Geological context of water-altered minerals in Valles Marineris, Mars

摘要

Greater than 15,000 km2 of the layered deposits within Valles Marineris are associated with water-altered minerals, yet their origin and history of alteration remain a mystery. There are numerous competing hypotheses for the formation of the interior layered deposits including aeolian, lacustrine, and volcanic. Recent orbiter spectroscopic data have indicated that water has played a role in their geological history. Thermal Emission Spectrometer (TES) measurements have revealed significant crystalline hematite-bearing deposits within Valles Marineris, typically related to interior layered deposits. These hematite deposits, found with a wide range of albedo values, are associated with relatively steep bedrock exposures but can also be seen downslope on flat surfaces where they may be a lag deposit. More recently, Observatoire la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) data have shown hydrated sulfates covering more than 13,000 km2 area of Valles Marineris. Sulfates are found in numerous topographic settings and geological units, but are typically located along the flanks of interior layered deposits and nearby low-lying floor units. Here we study the detailed morphologies of hematite and sulfate-bearing units such as mantled wall units, mass-wasting blocky deposits, massive floor deposits, and tectonically altered floor units. All of these terrains have diverse erosional styles and varied crater populations. In both hematite- and sulfate-bearing units, occasionally found in conjunction with one another, formation processes require contributions from water. The results indicate a wide range of diversity within an individual mineral class, between mineral classes, and also among morphological types. The diversity of geological settings and properties suggest that any single, unified formation mechanism is improbable.