Evidence for a changing Martian climate from the mineralogy at Mawrth Vallis

• Poorly crystalline Al/Si materials are abundant at Mawrth Vallis, Mars.
• Allophane and imogolite define a distinct stratum in Mawrth Vallis.
• The presence of allophane and imogolite suggests an explosive volcanic origin.
• Allophane and imogolite are indicators of climate change at Mawrth Vallis.
• Allophane and imogolite above smectite clays mark an end to abundant liquid water.

Layered outcrops in the Mawrth Vallis region of Mars contain the greatest diversity of aqueous alteration products on the planet, and these materials are used to infer past aqueous environments. Orbital investigations indicate Al/Si-rich clay-bearing units overly an Fe/Mg-smectite-rich unit. Many different secondary minerals have been identified in the upper Al/Si-rich clay units, but the presence of poorly crystalline phases has not been previously investigated. Identification of ∼10–30% allophane and imogolite in the clay-bearing units resolves previous mineralogical discrepancies between TES and CRISM of clay-bearing units on Mars. We demonstrate here that the poorly crystalline aluminosilicates allophane and imogolite comprise a significant portion of the uppermost stratum of the Al/Si-clay-rich units. These phases are unique to immature soils derived from volcanic ash in well-drained, mildly acidic environments on Earth, and we hypothesize that the deposits discovered here originate from supervolcanic activity in nearby Arabia Terra. The transition through time from smectite-bearing units to the uppermost allophane/imogolite unit in Mawrth Vallis signifies a change in climate from a warm and wet environment to one where water was sporadic and likely depleted rapidly.

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