Sandy ripples point to Mars’s past

Ripples 3 meters apart mingle with centimeter-scale ripples on the flanks of Mars’s Namib Dune, which spans hundreds of meters.

NASA/JPL-CALTECH/MSSS

Rover reveals puzzling sand dunes on Mars

The Martian was only a movie, but the blowing sand that drove the plot really does influence Mars today—and is now a source of intrigue for scientists studying the planet. Investigators with NASA’s Curiosity rover are exploring the planet’s dark sand dunes and have discovered structures thought to be unlike any on Earth: ripples spaced about 3 meters apart, intermediate in size between the little ripples and big dunes found on both planets. Scientists aren’t sure how they form, but they think the density of the thin martian atmosphere plays a role in shaping them.

“Something in the martian environment is begging for these to form,” says Lori Fenton, a planetary scientist who studies dunes at the SETI Institute in Mountain View, California, and who wasn’t involved in the research. Even before scientists can puzzle out what that is, they hope to use fossilized ripples in rocks that hardened from ancient dunes to glean clues about the thicker atmosphere of early Mars, according to Mathieu Lapotre, a graduate student at the California Institute of Technology (Caltech) in Pasadena, who presented the work here last week at the Lunar and Planetary Science Conference (LPSC).

Researchers had seen the large ripples in pictures taken from orbit, “but nobody thought hard enough” about them, Fenton says. In the end it took up-close observations by Curiosity, which is exploring Gale Crater. When the rover neared dark, billowy dunes girding the mountain that it aims to climb, some researchers on the rover’s science team considered them a potentially hazardous obstacle. But others saw an opportunity. “We have not visited an active sand dune field on another planet,” says Bethany Ehlmann, a participating scientist on the rover team at Caltech. “I don’t think it’s conscionable to be 100 meters away and drive by without stopping to take a look.”

So Ehlmann and others pushed for a campaign of pictures, wind measurements, and chemical analyses over several weeks in December 2015 and January (see map, below). The scientists presented their initial results at the LPSC: images of 5-meter-tall dunes that would not look out of place in Namibia. Imprinted on top of the large dunes were typically tiny ripples and the mysterious, larger ones.

On terrestrial beaches and deserts, small sand ripples form centimeters apart. They take shape when wind-borne sand grains hop and strike sand downwind, scattering yet more grains. Once a ripple begins to form, it protects sand on its downwind slope from being dislodged by further impacts. Meanwhile, scattered sand continues to be deposited on the upwind side of the ripple. Larger dunes, which can sit hundreds of meters apart, grow through a different process involving the aerodynamics of the atmosphere. As wind approaches a pile of sand, its streamlines are compressed, whipping up speeds and piling up more sand toward the crest. Eventually, the leading edge becomes too steep and collapses, allowing a new crest to form and the dune to inch along.

In the solar system, dunes are found on Earth, Mars, Venus, Titan, and perhaps even Pluto. Planetary scientists study them from orbit to infer wind directions—like a “free wind sock,” says Jani Radebaugh, who studies Titan’s dunes at Brigham Young University in Provo, Utah. They can also use them to infer wind speed. From theoretical models and wind tunnel tests using crushed walnut shells to mimic sand blowing in Mars’s weak gravity, researchers have estimated that wind speeds need to reach about 15 to 20 meters a second for martian dunes to form. Researchers hope to check those numbers by studying a handful of Curiosity’s images of moving dunes snapped on days when the rover also took wind measurements, says Nathan Bridges, a planetary scientist at Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.

Since landing in 2012, NASA’s Curiosity rover has trundled toward its mountain target, Aeolis Mons (also called Mount Sharp), but it must cross potentially hazardous sand dunes.

Since landing in 2012, NASA’s Curiosity rover has trundled toward its mountain target, Aeolis Mons (also called Mount Sharp), but it must cross potentially hazardous sand dunes.

NASA (3)/(DIAGRAM) C. BICKEL/SCIENCE

Other scientists on the Curiosity team are studying the size, chemistry, and color of martian dune sand. On Earth, dunes consist mainly of quartz grains, eroded from the granite that makes up much of the 
continental crust. On Mars, by contrast, dunes are made up mostly of dark, dense minerals like olivine, from volcanic basalt. The dunes get darker with time as finer dust, which contains lighter-colored feldspar minerals, blows away. Ehlmann says studying how the martian wind sorts grains by size and composition will help researchers understand other changes that occur as the sediment turns into stone.

But at the LPSC, scientists were most excited by the discovery of the large ripples. In addition to spotting them in the Curiosity images, Lapotre and his colleagues have identified them at dozens of sites all over the planet in images taken from orbit. They found that at higher elevations, where the atmosphere is thinner, the ripples are farther apart. If these ripples can be found in old rocks, Radebaugh says, “you can get a good sense of what the paleoatmosphere was like.”

If the large ripples are forming because of atmospheric influences rather than the impact mechanics of hopping sand grains, then, technically, they would be dunes rather than ripples. Lapotre says they seem to be similar to dunes found in riverbeds on Earth. Just as waves on the water’s surface shape the bottom, invisible “boundary layers” in the atmosphere may sculpt ripples on the surface of Mars.

Not everyone is convinced that the ripples are a new type of dune. “They could just be big ripples” left over from a windier martian past, says Ralph Lorenz, an APL planetary scientist. But Fenton says Lapotre and colleagues have spotted slip faces—signs of miniavalanches—on the lee sides of some of them. Larger dunes share such slumping features, but small ripples lack them. The new-dune scenario is “plausible,” she says. “I’m happy that somebody has put forward an explanation, because it has bugged me.”