Both Earth and Mars feature large sand dunes and small sand ripples. Recently, NASA's Mars rover Curiosity discovered and studied an expanse of medium-sized sand waves unlike any sand formations found on Earth.

The novel Bagnold Dunes are situated on the northwestern flank of Mount Sharp. Curiosity's examination of the modestly sized dunes marked the first in-depth scientific study of active sand dunes anywhere other than Earth.

Researchers believe the dunes' unique size could offer clues to the idiosyncrasies of the Martian atmosphere as well as to the geologic history of the Red Planet.

True dunes are massive sand dunes with steep downwind crests created by avalanches. Much smaller impact ripples, with troughs just a few inches apart, are created by the constant collisions between windblown sand particles and stationary particles on the ground.

The ripples found at Bagnold Dunes feature characteristics of both types of waves -- larger impact ripples with one side of each dune steeper than the other. Though the dunes don't resemble any windblown sand waves on Earth, they do recall sand ripples created by water. The crest lines of the dunes are sinuous, like those made by moving water on Earth.

But the Bagnold Dunes weren't created by water; they were created by the Martian atmosphere. Researchers dubbed them "wind-drag ripples."

"The size of these ripples is related to the density of the fluid moving the grains, and that fluid is the Martian atmosphere," Mathieu Lapotre, a graduate student at Caltech and scientist on the Curiosity mission, said in a news release.

"We think Mars had a thicker atmosphere in the past that might have formed smaller wind-drag ripples or even have prevented their formation altogether," Lapotre continued. "Thus, the size of preserved wind-drag ripples, where found in Martian sandstones, may have recorded the thinning of the atmosphere."

Researchers were able to find similarly sized wind-drag ripples preserved in ancient sandstone near Bagnold Dunes, suggesting the Red Planet lost its atmosphere rather early in its planetary evolution.

Researchers detailed their analysis of the ripples found at Bagnold Dunes in the journal Science.