Marching Sand Dunes Create Mars Mystery
Sand ripples imaged by the Opportunity rover across the plain known as Meridiani Planum on Mars.
Credit: National Aeronautics and Space Administration/Jet Propulsion Laboratory. Image courtesy of National Academy of Sciences, PNAS (Copyright 2008)

Sand grains stirred up by the winds of Mars are tossed higher and farther than those kicked up by winds on Earth, a new study finds. The results could help explain how dunes migrate across the Martian surface as well as what whips up dust storms that blow across the red planet.

Scientists first noticed dunes on the Martian surface in pictures taken by NASA's Mariner missions in the 1970s and have seen dust storms of all sizes spread across the planet ? one major storm in 2005 was even visible through a simple backyard telescope. But these features have puzzled astronomers because Mars has almost no atmosphere and very weak winds that seem unlikely to be able to sculpt dunes or whip up storms.

To help solve this conundrum, a team of scientists recently conducted wind tunnel simulations of windblown sand grains under the conditions found on both Earth and Mars to figure out how the particles would behave on these planets with vastly different atmospheres. Their results are detailed in the April 28 issue of the journal Proceedings of the National Academy of Sciences.

Sand collisions

Winds tend to move particles in one of three ways. When the particles are heavy, "they cannot be lifted off from the ground by the wind, so they remain very close to the ground," a process aptly called "creep," explained study team member Eric Parteli of the Universidade Federal do Cear? in Brazil. On the other hand, lighter particles can hang around in the atmosphere for awhile and travel over long distances.

In between these extremes, a process called "saltation" reigns. Wind turbulence kicks up a sand particle and carries it along in a series of jumps, finally dropping it to the ground where it collides with other particles and in turn knocks them up into the air, Parteli said.

Saltation is what creates the iconic sand dunes of the Sahara and other deserts on Earth as the sand particles accumulate into mounds, usually with gentler slopes on their windward sides. With the discovery of dunes on Mars, scientists thought the process might occur on our red neighbor as well.

"We only know that saltation occurs on Mars because we see that there are dunes," Parteli said.

But scientists were unsure if the dunes were simply relics of Mars' past, when its atmosphere was denser and capable of generating stronger winds than it is now. The planet's current atmosphere is less dense than Earth's, and wind speeds 10 times faster than those on Earth would be required to pick up Martian sand particles.

"So we know actually that the winds of Mars are actually very weak," Parteli told SPACE.com. "They remain very much below the minimum threshold for saltation all the time."

Dune migration, also caused by saltation, has only been noticed on the red planet in a few before-and-after images in the last couple years, which seemed to indicate that saltation could still be going on, at least in some areas of the Martian surface.

Parteli and his colleagues used wind tunnel simulations that took into account the difference between the Martian and terrestrial atmospheres (in terms of such parameters as pressure and temperature) to see if Martian winds could generate saltation events.

Their experiments and models showed that the winds could in fact eject particles from the surface. And not only that ? the particles went higher and traveled farther on Mars than on Earth, because Mars' gravity is weaker (approximately one-third of Earth's gravity).

"The particle is allowed to remain longer in the atmosphere," Parteli said. "It is not pushed downward by the gravity so strongly as it is on Earth."

The research was funded by the Conselho Nacional de Pesquisas (CNPq), the Comissao de Aperfei?oamento de Pessoal de Nival Superior (CAPES), the Funda??o Cearense de Amparo ? Pesquisa (FUNCAP), the Volkswagenstiftung and the Max-Planck Prize.

Dunes and dust storms

Though Parteli and his team's results show that saltation can occur if the winds are strong enough, the winds on Mars rarely reach above the proper threshold, which makes saltation "a very seldom event on Mars," Parteli said.

Strong winds could be confined to particular regions of Mars, said Kevin Williams of Buffalo State College, who was not affiliated with the new study. These winds could rush down from the polar ice caps and stir up sands in nearby regions, he noted. Another possibility is that it might be easier for wind to move faster in low-lying areas where the air is slightly denser, he told SPACE.com.

The new study provides "progress in understanding the overall process of sand saltation on Mars," Williams said.

The shapes of dunes in certain areas of Mars could shed light on the winds there, as dune shape is largely a function of wind direction. The most common type of dune (on both Earth and Mars), called barchans, are shaped by winds blowing in just one direction, while another type, called longitudinal, are formed by winds blowing in two directions. The shapes of dunes on Mars have been likened to the video game character Pac-Man, horseshoe crabs and worms.

Wind speed can also affect dune shape, with stronger winds producing taller, narrower dunes than lighter winds.

Saltation is also a suspected cause of the ruddy dust storms that can blow across the Martian landscape. The saltating sand particles could eject dust from the surface as they are dropped, Parteli said, though he noted that other factors, such as temperature change, are also suspected causes. Ultimately, "how dust storms form is actually an open question," he said.

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