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Unexplained / Enigmatic nature / Abiotic environment / Shifting sand dunes on Mars / 


Shifting sand dunes on Mars

Mars sands

After analyzing hundreds of high-resolution pictures of the Martian surface taken by the orbiting Mars Surveyor spacecraft, a team of researchers finds that weathering and winds on the planet create landforms, especially sand dunes, remarkably similar to those in some deserts on Earth.

Newly identified smaller, bright sand dunes, the researchers speculate, are formed of sulfates, possibly gypsum. These dune fields, which are a few miles across, appear bright in photographs because of the way in which the sand particles reflect light. The much larger dark dune fields, up to tens of miles across, are most likely made of grains eroded from lava flows.

At certain times of the Martian year, the winds are strong enough to move these sand particles to many areas of the now-dry planet.

"We can see from the high resolution that the winds are sorting these different particles out, and they haven't been homogenized with the rest of the planetary dust. In one picture we can actually see the bright stuff blowing off the dunes," said Peter C. Thomas, a senior researcher at Cornell's Center for Radiophysics and Space Research. Thomas and his colleagues, including Joseph Veverka, Cornell professor of astronomy, presented their findings on the shifting dunes of Mars in the Feb. 18 issue of the journal Nature.

Thomas speculates that an observer standing amid the sand dunes would be "sand blasted" at particular seasons of the year, as the winds moving through the thin Martian atmosphere whip the particles along at tremendous velocity. But at other seasons of the Martian year, he said, "there would be nothing blowing at all."

The researchers studied high-resolution pictures taken by the Mars Orbiter Camera (MOC) on board the Mars Global Surveyor spacecraft, which took images periodically while shrinking its orbit toward the final mapping orbit reached this month. The spacecraft, which entered Mars orbit in September 1997, now takes 118 minutes to circle the planet. The MOC takes images of the surface with a resolution as much as 50 times greater than the previously available pictures taken by the Viking spacecraft more than two decades ago.

In many of the hundreds of pictures studied, said Thomas, the dark sand dunes, which were also visible in the Viking pictures, are sharply etched. But on eight of the images, much brighter dunes, only hinted at in one or two blurry Viking images, also are clearly visible.

"We don't really know the composition of the bright dunes, but we are inferring sulfates because Viking and other data showed a lot of sulfates in the Martian soil. Gypsum would be bright and soft enough to erode quickly downwind as the images indicate," said Thomas. However, he added, the bright dunes could be formed of particles cemented by sulfates, or other minerals.

The winds on Mars appear strong enough to transport sand only at certain times of the year. A major reason is the planet's eccentric orbit, which produces strong winds primarily when Mars is closest to the sun. Strong winds also are due to the fact that a third of the atmosphere falls out of the carbon dioxide polar caps, and the motion of the gas creates very strong, highly seasonal and one-directional winds at high latitudes.

Until now, said Thomas, the model of the Mars atmosphere "was one of winds moving sand not very often and then pretty much in one direction at a particular spot -- and that's exactly what we are seeing." On a broad level, he said, modelers of the Martian atmosphere have the picture right.

As on Earth, the winds are selective in the particles they transport, by mass and composition. Much of the red planet is blanketed in a ubiquitous layer of dust, probably composed of rusty mixtures of clays and iron oxides. In certain areas, though, said Thomas, the wind is "cleaning off this dust" and replacing it with other particles to create the bright dunes.

The sulfates, he speculates, originally could have come from weathering in ancient dry river and lake beds, from ground water or from hot springs.



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