NASA’s Curiosity Mars rover discovered mid-sized sand dunes on Mars not found on our planet at the “Bagnold Dunes” on the northwestern flank of the planet’s Mount Sharp. Curiosity began its mission on this area six months ago and found distinct mid-sized ripples on Mars.

As stated by the study published on July 1 in the journal Science, both Mars and Earth have true dunes as big as a football field and as small as 30 centimeters. The downwind faces are shaped by sand avalanches, making them steeper than the upwind faces.

However, the rover found mid-sized ripples on Mars. These cannot be found on Earth.

“Earth and Mars both have big sand dunes and small sand ripples, but on Mars, there’s something in between that we don’t have on Earth,” says Curiosity mission team member Mathieu Lapotre, a graduate student at Caltech in Pasadena, California.

The team confirmed that the deposits were formed by wind blowing over the sand. However, the sand dunes on the Red Planet were more similar in terms of dune shape and spacing to ripples that normally form underwater here on Earth. They call these wind-drag ripples.

The wind on Mars drag the sand particles in the same way flowing water drags sand particles here on Earth. The size of these ripples is related to the density of the Martian atmosphere.

The only possible explanation for the formation of these meter-scale ripples is Mars’ thick atmosphere. Unlike now, Mars used to have a thicker atmosphere. The size of these ripples could have recorded the thinning of the atmosphere.

Analysis on ripple textures preserved in sandstone aged more than three billion years are the same size as the newer ones.

Curiosity Project Scientist Ashwin Vasavada of NASA’s Jet Propulsion Laboratory in Pasadena says that the mid-sized dunes are just a few reminders of how different Mars is from Earth. When they visited the Bagnold Dunes, it seemed very similar to Earth despite the two planets’ differences in gravity and atmosphere.

Curiosity is now at the lower region of Mount Sharp. At the moment, the rover is trying to find evidence that can shed light into how and when the Martian environment evolved from being life sustaining into the drier harsh planet we know of now.