Mars is a sandy planet and the HiRISE camera on the Mars Reconnaissance Orbiter (MRO) has given us tons of beautiful pictures of Martian sand dunes. But Mars’ dunes are much different than dunes here on Earth. Their movement is governed by different factors than Earth dunes.
The movement of sand dunes on Mars is of interests to scientists. How far the winds move them, and where they’re deposited, are some of the important questions. The study of all dune processes contribute to atmospheric and sedimentary science.
A team of planetary scientists at the University of Arizona Planetary Sciences Lab performed a detailed analysis of sand dunes on Mars. Matthew Chojnacki, associate scientist at the U of A, led the study, which was published in the journal Geology. The paper is called “Boundary condition controls on the high-sand-flux regions of Mars.”
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The study found that large-scale features on Mars, and the temperature differences of landforms, play a strong role in Martian dunes. The same is not true here on Earth.
The team focused their efforts on regions of Mars with large sand dunes. “Because there are large sand dunes found in distinct regions of Mars, those are good places to look for changes,” said Chojnacki.
“We wanted to know: Is the movement of sand uniform across the planet, or is it enhanced in some regions over others?” Chojnacki said. “We measured the rate and volume at which dunes are moving on Mars.” The researchers mapped sand volumes, dune migration rates and heights for 54 dune fields, encompassing 495 individual dunes.
The team relied on HiRISE (High Resolution Imaging Science Experiment) to study the dunes. HiRISE is on the Mars Reconnaissance Orbiter. It’s mapped about 3% of the Martian surface in high-resolution images.
“This work could not have been done without HiRISE,” said Chojnacki, who is a member of the HiRISE team. “The data did not come just from the images, but was derived through our photogrammetry lab that I co-manage with Sarah Sutton. We have a small army of undergraduate students who work part time and build these digital terrain models that provide fine-scale topography.”
What did they find?
In this study, the team found observed dunes that ranged from 2 meters to 122 meters tall (6 to 400 feet). The dune movement was clocked at about 0.6 (2 ft) per Earth year. This is in stark contract to dunes on Earth. Some of the fastest-moving dunes on Earth are in North Africa and move at about 30.5 meters (100 ft.) per year.
Planetary scientists have debated the nature of Martian dunes, wondering if they’re relics from the ancient past, or if they’re still being actively created and moved around the surface. Now we know. Mars may be a lazy planet in terms of dune movement, but it’s still active.
On Mars, the atmosphere is much thinner than here on Earth, and that’s key to understanding these results. Basically, the wind isn’t powerful enough to move sand dunes the same way it does on Earth. There must be other factors.
Across Mars, the survey found active, wind-shaped beds of sand and dust in structural fossae – craters, canyons, rifts and cracks – as well as volcanic remnants, polar basins and plains surrounding craters.
But it also found, surprisingly, that the largest movements of sand are near three distinct landforms: Syrtis Major, Hellespontus Montes, and the North Polar Erg.
Syrtis Major is a dark spot on Mars called an albedo feature. It’s just west of the Isidis Impact Basin. It’s dark because of the basaltic rock in the region and the lack of sand cover. The authors say that sand movement here is strongly influenced by the nearby Isidis Basin, which is 4 to 5 km deep.
Hellespontus Montes is a mountain range 711 km long, running roughly north-south. It’s also an albedo feature. It’s located in the Noachus Triangle. The team found that seasonal CO2 volatility played a role in dune formation here.
The North Polar Erg is a sand sea high in the northern latitudes. It’s also known as the Vastitas Borealis. It encircles the entire polar region. The North Polar Erg is the most active dune region on Mars. The team found that seasonal CO2 contributes to the movement here. The sand is largely locked in place when the CO2 is frozen, and then the melt contributes to sand movement, largely due to the lowered albedo.
Why did these three large regions see the greatest dune movement? What sets them apart? Stark transitions in geography, for one thing. Also, surface temperatures. On Earth, neither of these factors shapes sand dune movement.
“Those are not factors you would find in terrestrial geology,” Chojnacki said. “On Earth, the factors at work are different from Mars. For example, ground water near the surface or plants growing in the area retard dune sand movement.”
The team concluded that large transitions in geological formations shapes Martian sand dune migration. It’s aided by temperature changes near albedo features like Syrtis Major.
The team also found that sand movement is greater near small basins filled with bright dust. “A bright basin reflects the sunlight and heats up the air above much more quickly than the surrounding areas, where the ground is dark,” Chojnacki said, “so the air will move up the basin toward the basin rim, driving the wind, and with it, the sand.
This study makes it clear that “large-scale topographic and thermophysical variabilities play a leading role in driving sand fluxes on Mars,” as the authors say in their paper. The authors also say that the results of this study will help in the planning of future missions to areas that aren’t easily monitored and may have implications for studying ancient potentially habitable sites.