Models of Thermal Emission Spectrometer (TES) data suggest that poorly-crystalline weathering products allophane and aluminosilicate gel occur in several low-albedo regions of Mars. The presence of allophane in TES models indicates that the martian surface experienced low-temperature chemical weathering at low water-to-rock ratios and mildly acidic to neutral pH on regional scales. The allophane and gel may be ancient and preserved by a persistently dry martian climate. Alternatively, evidence for recent ground ice in these regions suggests that pedogenic processes causing the formation of poorly-crystalline aluminosilicates could be late Amazonian in age and may be active today. While previous models have suggested that global-scale acidic weathering has occurred on Mars for the past 3.5 billion years, the presence of allophane indicates that acidic weathering was not occurring in these low-albedo regions and that mildly acidic to neutral weathering has been an important regional-scale weathering process on the martian surface.

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