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Why might lakes have existed inside Valles Marineris?
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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.
The presence of thick, sulfate-rich sedimentary sequences within the canyons of Valles Marineris suggests that lakes may have formerly occupied these depressions on Mars.12
These geological deposits indicate that water once collected in the canyon system before potentially draining away in catastrophic flood events toward the east, where evidence of large outflow channels remains. While the canyon system itself was primarily formed by faulting related to the Tharsis volcanic region, these sedimentary layers provide key evidence for the historical presence of liquid water within the Martian landscape.12