When you’re picturing dunes your mind might think of the Sahara or a white sandy beach (or at most be reminded that "the spice must flow"). Either type is seen on Earth: small decimeter-size ripples or large waves of sand hundreds of meters long.
Both types of dunes can be found on Mars, but researchers were surprised to find a third type. An international group of astronomers combined data from NASA’s Curiosity Rover and the Mars Reconnaissance Orbiter and discovered that the Red Planet sports mid-size dunes, too. Their findings are published in Science.
At first, these large Martian ripples were thought to have a similar origin to the small wind dunes found on Earth’s beaches, and the reason why they are larger on Mars is because the Martian sand grains hop in a different way.
If this was the case, there would be no small-scale dunes; instead, all three types co-exist peacefully. No existing model can explain how the winds have formed these meter-size dunes, but the team realized that their stable size, sinuous crest, and asymmetric profiles were akin to underwater current ripples.
Subaqueous dunes are well understood and there are decades of experiments that have modeled them in fine detail. The team developed a model based on the underwater ripples found on Earth, and once applied on the mid-size Martian dunes, it was found to accurately predict their crest-to-crest distance.
This discovery of the wind-drag ripples is not just about the current state of the Martian surface, but it could have an impact on our understanding of Mars geological and atmospheric past.
These ripples were observed in sedimentary rocks in the Victoria Crater by Opportunity and, since their size depends on the atmospheric density, the model can be used to estimate how dense the atmosphere was when those sediments formed.
Mars used to look more like Earth, and understanding how quickly it lost its atmosphere is important in furthering our understanding of the evolution of planets.