DISTINCT ALTERATION ENVIRONMENTS IN TERRA SIRENUM, MARS

Data returned from a variety of satellite missions to Mars have shown that the Terra Sirenum region of the southern highlands is unique area. Here, a diverse array of secondary minerals are found in close proximity, including chlorides, aluminum and iron/magnesium phyllosilicates, acidic sulphates (alunite and jarosite), as well as several calcium/magnesium sulphates. These minerals require the presence of water to form, so can help us understand the history of water in Terra Sirenum.

We combine topographic, imaging, and remote sensing data (MOLA, CTX, CRISM, THEMIS) to investigate spatial patterns of secondary minerals. We use previously published mineral identifications as well as new automated detections using an algorithm to find rare mineral phases in CRISM data. This allows us to incorporate minerals such as alunite and jarosite, which point to acidic water conditions at the time of formation.

Preliminary results indicate that different secondary minerals have distinct elevation distributions, suggesting that separate processes are involved in their formation. Phyllosilicate minerals are found throughout the study area, at all elevations, whereas sulphates are most often found at lower elevations. Chlorides occur at both high and low elevations, in local topographic depressions. These results point to at least three distinct alteration environments. First, widespread water is necessary to form pervasive phyllosilicate deposits. Next, the restricted range and low elevation of most sulphates points to a subsurface water source for their formation. Although not spatially extensive, alunite and/or jarosite are sometimes found in association with other sulphates throughout the study area, indicating that the water involved was acidic. While sulphates are most commonly found in distinct layers/patches in craters, chlorides are most often found draping other units, suggesting surface runoff rather than groundwater origin for these deposits. Since the chloride deposits appear in local depressions, rather than finding their way to the lowest regional elevations, it is likely that they precipitated from small volumes of brine rather than a regional source. For high elevation chloride deposits to occur, the water source must also have been at high elevation (e.g. glacial runoff or precipitation).