Pretty Green Mineral -- Pretty Dry Mars?
This is a photograph of a typical Hawaiian olivine basalt. The rock is 14 centimeters across and contains about 15 to 20% olivine. A weathered face oxidized to a brownish-red is just visible at the bottom.
Olivine composition mapped in the Nili Fossae region. Map on the left shows the location of the enlarged area shown on the right. Hoefen and coworkers see a trend toward lower Fo values (higher FeO content) to the northeast. They counted the pixels mapped as olivine in the map and concluded that the Nili Fossae olivine exposure covers about 30,000 square kilometers.
Based on the presumed age of ~3.6 billion years for the Nili Fossae region, Hoefen and colleagues think this could be the upper limit to when the olivine was exposed at the surface. Because olivine weathers rapidly to clays and iron oxides, this implies that no water has flowed there since then. Alternatively, the olivine may have been uncovered more recently, in the past few thousand years or so, and the current cold and dry conditions have slowed or limited chemical weathering. What's needed is a better constraint on how long olivine can exist.