Mawrth Vallis (Mawrth means "Mars" in Welsh) is a valley on Mars at 22.3°N, 343.5°E with an elevation approximately two kilometers below datum. It is an ancient water outflow channel with light-colored clay-rich rocks.
Mawrth Vallis is one of the oldest valleys on Mars. It was formed in and subsequently covered by layered rocks, from beneath which it is now being exhumed.
The Mawrth Vallis region holds special interest because of the presence of phyllosilicate (clay) minerals which form only if water is available, first identified in data from the OMEGA spectrometer on the European Space Agency's Mars Express orbiter. Mars Reconnaissance Orbiter's Compact Reconnaissance Imaging Spectrometer for Mars has identified aluminium-rich and iron-rich clays, each with a unique distribution. Some of the clays recently discovered by the Mars Reconnaissance Orbiter are montmorillonite and kaolinite, and nontronite. Since some clays seem to drape over high and low areas, it is possible that volcanic ash landed in an open body of water. On Earth such clays occur in (among other environments) weathered volcanic rocks and hydrothermal systems, where volcanic activity and water interact. Mawrth Vallis was at one point considered as a landing site for the Mars Science Laboratory, which ultimately landed at Gale Crater. Clays minerals easily preserve microscopic life on Earth, so perhaps traces of ancient life may be found at Mawrth.