BY DR EMILY BALDWIN
Posted: 09 January, 2009
Contrary to a previous explanation that suggested high speed winds were responsible for rolling rocks around the red planet, a new model shows that a much more ordered system transports rocks upwind.
Recent images taken by the Mars Exploration Rover Spirit show small rocks regularly spaced about five to seven centimetres apart, in organised patterns, on the plains between Lahontan Crater and the Columbia Hills. One previous explanation suggested that the rocks – some of which are as large as footballs – were picked up and carried downwind by winds reaching several hundred metres per second (at a height of two metres above the ground). This is much higher than wind speeds observed on Mars today, so the authors concluded that the patterns were created in a much windier Mars of the past.
The Spirit rover took these images of evenly space rocks on Mars. Image courtesy J. Pelletier, UA.
In a new study, Jon Pelletier, associate professor of geosciences at the University of Arizona, suggests that the wind works in a different way to slowly tease the rocks from one place to another. Moreover, Pelletier’s model sees the rocks moving upwind. "My model requires only enough wind to pick up sand, about 10-20 metres per second at two metre height," he tells Astronomy Now.
"You get this happening five, 10, 20 times then you start to really move these things around. They can move many times their diameter,” says Pelletier. "In a wind tunnel it takes about 10 minutes [to move their own diametrer]. On Mars it would take about the same time if the wind was strong enough. Winds are variable, so it's hard to say exactly, but if the winds are moderately strong and steady, it takes very little time."
A schematic to explain how the rocks move upwind, by rolling into pits carved out by winds blowing across the Martain plains. Image courtesy J. Pelletier, UA.
The process is similar for rock clusters, too. However, rocks in the front of the group shield those in the middle or on the edges from the wind. Because the middle and outer rocks are not directly hit by the wind, the wind creates pits to the sides of those rocks. Therefore, they roll to the side, not directly into the wind, and the cluster begins to spread out.
The finding supports previous work that one of Pelletier’s study team members had conducted thirty years ago. James Steidtmann of the University of Wyoming had studied upwind migration by using a wind tunnel to see how pebbles on sand moved in the wind. Steidtmann's research showed that the rocks moved upwind and that over time, a regular pattern emerged.
To investigate the regular patterns of the rocks on Mars, Pelletier combined three standard numerical computer models to take into account air flow, erosion and the deposition of sand, and the rocks' movement. He also conducted what is known as a Monte Carlo simulation, which applies his model over and over to a random pattern of rocks to see how they ultimately end up. Out of 1,000 simulations the rocks formed a regular pattern 90 percent of the time. As an independent verification, he also compared the pattern predicted by the numerical model to the distances between each rock and its nearest neighbour in the Mars images, and found that they were well matched.
Simulations also predict the movement of rocks from an initially random configuration (left) into a more ordered pattern (right). Image courtesy J. Pelletier, UA.
Upward migration of rocks also occurs on Earth, but is difficult to study due to interference from plants, wildlife and humans. Pelletier says that this migration could still be observed on Mars today. "If a current or future rover took images of the same location with a time span of a significant wind storm, it could be observed," he says. "The only caveat is that once the rocks develop a more ordered pattern (from a random initial one) they do not rearrange as often."
The work is reported in the January issue of the journal Geology.