Water on ancient Mars: Forming the phyllosilicates at Mawrth Vallis.

摘要

The clay units at Mawrth Vallis are among the most extensive on Mars. Here we characterize the mineralogy and texture of these units through analyses of data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and High Resolution Imaging Science Experiment (HiRISE) instruments and through laboratory studies of mineral mixtures.Analyzing CRISM data, two main phyllosilicate-bearing units have been identified: a nontronite-bearing unit overlain by an Al-phyllosilicate-bearing unit. This upper unit contains a lower layer containing montmorillonite plus hydrated silica and an upper layer containing kaolinite plus hydrated silica. A ferrous mineral has also been identified near the boundary between and mixed with the nontronite- and Al-phyllosilicate-bearing units and saponite has been identified on a crater floor. These individual minerals are identified and distinguished by features at 1.38--1.42, &sim1.91, and 2.17--2.41 microm.To help determine what type of hydrated silica is mixed with the montmorillonite and kaolinite, spectra of mixtures of kaolinite-opal-A, kaolinite-montmorillonite, montmorillonite-obsidian, montmorillonite-hydrated silica (opal), and glass-illite-smectite (where glass was hydrothermally altered to illite-smectite) were measured and analyzed. The best spectral matches with the Mawrth Vallis data are mixtures of obsidian with 20 to 50 % montmorillonite or mixtures of montmorillonite with 25 to 50 % kaolinite. The spectra of clay mixtures with opal and of hydrothermally altered glass-illite-smectite did not represent as well the Martian spectra observed in this region.Finally, HiRISE data was examined to characterize the textures and morphologies of the Mawrth Vallis clays and determine if there are specific textures that correlate with mineralogy. Nontronite-bearing rocks appear tan in HiRISE COLOR data and exhibit irregular polygons 2--5 m across. Montmorillonite-bearing rocks generally appear blue and exhibit regular polygons 0.5--1.5 m across. Kaolinite-bearing rocks are the brightest clay-bearing rock in HiRISE data and are fractured non-polygonally. Hydrated silica and the ferrous mineral exhibit no consistent textures.The nontronite- and montmorillonite-bearing units are remarkably consistent in both mineralogical and textural stratigraphy across the Mawrth Vallis region, suggesting they were formed by a regional process and exposed to regional environmental conditions post-deposition.