The surface of Mars is dotted with large, wavy structures in the sand called mega-ripples. The latter are also present in deserts on Earth, where they move continuously in the form of waves of sand under the effect of the winds. But unlike our planet, astrophysicists have always believed that Martian mega-waves were stationary, due to weak Martian winds. Recently, planetologists have detected for the first time the migration of certain mega-waves on the red planet, calling into question the current Martian atmospheric models.
Researchers have spotted moving large waves of Martian sand for the first time. The discovery dispels the long-held belief that these “mega-ripples” have not moved since their formation hundreds of thousands of years ago. They are also evidence of stronger winds than expected on the Red Planet.
It’s pretty amazing that humans can detect these changes on Mars, says Ralph Lorenz, a planetologist in the Applied Physics Laboratory at Johns Hopkins University. ” We can now measure processes on the surface of another planet that are just twice as fast as our hair grows ».
Still-appearing Martian mega-waves
Mega-ripples are found in deserts on Earth, often between dunes. Waves in the sand spaced several tens of meters apart, they are a larger version of the ripples that occur about every 10 centimeters on many land dunes. But unlike dunes, mega-ripples are made up of two sizes of sand grains.
In 2016, the Curiosity rover was taking images of Martian mega-waves. But no noticeable movement suggested that these structures were mobile. Credit: NASA / JPL
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Bigger and heavier grains cover the ridges of mega-waves, making it more difficult for the wind to move these structures, says Simone Silvestro, a planet specialist at the Italian National Institute of Astrophysics. Since the early 2000s, Mars rovers and orbiters have repeatedly spotted mega-ripples on the Red Planet.
But they didn’t seem to change in any measurable way, leading some planetologists to think of them as relics of Mars’ past, when its thicker atmosphere allowed for stronger winds. Using images captured by NASA’s Mars Reconnaissance Orbiter (MRO), Silvestro and his colleagues have shown that some mega-waves move very slowly.
A migration phenomenon similar to that of land mega-waves
The researchers focused on two sites near the equator of Mars. They analyzed approximately 1,100 mega-waves in the McLaughlin crater and 300 in the Nili Fossae area. They looked for signs of movement by comparing pictures time-lapse of each site – taken respectively 7.6 and 9.4 years apart. Mega-waves in both regions have advanced about 10 centimeters per year, the team reports in the review Journal of Geophysical Research : Planets.
Images of mega-waves on the surface of Mars taken by the Mars Reconnaissance Orbiter in 2019. Credits: NASA / JPL
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This is the speed at which mega-waves move in the Lut desert in Iran. Just a few decades ago there was no evidence that the sands on Mars were mobile, planetologists didn’t think the winds were strong enough. Winds on Mars could accelerate small grains of sand, suggest Silvestro and his colleagues.
Images proving the slow migration of Martian mega-waves in the region of Nili Patera. Credit: University of Arizona
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On the same subject : Geologists bring new evidence of ancient rivers on Mars
Once these grains start to roll or bounce, they can act like rams, hitting larger grains and setting them in motion. This process, known as impact-induced sliding, has been observed on Earth. Sand dunes on Mars likely provided the grains needed for mega-ripples to migrate, the team concludes, as nearby dunes have moved in the same direction as the mega-ripples.
A necessary reassessment of the impact of Martian winds and sandstorms
Atmospheric models of Mars suggest that winds capable of moving sand are rare. This discovery of migrating mega-waves will force these models to be revised. Silvestro plans to expand his search for migratory mega-waves to the entire planet. He suspects that the fastest will be near the fastest dunes on Mars.
The moving mega-ripples could in turn trigger dust storms, the researchers suggest. Airborne dust can coat solar panels, reducing their efficiency, and can also jam mechanical parts like gears. This is bad news for Martian rovers and future human settlers.
Sources : JGR Planets
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