Razor-blades & recent water... (2) - Sol 791

In June, the sand was dry and cracks were filled in or worn away.
Cracks do form in drying mud, but this is not mud, Sumner said. There was no explanation for the formation of these types of cracks in sulfate sands before Opportunity landed, she said. Chavdarian also looked at two types of thin, brittle fins poking a few inches out of the desert sand, usually facing into the wind. The most common type was found only in January, when the sand was moist. Chavdarian and Sumner think that the fins are formed when water seeps into cracks in the sand, carrying minerals with it. The water evaporates away, leaving behind those minerals, which are exposed as the wind blows sand away. Windblown material sticks to the exposed fin, making it larger and stronger.
If the cracks and fins seen by Opportunity on Mars are formed in the same way as the features at White Sands, it would provide evidence for water at the surface of Mars away from the Polar Ice Caps, which are mostly formed by CO2 and water ice.