Death Valley, in California’s Mojave Desert, is famous for its extreme heat, with ground temperatures that have been recorded as high as 201 degrees F. In the northwestern part of the valley, there’s a natural recurring phenom- enon, documented by scientists since the 1940s but, until recently, unexplained. “Sailing stones” move across Racetrack Playa seemingly of their own volition, leaving deep tracks in their wake, often up to hundreds of feet long and between three and 12 inches wide. Many of the stones weigh as much as 700 pounds, and are so heavy that even the powerful winds should have difficulty moving them.

Nobody has yet seen these stones, mostly dolomite, move — just the evidence indicating movement — but over the years, many theories about the phenomenon have been proposed. Now, NASA researchers believe that they have found the explanation — it’s a combination of meltwater, frost and wind.

MYSTERY SOLVED
In the summer of 2010, a group that included senior NASA Goddard scientists, undergraduates and graduate students visited Racetrack Playa to study the phe- nomenon. Three months before, one of the group’s leaders — NASA Goddard Space Flight Center geophysicist Gunther Kletetschka — had buried a large number of sensors, known as Hygrochrons, across the plain. Buried at depths of 3∕4 inch and 3 inches, respectively, the devices were able to measure temperature and water content accurately. When the scientists dug them up, data revealed that in March, there were freezing temperatures in the top layer of soil, while in April, the soil was wet at depths down to three inches. Based on this information, Kletetschka and the rest of the team concluded that in March, the plains of Racetrack Playa are covered with pools of meltwater runoff from the mountains surrounding the plain after winter. At night, the water freezes around the bottom of the rocks, creating an “ice collar.” In April, when more runoff arrives on the plain, these ice collars help float the rocks. In some cases, the wind lends a helping hand, but Kletetschka says that it’s not a necessary component.

“We found a new parameter allowing this phenomenon to occur without strong winds,” he says, and he’s currently organizing a collaborative paper that describes the phenomenon.

Kletetschka and his colleagues’ theory can also explain why, in some cases, scientists find tracks that seem way too big for the rocks that have made them: Tracks are made by the ice block that circled the rock, only to melt away later, leaving the rock as evidence.

The new theory could put an end to years of speculation about what makes the rocks move. In the past, animals or gravity were thought to be responsible; aliens have also been credited, as well as students at the University of Nevada at Las Vegas engaging in fraternity pranks.

The factor that the previous explanations could not account for is that footprints made on the surface of the wet, muddy plain would typically be visible for years, and no tracks except for the ones made by the rocks have ever been found on the playa. Another theory, that earthquakes might move the rocks, has also been dismissed, since all quakes in the area are recorded, and none has coincided with new tracks being made by the rocks.

ROCKS MOVE UPHILL
Activity related to magnetic fields has also been proposed as an explanation; two of the students who participated in the summer study tried to detect magnetic fields in the area but found none. The slight incline of the plain doesn’t explain the 

phenomenon either, as the scientific measurements indicate that most of the rocks actually move upward. The one-inch incline is so insignificant that the wind can push the ice-covered rocks upward as they sail on the slippery, friction-free ground.

To determine how the rocks move, the scientists marked all rocks and tracks with colored pushpins and measured the incline of the track and ground nearby using a builder’s level. Individual rocks were numbered and tagged using GPS technology. Finally, researchers took photographs of both the rocks and the tracks to closely monitor the path of each individual rock.

In a related project, two students photographed the Cottonwood Mountains and Nelson Range surrounding Racetrack Playa and correlated the photos with GPS coordinates. They entered the information into a program that makes it possible to determine the location of a rock based on a photo alone. Using this program, future visitors to Racetrack Playa can upload their private photos to a project website (racetrackplaya.org) and help the scientists monitor the rocks’ movements.

To date, there’s no film of the moving rocks, though several scientists have tried to capture their progression.The rocks don’t move every year, which complicates the effort, but nevertheless, several teams of scientists plan to set up camera equipment on the plain.

INDUSTRIAL APPLICATIONS
Apart from solving the mystery of how the rocks move, the new findings may be used in manufacturing. Microelectromechanical systems (MEMS) technology, used in many different industries, employs miniature nanotechnology components, which often have problems with dust particles collecting on the thin silicon wafers.

“I think the new knowledge about friction gained from Racetrack Playa can help with this problem,” Kletetschka says.

FACTS: RACETRACK PLAYA
Length: 4.5 miles
Width: 2 miles
Eleavation: 3,707 feet above sea level
Incline: 1 inch