Outside of Earth, there are only three other bodies that have sandy dunes: Titan, Venus, and Mars. Mars even has a type not seen anywhere else in the Solar System. Between February and April of this year, NASA's Curiosity rover was busy investigating four of its dune sites and is now analyzing the sand collected there.

This is the second part of a long-term study on the dunes of Mars. The first study took place between the end of 2015 and the beginning of 2016. It focused on crescent-shaped dunes, the more common and stereotypical dune shape. The latest investigative phase focused on linear dunes, straight dunes much longer than they are wide.

"At these linear dunes, the wind regime is more complicated than at the crescent dunes we studied earlier," Mathieu Lapotre of Caltech, in Pasadena, California, who helped lead the Curiosity dune campaign, said in a statement. "There seems to be more contribution from the wind coming down the slope of the mountain here compared with the crescent dunes farther north."

Both these linear and crescent dunes are part of a complex dark band of sand known as the Bagnold dunes, where the rover discovered a type of dune exclusive to Mars, which are only a few meters in length. On Earth, dunes are either decimeter-size or hundred of meters wide. The third option found on Mars is only possible due to the characteristics of the Martian atmosphere and the interaction between sand and wind.

And the wind is crucial to understanding dunes. The probe took pairs of images of the same area and measured how the grains of sand moved, which allowed them to work out wind direction and strength.

"There was another key difference between the first and second phases of our dune campaign, besides the shape of the dunes," Lapotre continued. "We were at the crescent dunes during the low-wind season of the Martian year and at the linear dunes during the high-wind season. We got to see a lot more movement of grains and ripples at the linear dunes."

How dunes form and change is very important to understand the geology of the Red Planet. Martian sandstone was created from dunes just like these and the wind would affect its mineral composition.

The dunes are on the side of Mount Sharp, the feature that Curiosity has been studying since landing on Mars in 2012. Mount sharp is at the center of an ancient lake and its rocks are providing a window into when Mars was a water-rich world.