Researchers have seen large waves of Martian sand migrating for the first time. The discovery dispels the traditional belief that these "megaripples" have not moved since they were formed hundreds of thousands of years ago. They are also evidence of stronger than expected winds on the Red Planet.
It's pretty amazing that humans can detect these changes on Mars, says Ralph Lorenz, a planetary scientist at the Johns Hopkins University Laboratory of Applied Physics who was not involved in the research. "We can now measure processes on the surface of another planet that are only a couple of times faster than our hair."
Megaripples are found in the deserts of Earth, often among the dunes. The waves in the sand spaced tens of meters apart are a larger version of ripples that ripple every 10 centimeters in many sand dunes.
But unlike dunes, megaripples are made up of two sizes of sand grains. The thicker, heavier grains cover the ridges of the megaripples, making it difficult for the wind to move these characteristics, says Simone Silvestro, a planetary scientist at the Italian National Institute of Astrophysics in Naples.
Since the early 2000s, Mars rovers and orbiters have repeatedly seen megaripples on the Red Planet. But they didn't seem to change in any measurable way, leading some scientists to think they were relics from Mars' past, when their thicker atmosphere allowed for stronger winds.
Now, using images captured by NASA's Mars Reconnaissance Orbiter, Silvestro and his colleagues have shown that some megaripples crawl slowly, very slowly.
The researchers focused on two sites near the equator of Mars. They analyzed approximately 1,100 megaripples in the 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 about 10 centimeters per year, reports the team in the Geophysical Research Magazine: Planets. That's how fast megaripples move in Iran's Lut desert.
It is surprising that megaripples move 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 the sands on Mars were mobile, he says. "None of us thought the winds were strong enough."
Winds on Mars may be accelerating small grains of sand, Silvestro and colleagues suggest. Once these grains start to roll or bounce, they can act as battering rams, hitting larger grains and setting them in motion. This process, known as impact-driven creep, has been observed on Earth.
The sand dunes on Mars likely donated their grains to the migration of the megaripples, the team concludes, because the nearby dunes moved in the same direction as the megaripples.
Atmospheric models on Mars suggest that winds capable of moving sand are rare. This discovery of migrations of megaripples will force a review of those models, the team suggests.
Silvestro plans to expand his search for migrations of megaripples to the entire planet. He suspects that the fastest megaripples will be near the fastest dunes on Mars. Moving megaripples are beacons of windy conditions, which in turn could trigger dust storms, the researchers suggest. Dust in the air can cover solar panels, reducing their efficiency, and can also adhere mechanical parts such as gears. That's bad news for Mars rovers and human habitats alike.
