Ripples imaged on the surface of sand dunes on Mars are unlike any seen on any land form of Earth. Researchers believe investigation of these ripples may provide astronomers with a look into the history of the climate on the Red Planet. But, what are these strange formations, and what conditions formed the unique Martian features?

The Curiosity rover on Mars photographed sand dunes on the Martian surface in December 2015, and astronomers determined one type of ripple was not like any recorded on land here on our home planet. The images recorded by Curiosity represented the first close-up examination of sand dunes on the surface of an alien landscape.

The features are located at the northwestern flank of Mount Sharp, in an area known as the Bagnold Dunes. Although small and large ripples are seen on our own world, the sand seen at that location on Mars also shows medium-sized ripples, a phenomenon not recorded here on the dry land of Earth.

"As Curiosity was approaching the Bagnold Dunes, we started seeing that the crest lines of the meter-scale ripples are sinuous. That is not like impact ripples, but it is just like sand ripples that form under moving water on Earth. And we saw that superimposed on the surfaces of these larger ripples were ripples the same size and shape as impact ripples on Earth," Mathieu Lapotre of Caltech said.

Mars and Earth each exhibit large-scale sand dunes, formed by the collapse of sand, which can stretch 300 feet or more in length. Impact ripples, formed by the collision of small particles of airborne sand with the ground, measure less than 12 inches from one leak to the next. They are commonly seen on beaches here on Earth. For years, astronomers have seen intermediate-sized ripples on the surface of Mars, but assumed these features were larger versions of the familiar impact ripples seen on Earth. This close-up view shows impact ripples are nearly the same size as those on Mars, meaning the 10-foot-wide features are a different form of geological feature.

These formations may have been the result of a geological past in which Mars possessed a significantly thicker atmosphere than that which surrounds the planet today. If so, the variety of ripples could provide a record of thinning conditions in the Martian atmosphere over the course of billions of years.

Discovery of the unique ripple patterns on the surface of Mars and analysis of the features are profiled in the journal Science.