Rock-Walkin’ on Mars
It’s all in the Wind Action
Five years of rover studies on Mars have uncovered some cool sights on the Red Planet. Through their eyes we’ve seen the planet change in subtle and not-so-subtle ways, and examined the tiniest details allowed by the resolution of the rover instruments. This is important in planetary studies. Sure the big pictures are important, but the true history of a planet lies in its day-to-day, minute-by-minute changes across small areas of the surface.
These Spirit Rover camera images of an intercrater plain between Mars' Lohontan Crater show uniformly-spaced small rocks called clasts. Courtesy Geological Society of America.
So it goes with the distribution of rocks across the planet’s surface. It may surprise you to know that rocks on Mars are walking across the surface, with the help of the ever-present Martian winds.
If you examine the images of rocks strewn across an intercrater plain studied by the Mars Spirit rover, you can see that they are pretty much evenly scattered, suggesting that something — a mechanism like wind action — has acted to distribute them in regular spacing — about 5 to 7 centimeters (a few inches) part – as you see them here.
At first, scientists thought that perhaps high-speed winds in the past might have somehow tumbled these rocks into place. But, new research shows that it would be difficult for winds to carry these small rocks — which range from quarter-sized chunks to softball-sized blocks — very far. Jon D. Pelletier (associate professor of geosciences as the University of Arizona in Tucson) and his colleagues looked at images of such rock plains taken by the Spirit rover and came up with an alternate mechanism for “rock walking” that makes more sense.
They suggest that the Martian wind blows sand away from in front of a rock, which creates a small pit. The sand whirls around and ends up behind the rock, forming a sort sheltering wall. Eventually the rock rolls forward into the pit, actually moving into the wind. As the wind continues to blow, the process gets repeated and the rocks make their way across the surface, tacking into the wind centimeter by centimeter.
Clusters of rocks go through the same process with almost similar results. The difference is that the forward “wave” of rocks in the cluster shelter those in the middle or the sides from the wind. The sheltered rocks aren’t hit directly by the wind and the pits that are created lie to the sides, not the front. So, when those rocks start moving, they fall sideways. Over time the side-winding motion scatters the rocks out across a wider area.
These images show how simulated rocks end up being spaced by simulated winds like thos eon Mars. The image on the left is the starting position of the rocks; the image on the right shows where they end up after being affected by the wind. The yellow streaks represent the sand piled behind the rocks by the wind. Courtesy Jon D. Pelletier, University of Arizona.
How did Pelletier figure this out? He constructed a computer simulation of Martian winds acting on rocks and sand.
He ran the simulation a thousand times, and 90 percent of the time the rocks ended up in a regular pattern.
As an independent verification, he also compared the pattern predicted by the numerical model to the distances between each rock and its nearest neighbor in the Mars images. The patterns of the Martian rocks matched what the model predicted.
Now, this upwind migration doesn’t just happen on Mars. Geologists also see it happening on Earth.
Pelletier’s colleague and co-author Andrew Leier pointed out that this mundane distribution of rocks on a sandy, wind-blown surface can actually tell a lot about how wind-related processes operate on a place as familiar as the Earth and as alien as Mars. On Earth, the process is complicated by life because plants and animals can alter wind patterns and rearrange rocks, which actually makes it tougher to study here than on Mars.
Next up after the rock-walking study for Pelletier is to apply this same technique to larger wind-blown features on Mars. So, in the near future, look for a deeper understanding of the motions of sand dunes, wind-sculpted valleys and ridges (referred to by geologists as “yardangs”).
Studies such as Pelletier’s are small steps toward a deeper understanding of the climate history of other planets and particularly where those climates where those climates went awry. Such information tells us much about those planets — and about our own planet and its climate, too.
