This just in! Sand dunes are cruising on Mars!
Fact: The surface of Mars shows massive erosion and huge fields of sand dunes.
Problem: Mars hasn’t had liquid water for more than a billion years. High winds are rare and its atmosphere is thin. Is the erosion due to ancient water or modern wind?
Solution: The sand dunes are blowing in the wind, moving much like dunes on Earth.
In a study posted online May 9, Nathan Bridges and colleagues analyzed data from an eye-in-the-sky called Mars Reconnaissance Orbiter. Using a high-resolution telescope, the researchers measured the movement of sand dunes over a 105-day span.
The fine-grained images showed that the dunes are indisputably on the move, says Bridges, a senior scientist at the Applied Physics Laboratory at Johns Hopkins University. “Even though Mars has a very thin atmosphere and high-speed winds are rare, the dunes are moving.”
The research group saw movement both in entire dunes, and in the ripples on their surface. Across one meter of dune front, they calculated an annual sand movement totaling about 2.3 cubic meters. “If you had a children’s sandbox, that would fill it with sand in a year,” Bridges says.
On Mars, as on Earth
And that, he adds, is within the range of movement seen in some Earthly dune fields. “We are not making the case that Mars has the fastest dunes, but they do move like some on Earth. Mars is an active planet, maybe not as active as Earth, but we are seeing significant movement.”
How much wind is needed to move sand when the atmosphere is less than one percent as dense as Earth’s? The grains would start moving in a wind of about 20 to 30 meters per second (40 to 50 miles per hour, measured at a height of 1 meter), Bridges says. “That is about 10 times what you need on Earth, due to the atmospheric density difference.”
Such winds do blow — rarely — on Mars, but once the sand starts moving, it’s easier to keep it rolling, he says. “Recent research by my colleagues has found … a lower-speed wind can sustain the movement.” Under the reduced gravity of Mars, a grain stays aloft longer, giving the wind more time to accelerate it. When the high-speed grain hits the sand bed, a high-energy collision impels more sand grains into motion.
Mars: A moving planet
At any rate, the discovery proves that wind needs no help from water in moving dunes, Bridges says. “We have seen dunes in images since the 1970s, but there was a question, were they currently active, moving? Mars has a very thin atmosphere and it would need high-speed winds to move sand, and those are very rare. So it’s been an open question, how much sand is moving now, and was more moving in the past?”
On Earth, water is highly erosive, but Mars has no liquid water, “so one agent of erosion on Earth is lacking,” says Bridges. “There is a lot of evidence for erosion — craters that appear to be filled in with dirt, and the primary mechanism is wind.”
And lasting sandblasting
Wind does not just move sand — it also creates sand, Bridges says. His group calculated that the natural Martian sandblaster sand would erode 1 to 50 microns off rock per year, about the same rate as in Victoria Valley.
That sandblasting would provide a source of the sand that litters so much of the red planet, Bridges says. “Erosion is occurring today, so wherever you have sand, and moderate winds, you are likely to get significant amount of erosion from rocks.” That could then create silt or more sand.
When we see all these eroded terrains, “you don’t have to evoke any past climate to explain this,” he says. “It’s a current process, and it was likely occurring for billions of years.”
— David J. Tenenbaum
- Earth-like sand fluxes on Mars, Nathan Bridges et al, Nature, published online ahead of print 9 May 2012, doi:10.1038/nature11022 ↩
- Mars Reconnaissance Orbiter↩
- Visiting the Antarctica’s dry valleys ↩
- Types of sand dunes ↩
- The sands of Mars ↩
- Mars and Earth comparison table ↩
- NASA Orbiter Reveals Big Changes in Mars’ Atmosphere ↩
- Facts about the Martian atmosphere ↩