in the Martian past, and that water still cycles between reservoirs in the ground and atmosphere today.
Unlike other planets such as Mercury, it is difficult to lose water from the atmosphere of Mars into space. Therefore much of the water Mars started with is still there. Mars is much smaller than the Earth, and Mars is farther from the sun that either the Earth or Venus. These facts mean that the surface of Mars cooled off more rapidly than the other two planets. In fact, recent measurements of the Martian surface show just how cold it can be. Because Mars is colder, it fits what some scientists call the "Goldilocks" phenomenon.
On a cold planet, frozen water can be trapped within the ground. This ground water can be released however when Mars experiences a warming change in climate. Future exploration of Mars will be directed at answering questions about what is the possible Martian water cycle, ie. what has happened and what does happen to the Martian water. Answers to these questions will help scientists better understand the Martian climate history.
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