Exploring the rippling dunes of our solar system

That flight relies on wind speed: Wind tunnel experiments suggest that getting a grain of sand moving on Mars takes a gust 10 times stronger than required for the same grain on Earth. But once the sand starts moving, it’s easier to keep it going, thanks to the planet’s lower gravity. “The big unknown for Mars is the threshold you need to start the movement of sand,” says Simone Silvestro, a researcher at the National Institute for Astrophysics (INAF) in Italy.

For decades, scientists suspected the dunes they saw on Mars were ancient relics from a past featuring a thicker atmosphere and stronger winds. That changed in 2019, when Silvestro and his colleagues used NASA’s Mars Reconnaissance Orbiter to capture dunes creeping along near the martian equator. By comparing images taken more than seven years apart at two different sites, the team determined that these megaripples — which are the largest of dunes at around 3 feet (1 meter) in height — edge along at about 4 inches (10 centimeters) per year. 

Even today, scientists aren’t certain how strong surface winds are on Mars. They have mapped out global wind patterns based on the topography and how the landscape has formed, but the bulk of atmospheric measurements for the planet have been made by orbiters, which are limited to observing the upper atmosphere. Meanwhile, landers and rovers can only provide wind speed information at ground level; this leaves a wide stretch of sky empty of atmospheric measurements.

Ideally, a rover or lander would sit in one spot and stare at the sand movement constantly, rather than in timed snapshots, Diniega says. That would allow researchers to ground their models in truth. Given that windy, dust-filled air has the potential to clog and damage robotic explorers, it would be even better, Silvestro says, to send people to the planet to measure wind speeds and dune motion.