Megaripples near a sand dune on Mars.
NASA/JPL/University of Arizona
Researchers have spotted large waves of martian sand migrating for the first time. The discovery dispels the long-held belief that these “megaripples” haven’t moved since they formed hundreds of thousands of years ago. They’re also evidence of stronger-than-expected winds on the Red Planet.
It’s pretty staggering that humans can detect these changes on Mars, says Ralph Lorenz, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory who was not involved in the research. “We can now measure processes on the surface of another planet that are just a couple times faster than our hair grows.”
Megaripples are found in deserts on Earth, often between dunes. Waves in the sand spaced up to tens of meters apart, they’re a larger version of ripples that undulate every 10 centimeters or so on many sand dunes.
But unlike dunes, megaripples are made up of two sizes of sand grains. Coarser, heavier grains cap the crests of megaripples, making it harder for wind to move these features around, says Simone Silvestro, a planetary scientist at Italy’s National Institute of Astrophysics in Naples.
Since the early 2000s, Mars rovers and orbiters have repeatedly spotted megaripples on the Red Planet. But they didn’t seem to change in any measurable way, which led some scientists to think they were relics from Mars’s past, when its thicker atmosphere permitted stronger winds.
Now, using images captured by NASA’s Mars Reconnaissance Orbiter, Silvestro and his colleagues have shown that some megaripples do creep along—just very slowly.
The researchers focused on two sites near the equator of Mars. They analyzed roughly 1100 megaripples in McLaughlin crater and 300 in the Nili Fossae region. They looked for signs of movement by comparing time-lapse images of each site—taken 7.6 and 9.4 years apart, respectively. Megaripples in both regions advanced by about 10 centimeters per year, the team reports in the Journal of Geophysical Research: Planets. That’s about how fast megaripples move in the Lut Desert of Iran.
It’s a surprise that megaripples move at all on Mars, says Jim Zimbelman, a planetary geologist at the Smithsonian Institution’s Air and Space Museum. Just a few decades ago, there was no evidence that sands on Mars were mobile, he says. “None of us thought that the winds were strong enough.”
Winds on Mars might be accelerating small grains of sand, Silvestro and his colleagues suggest. Once these grains start to roll or bounce, they can act like battering rams, knocking into larger grains and setting them in motion. This process, known as impact-driven creep, has been observed on Earth.
Sand dunes on Mars likely donated their grains to the megaripples’ migration, the team concludes, because nearby dunes moved in the same direction as the megaripples.
Atmospheric models of Mars suggest winds capable of moving sand are rare. This discovery of migrating megaripples will force those models to be revised, the team suggests.
Silvestro plans to expand his search for migrating megaripples to the whole planet. He suspects the speediest megaripples will be near Mars’s fastest moving dunes. Megaripples on the move are beacons of windy conditions, which might in turn kick-start dust storms, the researchers suggest. Airborne dust can blanket solar panels, reducing their efficiency, and it can also gum up mechanical parts like gears. That’s bad news for Mars rovers and human habitats alike.