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martes, 14 de septiembre de 2010

What Drives Death Valley's Roving Rocks?

Desert Solitaire

Photograph courtesy Cynthia Cheung, GSFC/NASA

One of the mysterious peripatetic, or roving, rocks of Death Valley National Park in California and Nevada sits at the end of a curved track in a summer 2010 picture.

Found in the Racetrack—an aptly named dry lakebed, or playa—the moving rocks have stumped scientists since the 1940s. For instance, the rocks are thought to move as fast as a walking person, but they've never been seen in action. Previous studies have shown that gravity or earthquakes can't explain the objects' movements.

Now a student-research project led by NASA's Goddard Space Flight Center has lent support to the idea that, during wintertime, the rocks float down the playa on small "collars" of ice, which form around the stones when lake water flows down the surrounding hills and freezes on the lakebed, according to Cynthia Cheung, a principal investigator for the project. More water flows may allow the ice-collared rocks to "float."

A team of undergraduate and graduate students studied data from tiny sensors placed underneath the soil to monitor water flows. The team found that the sensors registered freezing water temperatures in March, which would provide the right conditions for ice collars to form.

Even so, the ice theory's not rock solid, Cheung noted: The harsh desert's many microclimates mean that "each rock ... may move by a different force, [and] there may not be one hypothesis that fits all the movements."

Traveling Companions

Photograph courtesy Maggie McAdam, GSFC/NASA

Sometimes the traveling rocks move in pairs (pictured in a summer 2010 photo), possibly as part of a single ice sheet, Cheung said.

Because the Racetrack is on protected federal land, the researchers are limited in their techniques, she added. For example, the rocks can't be disturbed, and the few cameras allowed to photograph the rocks over the winter have to be hidden as part of the landscape, she said.

So far, the cameras haven't snapped pictures of a rock in motion. But "newer technology may come to our aid," Cheung said. "We talk about various ways that we can embed things into rocks [so as] not to destroy the wilderness."

Extreme Beauty

Photograph by Pete Ryan

Above, the curved trail of a roving rock cuts across the Racetrack in a 2006 photograph. The Racetrack is a "place of stunning beauty and mystery" cradled into a remote valley between the park's Cottonwood and Last Chance ranges, according to the Death Valley National Park website.

Even so, the hottest and driest place in North America is "not very bearable," Cheung noted. Temperatures in July, for instance, can reach 115 degrees Fahrenheit (about 46 degrees Celsius).

And getting there isn't easy: Roads are washed out during wintertime, and in summer, Cheung said, "it's not a nicely paved road—even with four-wheel drive.

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