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What evidence shows that Mars once had flowing water?
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Mars
An especially serene view of Mars (Tharsis side), a composite of images taken by the Mars Global Surveyor spacecraft in April 1999. The northern polar cap and encircling dark dune field of Vastitas Borealis are visible at the top of the globe. White water-ice clouds surround the most prominent volcanic peaks, including Olympus Mons near the western limb, Alba Patera to its northeast, and the line of Tharsis volcanoes to the southeast. East of the Tharsis rise can be seen the enormous near-equatorial gash that marks the canyon system Valles Marineris.
Evidence that Mars once hosted flowing water is supported by a variety of geological features and surface observations:
- Valley Networks: The ancient cratered terrain is marked by extensive networks of dry valleys that resemble terrestrial river systems. These features, which often include deltas where valleys enter lowlands, indicate that water once flowed slowly across the surface, suggesting a warmer and wetter climate in Mars's early history.1
- Outflow Channels: These are massive flood channels, often tens of kilometers wide and hundreds of kilometers long. They were carved by catastrophic, high-volume floods, likely triggered by the sudden release of groundwater or the collapse of subsurface reservoirs.23
- Sedimentary Evidence: Robotic missions have provided direct surface analysis. For example, the Opportunity rover discovered rocks that appear to have formed along the shoreline of an ancient salty body of water. Similarly, the Curiosity rover identified water-transported gravel in Gale crater, indicating it was once the floor of an ancient stream.4
- Mineralogy: Spectroscopic data from orbiters has revealed the presence of clay minerals and other alteration products that typically form in the presence of liquid water.4