The Dynamic Albedo of Neutrons (DAN) instrument on NASA's Mars rover Curiosity detects even very small amounts of water in the ground beneath the rover, primarily water bound into the crystal structure of hydrated minerals. This graphic presents two types of modeling for how much of the detected water is very close to the surface and how much is deeper within the top 20 inches (half meter). At most places where the rover has made DAN measurements, the best fit for the data is one with less water in the upper layer than in the lower layer.
Image Credit: NASA/JPL-Caltech/Russian Space Research Institute
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