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Gullies are common on steep slopes of many impact craters on Mars. When gullies were first observed, there was a lot of excitement surrounding them because similar features form on Earth through the action of liquid water. However, liquid water is currently unstable on the surface of Mars. Long-term observations have prompted many scientists to question a liquid water origin for the gullies, and alternative ideas have been suggested. These include flows of salt-rich (briny) water, as the salt would allow water to be liquid under lower temperatures than those for pure water. Also "dry" process…
