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Physics

Moving sculptures

Computer simulations explain how the giant dunes on Mars emerge and evolve

nasaNot always frozen: Some of the Martian dunes may have emerged in the last couple thousands of yearsnasa

More than plain hills of sand, the dunes are wind-shaped sculptures that preserve in their curves the history of a planet’s climate. In the case of Mars, one of the Earth’s neighbors in the solar system that has attracted a lot of attention from researchers in the last few years, the dunes reveal a past full of storms and strong, swift winds that occur approximately every five years. Analyzing the characteristics of the atmosphere and the soil of the red planet and comparing them with what they know about terrestrial dunes, physicists Eric Parteli, from the State of Pernambuco, and Hans Herrmann, from Germany, have been able to reproduce with the help of a computer the shapes of the Martian dunes. In a paper to be published in the Physical Review Letters, they describe the first steps to explain how the dunes on Mars emerge and evolve, thus contributing to undo a mystery that has intrigued physicists and astronomers for nearly thirty years: whether the dunes are indeed frozen or whether they move.

Up to the mid-nineties, physicists and astronomers believed it was highly unlikely that the current atmosphere of the red planet, one hundred times thinner than the Earth’s atmosphere, would allow the emergence of the vast fields of Martian dunes, which are among the largest in the solar system. If this were true, then it stands to reason that the Martian dunes were relics from billions of years ago, when the planet’s atmosphere was thicker and the frequent impacts of meteors had not yet expelled its gases into outer space. This belief began to be reviewed very recently, thanks to the clearer, higher-resolution images obtained by NASA’s Mars Global Surveyor space mission, which continuously monitored the Martian atmosphere and surface from 1997 to 2006.

Past and Present
The new images – some of them revealing dune shapes never seen on Earth – attracted the attention of Herrmann, a professor at the Federal University of Ceará and at the Polytechnic School of Zurich, Switzerland. An expert on the physics of terrestrial dunes, the German physicist decided to test a mathematical model on Mars. The professor has been working on the development of this mathematical model since 2000 and he had already reproduced, with great precision, the movement of the dunes that exist in the Moroccan desert and on Jericoacoara Beach, on the coast of the State of Ceará. By using this mathematical model, which is actually a computer program, Herrmann wanted to discover whether it was possible for Martian dunes to emerge under the current atmospheric conditions.

In 2004 Herrmann and Parteli, who was a doctoral student at the time, began to analyze the data transmitted by the Mars Global Surveyor, looking for information that would help them simulate the Martian environment. They were able to discover the density of the air and the size of the sand particles, but they still lacked data on the power of the wind’s interaction with these particles of sand. As they did not have this data, the two physicists simulated this interaction following to the same magnitude at which it occurs on Earth. However, nothing appeared on the computer screen. When they increased the intensity of the interaction by a factor of ten, they were able to create virtual dunes. “Based on this result, we began to look for an explanation for this successful hunch,” says Parteli, who is currently a visiting researcher at the University of Stuttgart, in Germany.

nasaMartian Fields: crescent-shaped dunes were shaped by winds coming from only one directionnasa

By using the equations of the model, Parteli and Herrmann were able to calculate how the sand particles are launched into the air by the action of the Martian winds. They found that the Martian winds seemed to be more efficient than those on Earth. A strong wind on Earth lifts the sand particles up by only a few centimeters above the ground and carries them for approximately 10 meters, whereas on Mars winds of the same intensity are able to lift the particles up to nearly one meter from the ground and carry them much further, because of the thinner atmosphere and the low gravity. Under Martian conditions, every effect is multiplied by ten. The sand particles travel ten times faster and, when they drop, ten times more particles fall on the ground, forming sand clouds that lie very close to the ground, much like the sand particles that hurt the feet and ankles of tourists who visit the dunes in Brazil’s Northeast region.

This information, however, did not prove that the recent atmospheric conditions of the red planet had led to the emergence of dunes in the last hundreds of thousands of years. It was still necessary to discover how fast the winds blow on Mars. Parteli and Cuban physicist Orencio Durán, also from the University of Stuttgart, published an article in this January’s edition of Physical Review, showing that the speed of the Martian winds is etched in the shape of the dunes, establishing the minimum size that the crescent-shaped dunes can achieve.

These crescent-shaped dunes – common to the Earth and to Mars – are formed in places where the winds always blow in the same direction and where there is relatively little sand. Analyzing images of these crescent-shaped dunes in two regions that lie next to the Martian North Pole and to the Arkhangelsky crater in the Southern Hemisphere, Durán and Parteli verified that those three groups of dunes had been shaped by winds blowing at approximately 125 kilometers per hour. When they inserted this data into the computer program, Herrmann and Parteli saw that the crescent-shaped dunes that were being formed were ten times higher than those found on Earth – a sign that giant dunes exist on Mars. This was the missing link that proved that the Martian dunes could indeed have emerged in the last couple of thousands of years.

Rare Winds
But how would they have been recently formed if they seemed to be frozen? Parteli may have found the answer when he analyzed the frequency with which such strong winds blow on Mars: possibly once every five years and then for only half a minute. With this frequency, a 200-meter crescent-shaped dune would take four thousand years to move only one meter, an extremely low speed as compared to the speed at which the Earth’s dunes move: from 5 to 20 meters a year.

The images sent by the Mars Global Surveyor seem to confirm this. Geologist Kenneth Edgett, from U.S. company Malin Space Science Systems, analyzed the images registered by this space probe one by one and concluded that only small stretches of sand probably move on Mars now. “There is fairly good evidence that not all dunes are frozen,” says Mary Bourke, from Oxford University in the United Kingdom. She recently observed that small dunes in the Martian North Pole had shrunk and disappeared in less than two years. Most of the dunes, however, are indeed frozen, like the ones Mary discovered inside the Kaiser Crater in 2005.

After reproducing the Martian crescent-shaped dunes, Herrmann and Parteli tried to model other types of dunes, similar to those that are shaped on Earth when the winds come from two different directions and that alternate cyclically. With winds blowing at 125 kilometers an hour, they were able to reproduce three other shapes of Martian dunes. Based on these simulations, the physicists calculated that the strong winds on Mars that produced the dunes probably change direction once every dozens of millions of years. This long interval suggests that a change in the direction of the winds is associated with the precise movement of the planet, which oscillates like a spinning top during  its 51 thousand year journey around the Sun – alternating the exposure of the Northern and Southern Hemispheres to the heat, one hemisphere at a time.

The mathematical model developed by Herrmann and Parteli based on the study of the Earth’s dunes can also help one to understand the atmosphere and some characteristics of the surfaces of other planets in the solar system, such as Venus, or Titan, Saturn’s biggest moon. “We can adjust this model to these celestial bodies”, says Parteli. But this might not be as simple as it sounds. “The atmosphere of Venus and Titan is much thicker than the Earth’s atmosphere”, says the physicist from Pernambuco, “and we are still unable to understand how the particles of sand are transported under those circumstances.”

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