The Curiosity rover is sending back photos of things mission scientists are struggling to understand.

It looks like there was standing water on Mars, but geologists can’t picture any circumstances where large amounts of water could have survived for long. The sun should have been cooler, for one thing, in the early years of the solar system. Another curiosity is that Curiosity (the name of the Mars Science Mission rover), has failed to find evidence of carbonates on the surface, yet those would be expected if Mars had more carbon dioxide in the past to act as a greenhouse gas to keep the surface warm enough for liquid water. These paradoxes, NASA’s Curiosity page says, make “climate modelers struggle to produce scenarios that get the surface of Mars warm enough for keeping water unfrozen.”

The dilemma has been building for years, the Mars rover scientists say. From the photos, scientists see evidence of river channels and lakebeds on Mars, but chemically, the rocks don’t match. Dissolved CO2 in water helps minerals like iron and magnesium precipitate into carbonate rock. Orbiters and rovers have been unable to find carbonates above a low threshold.

We’ve been particularly struck with the absence of carbonate minerals in sedimentary rock the rover has examined,” said Thomas Bristow of NASA’s Ames Research Center, Moffett Field, California. “It would be really hard to get liquid water even if there were a hundred times more carbon dioxide in the atmosphere than what the mineral evidence in the rock tells us.” Bristow is the principal investigator for the Chemistry and Mineralogy (CheMin) instrument on Curiosity and lead author of the study being published this week in the Proceedings of the National Academy of Science.

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