Once Martian sand grains hop, they don’t stop.
That’s the conclusion of a new study that finds sand can move on Mars without much windy encouragement.
Mars’ sandy surface has clearly been shaped by wind. Its characteristic dunes and ripples are the kind formed by sand particles taking short wind-borne hops, a process called saltation.
But atmospheric simulations and landers’ direct measurements of wind speed have found that the Martian wind hardly ever blows hard enough to kick sand grains off the ground in the first place.
The new paper, to appear in an upcoming Physical Review Letters, suggests a solution to this paradox: a kind of billiard-ball effect in which one sand particle knocks the next one into motion. “It’s much easier to keep this process going than it is to start it in the first place,” says study author Jasper Kok, an atmospheric physicist at the National Center for Atmospheric Research in Boulder, Colo., who did most of this research while at the University of Michigan in Ann Arbor. “It’s like when you ride a bike: It costs a lot of exertion to get it going, but once you’re going it’s easier to keep going.”
Kok modified a numerical model, previously applied to geological processes on Earth, to include Martian gravity and atmospheric conditions. Unlike in other models, Kok simulated a process called splashing, in which a flying sand particle knocks at least one new grain into the air as it smacks into the ground.
“That’s hard to study in a wind tunnel,” notes planetary scientist Robert Sullivan of Cornell University. The study “goes numerically where we have a hard time going with wind tunnel experiments,” he says.
The way sand grains knock each other around turns out to make all the difference, Kok says. Because Martian gravity and air density are so much lower than Earth’s, a small kick from the wind sends sand particles on Mars flying much higher, up to a meter off the ground.
“It’s like playing golf on the moon,” Kok says. Particles get caught in stronger winds as they rise, causing them to pick up speed and ultimately slam into the ground, where they kick up more particles and start the cycle over. “This splashing process is really efficient,” Kok says. “It can keep saltation, or sand blowing, going on Mars at relatively low wind speeds.” These jumping sand grains can create ripples over time even without high sustained winds, he says.
The finding could help solve other puzzles in the Martian landscape. Earlier models predicted that crescent-shaped sand dunes called barchan dunes should grow to at least 500 meters long — but many are only 100 meters. And the Mars rover Opportunity has found sand ripples made up of particles only 100 micrometers in diameter, so small that scientists had expected them to stay aloft once kicked up. The new model could explain both riddles by showing that splashing can keep particles moving at low wind speeds. Slow-moving sand grains don’t travel far and therefore make short dunes, but even tiny particles can get pushed into ripples, Kok says.
“This study is very welcome, very informative,” Sullivan says. “The results go a long way toward explaining several mysteries.”