In a particularly parched region of an extraordinary planet, rocks big and small glide across a mirror-flat landscape, leaving behind a tangle of trails. Some rocks travel in pairs, their two tracks so perfectly in synch along straight stretches and around curves that they seem to be made by a car. Others go freewheeling, wandering back and forth alone and sometimes traveling the length of several football fields. In many cases, the trails lead right to resting rocks, but in others, the joyriders have vanished.
This may sound like an alien world, but it's actually Racetrack Playa in Death Valley, Calif. Since the 1940s, researchers have documented trails here and on several other playas in California and Nevada. Seventeen undergraduate and graduate students from the Lunar and Planetary Sciences Academy (LPSA) at NASA's Goddard Space Flight Center in Greenbelt, Md., traveled to the Racetrack and nearby Bonnie Claire playas this summer to investigate how these rocks move across the nearly empty flats.
Some rocks are thought to have moved nearly as fast as a person walks. But nobody has actually seen a rock in motion, and scientists haven't deduced exactly how it happens. The easy explanations—assistance from animals, gravity, or earthquakes—were quickly ruled out, leaving room for plenty of study and irresistible speculation over the years.
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Sporting sunhats and carrying lots of water, the students arrived around 7 a.m. for their day of data collecting. They broke into five teams, each led by a Goddard scientist, and took out their maps. Then they packed their equipment and headed in different directions in search of rocks and trails. ...
For each rock and trail, the students recorded GPS coordinates and snapped photos. They dug up small sensors called Hygrochrons that had been buried (with the required permission of the National Park Service) three months earlier by Gunther Kletetschka, one of the trip leaders. The interns captured the electronically stored temperature and humidity data. They marked the trail boundaries by slipping colored pushpins into cracks in the clay and measured each track's length, depth, and width. They confirmed earlier observations that some of the big rocks have moved farther than the small ones.
The interns also found small mounds at the ends of some trails. People speculate these were formed when the rocks ploughed into the clay and came to rest. Quite puzzling were the mounds at the ends of trails that had no rocks.
The students checked for unusual or changing magnetic fields. (Nope, no evidence of that.) One student conducted radiation measurements. (Nothing strange there, either.) They pulled out small levels to determine if the rocks might be moving along trails tilted ever-so-slightly downhill. Instead, "the general trend is that they move uphill," as reported by Andrew Ryan of Slippery Rock University in Slippery Rock, Penn., in a talk that the LPSA group gave later at Goddard. "But the slope is so insignificant that we don't think it would influence this movement."
Two interns, Kynan Rilee from Princeton University in Princeton, N.J, and Gregory Romine, a graduate student at San Francisco State University, got the special assignment of photographing the playa's skyline and correlating these pictures with GPS coordinates. Rilee later fed this information into a model that can be used to determine where on the playa a photo was taken even if no GPS coordinates were documented. Soon, any visitor to Racetrack Playa will be able to upload photos for analysis at
http://www.racetrackplaya.org.
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For a while, speculation was that the Racetrack Playa rocks have properties that help them move. But the rocks are just dark dolomite boulders that tumbled down from the mountain highlands. (That's not how the trails were made; those came after the rocks found a home on the playa.) "Dolomite is relatively common, and the rocks themselves are not unusual," explains Jackson. "It's where the rocks are located that makes them special."
Some of the rocks that have moved weigh less than a pound, but many are 25–30 pounds. One of the largest sliders, named Karen, has been estimated at 700 pounds. A powerful force is required to move rocks that big, and the obvious candidate is the fierce playa wind. "It's surprising when you see how big some of these boulders are," says Ryan. "You think, 'How can something that big get blown around?'"
Wind speeds of 150 miles per hour or more would probably be necessary to move most of the rocks. The wind speeds that graze the playa's surface are very fast, but not that fast, so the newer studies tend to ask how the friction between the rocks and the clay might be reduced.
The interns evaluated several hypotheses that have been offered over the years. ...
Investigators have thought for years that the friction is somewhat reduced when the playa's surface gets wet and the top layer of clay transforms into a slick film of mud. Algae may lie dormant in the dry clay and bloom when the surface wets, further reducing the amount of friction. The students performed water-absorption experiments at Bonnie Claire Playa and found that the clay does get slippery. Even so, the students concluded that most rocks could not move without other help.
The aid probably comes in the form of ice—in this high desert, winter brings snow to the mountains. The meltwater washes downhill and collects in huge, shallow pools that spread across the playa and freeze at night. Decades ago, researchers proposed that big sheets of ice might envelop clusters of rocks, then catch the wind and drag the rocks around together. This might explain the cases in which two tracks run perfectly alongside each other.
When an experiment ruled out the possibility that this happens in all cases, the concept was refined. Now it's thought that collars of ice can form around the lower parts of the stones, probably because the mass of a rock retains the cold. When more water moves in, the collar helps the rock partially float, so even a heavy rock might slide when the wind blows. The presence of ice collars could explain why some trails start narrow and get wider: the rock gradually sinks into the wet clay as its icy lifejacket melts away.
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Kletetschka is coordinating a research paper by the group that will present Hygrochron and other data and will suggest a slightly different mechanism for the rock movement. The rocks are still thought to be collared by ice, but the group has identified a new parameter that is critical in explaining why it is so easy to move the rocks and create trails. The paper will give the details, but the finding means that the wind speed doesn't have to be as great to move the rocks. "This idea would also explain the trails that don't have rocks," Kletetschka says. "The trails were made by rocks whose larger parts were made from ice."
A Sailing Stone in Death Valley
