A plucky droid on a mission traverses a field of sandy dunes, alone on a vast, alien desert sea.
It’s an image straight out of science fiction—but it’s also what happened on Mars in late 2015 and early 2016, when NASA’s Curiosity became the first rover to explore an active dune field on another planet. For about 3 months, Curiosity traversed the Bagnold Dune Field on its way to Mount Sharp, studying how the dune sands shift and move.
Curiosity took images at five sites as it roved dunes as high as 6 meters tall, some bigger than a football field. The changes it saw were limited, but a painstaking analysis by Bridges et al. provides insight into the physics of moving sand grains, how it has shaped Mars’s landscape, and how it compares to that on Earth. Despite Mars’s lower gravity and its much lower atmospheric density—just 1% of Earth’s—the physics is remarkably similar.
In recent years, the growing fleet of spacecraft at Mars has given scientists the opportunity to study and track Mars’s dunes from orbit. In particular, the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter has recorded dramatic images of dunes that over the course of several years, have captured their slow advance, the shifting ripples that form on their faces, and even avalanches as their faces erode.
The changes recorded by Curiosity were subtle, likely because the season when the images were taken typically has the lowest wind speeds of the year and because the rover spent no more than a few sols (Mars days) in any one place. In most cases, the movement amounted to the disappearance of individual grains from one image to the next, or grains moving a few pixels, usually in no apparent coherent direction.
But sometimes Curiosity saw clear evidence of the landscape’s movement. At one site, facing the steep leeward face of a 4-meter-high dune named Namib Dune, it saw small flows of sand: miniature avalanches less than a centimeter wide. From another position, it saw a sheet of sand slumping down the face by about a centimeter over 1 sol.
Curiosity’s anemometer recorded wind speeds that averaged around 3–9 meters per second, with gusts that probably exceed its present calibration limit of 20 meters per second. However, not all instances where changes occurred were linked to high winds. This finding suggests that other forces might also be at work, such as the electrostatic forces between grains that build up as they rub together.
What the team did not detect were any movements of ripples on the dunes or of the dunes themselves. This lack may be because according to atmospheric models and seasonal imaging from HiRISE, Curiosity happened to visit the dune field during local (southern hemisphere) fall/winter, the time of the Martian year with the lowest wind speeds. HiRISE images of the region have shown widespread ripple movement and dunes migrating up to 0.2 meter each year, but this movement happens during other seasons (around local summer), when winds are predicted to be stronger. (Journal of Geophysical Research: Planets, https://doi.org/10.1002/2017JE005263, 2017)
—Mark Zastrow, Freelance Writer