This image shows evidence for running water on the surface of Mars.
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Image from: NASA
There seems to be no running water on the surface of Mars today even though there is evidence for running water, including river channels such as those shown here, and there are frozen, icy polar caps. Features such as the streams shown here suggest that there was water present near the surface at some time in the Martian past. The atmosphere of Mars seems to contain a little water too. There are clouds and fog. Water in the atmosphere suggests that water still cycles between the ground and atmosphere today. Fog rises out of the ground and condenses back again, and water vapor in the atmosphere condenses and evaporates. In most respects, however, the possible Martian water cycle is nothing like it counterpart on Earth because Mars is so cold. Much of the water of Mars is frozen into the ground, and can only be released when Mars experiences a warming change in climate.
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 when it comes to the suitability of Mars to support life.
Future exploration of Mars will be directed at answering questions about exactly what has happened to the Martian water. Answers to these questions will help scientists better understand the Martian climate history.
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