Upwelling groundwater is responsible for the abundant evaporite deposits at Mars's Meridiani Planum, where NASA's Opportunity rover landed, according to a new analysis. The work solves the puzzle of how the deposits could have formed on the sloping plain, where there are no basins in which rainwater could pool and evaporate away.
After Opportunity landed in January 2004, it discovered Meridiani is composed of sedimentary, layered rock rich in sulphate salts and riddled with small spheres of haematite, nicknamed "blueberries". These signs ? along with a "cross-bedding" pattern in the upper-most layer of the rocks ? led the rover team to suggest they were formed by substantial amounts of acidic, salty water
"The environment and location where they're found doesn't match the typical location on Earth ? deep depressions where water can flow in but can't flow out," says Jeffrey Andrews-Hanna, a planetary scientist at MIT in Cambridge, US. "Meridiani doesn't occur in a closed basin of any kind. The entire surface is sloping gently down towards the north."
Now, he and colleagues Roger Phillips of Washington University in St Louis and Maria Zuber of MIT may have solved the conundrum. They performed computer simulations of the water cycle on Mars, beginning with what appears to have been a wet period in its early history, between 4.5 billion and 3.7 billion years ago.
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