Published online 30 November 2007 | Nature | doi:10.1038/news.2007.315

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The salt flat with curious curves

Mapping one of the flattest parts of the planet will help satellite calibration.

The survey reveals a total elevation range of less than 80 centimetres.Courtesy of the researchers

A precise topographical map has been made of one of the flattest places on Earth: the salar de Uyuni, a vast plain of white cemented salt in the mountains of Bolivia. The ground survey, aided by global positioning systems (GPS), shows variations in elevation of less than a metre across an area almost half the size of Wales.

Intriguingly, the work reveals bumps in the salt that lie above lumps of dense rock buried several kilometres below, just as water will bulge over a bump on the ocean floor. Knowing exactly where these bumps lie will help researchers to use the flat as a giant calibration device for satellite-based radar and laser altimeters.

Adrian Borsa, a geophysicist at the US Geological Survey in Pasadena, California, led the work that is now reporting the most precise GPS characterization yet of the salt flat, accurate to 2.2 centimetres. Borsa and his colleagues began the research in 2002, with a campaign that combined data from fixed GPS stations with ‘kinematic GPS’ data obtained by crisscrossing the salar in trucks fitted with GPS antennae. “It’s as if you’re on a white ocean with no waves,” says Borsa. “You see the horizon, the curvature of Earth. It’s absolutely featureless.”

“It’s as if you’re on a white ocean with no waves,” says Borsa.Courtesy of the researchers

The study was commissioned specifically as a way to calibrate NASA’s ICESat, launched in 2003 to study ice sheets in Antarctica and Greenland. Satellites can calibrate their altimeters by bouncing signals off the ocean surface, the height of which is well established. But because of atmospheric interference, tides and waves, there are uncertainties. Borsa says the salar, now so accurately mapped and with dry, clear skies, is about five times better than the ocean as a reference point.

The results have been helpful, says Sinead Farrell, a researcher at the US National Oceanic and Atmospheric Administration who is using ICESat to measure changes in sea ice. “It’s the first calibration surface that we look at,” she says. “The information that they provide is useful to all the other teams.” She adds that the salt flat is also being used for Envisat, a European satellite, and could be used to calibrate future orbiters such as Cryosat-2, a planned European ice-monitoring mission.

Not so flat

Whereas the Earth-observing community likes the flatness of the salar, geologists are interested in the tiny undulations.

The strength of gravity varies from place to place around Earth — imperceptibly to humans but in ways that scientists have mapped precisely — owing to the density of objects nearby. Borsa found that the salt had created faint mounds of some 40 centimetres in height over regions of slightly higher gravity, owing to high-density volcanic rocks kilometres below. “We found a rare breed of topography,” says Borsa, who says that this is the first time a land surface has been shown to undulate in step with the gravitational field like this. “Here’s a place where you see the imprint of the gravitational field due to buried mountain ranges.”

Borsa has a theory as to how this happened. When the salar floods, as it does every five years or so, salt dissolves and, in solution, flows to regions of higher gravity, he says.

This phenomenon is already known to oceanographers: the free-standing ocean surface changes slightly to match the pull of gravity underneath. So there are slight bulges in the ocean in places directly above submarine seamounts. “We know that ocean surfaces follow the gravitational potential surface: the geoid,” says Farrell, who was not a part of the study. “It is a new result to show that the salt flat also follows the geoid.”

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The next step for those with a geological interest, Borsa says, is to take out the portion of the elevation variations that are due to gravity and see what’s left. So far, he is seeing a strange, regularly patterned checkerboard surface. He doesn’t know what could produce it, but speculates that it could reflect the imprint of periodic pressures due to prevailing winds. Working out the forces that produce this pattern could be useful if similar phenomena are ever spotted on another planet, he says. “Now we’ve got an analogue on Earth.” 

  • References

    1. Borsa, A. A. et al. Geophys. J. Intl doi: 10.1111/j.1365-246X.2007.03604.x (2007)
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