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The image below from Mars Reconnaissance Orbiter shows gullies remarkably similar to gullies formed on Earth by flowing water, thus providing striking evidence that at some time in the past liquid water did flow on the Martian surface, something that is not possible now. Key quote from the caption:
The gullies shown in the subimage (approximately 710 x 1100 yards) have well developed alcoves, deeply incised channels, and large depositional fans, and are similar to terrestrial landscapes sculpted by surficial water.
The gullies shown in the subimage experienced several periods of activity, as evidenced by older channels cut by younger ones or by their deposits. The current and recent Martian temperatures and atmospheric conditions would not allow for water to be stable at the surface for extended periods of time: it is so cold that the water would freeze, and then it would sublime quickly, because the air is very thin and dry. These gullies could have formed under a different climate, or maybe by repeated bursts of transient fluids. Current leading hypotheses explaining the origin of gullies includes erosion from seepage or eruption of water from a subsurface aquifer, melting of ground ice, or dust-blanketed surface snow.