Editor’s Note: This is the second part of a story about similarities in minerals found at Searles Lake and the asteroid Bennu. Read Part 1.
At Searles Lake, water levels—and the compositions of brines—have varied dramatically throughout the Pleistocene due to the region’s topography and the availability of water. The lake lies in a closed basin in the Mojave Desert in southeastern California, along with four other lakes fed by the Owens River. The river’s flow has fluctuated sharply due to climate, tectonic, and volcanic changes.
Searles Lake and the adjacent playas in several nearby valleys are visible in the satellite images above. Though Searles is generally dry, the images were captured on February 11, 2024, after water had pooled on the playa following unusually wet weather.
In the natural-color image (left), Searles Lake appears green, and snow is white. In the false-color image (bands 7-2-1) on the right, it is easier to distinguish between water and land. Snow appears light blue, water is dark blue, and areas with vegetation are green. Some of the water on the playa may also be the result of brine pumped to the surface during mining processes. Both images were acquired by the MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA’s Terra satellite.
During high-flow periods on the Owens River, Owens Lake would spill into China Lake, then Searles Lake. Searles would overflow into Panamint Lake, then into Lake Manly in Death Valley. Water levels at Searles sometimes rose as high as 700 feet (200 meters) above the current basin floor. When the flow of the river slowed, Searles Lake dried, encouraging the large-scale formation of evaporites—sedimentary deposits of salts produced by the evaporation of water.
The Owens River provided another key ingredient for the formation of evaporites at Searles Lake—an unusually rich array of dissolved minerals. Weathering of volcanic rocks in Owens Valley and the Long Valley Caldera, a depression left by a massive volcanic explosion, was a key source of these mineral solutes. Geothermal springs rich in ions of sodium, potassium, bicarbonate, and calcium carbonate in the caldera flow into Owens River.
The Operational Land Imager (OLI-2) on Landsat 9 captured the image of Long Valley Caldera shown above on April 8, 2024. The lower-elevation caldera appears brown while the higher-elevation hills surrounding it are capped with snow. The Owens River flows southeast from Lake Crowley, a reservoir created by damming the river in 1941.
Though the properties of the evaporite minerals found on asteroid Bennu make clear that they formed in liquid water, the volume of that water is less well understood. “The water on Bennu’s parent body may have been in quite small pockets, perhaps a meter thick, surrounded by wet, drying mud,” said Tim McCoy, curator of meteorites at the Smithsonian’s National Museum of Natural History and coauthor of a new report about the evaporites found on Bennu published in Nature.
“We know Bennu’s parent body did not have surface lakes or rivers like you see at Searles Lake,” said Hannah Kaplan, a planetary geologist at NASA’s Goddard Space Flight Center. “The water on Bennu’s ancestor likely percolated through the subsurface, between the pores of rocks and through small fractures in hydrothermal systems.” Supporting that idea, images of Bennu obtained by OSIRIS-REx show carbonate veins ranging from 3 to 15 centimeters (1 to 6 inches) on some boulders. The image above shows one of these veins.
Clues to the nature of this ancient water might be found on the dwarf planet Ceres, where spacecraft have observed similar evaporite minerals on the surface, and on Enceladus, an icy moon of Saturn that emits plumes of water containing sodium salts, McCoy added.
When water levels were high at Searles Lake, the lake produced iconic evaporite formations—tall spires of calcium carbonate (tufa) known as the Trona Pinnacles. The spires, shown above, vary in size and shape, with some rising as high as 140 feet (40 meters).
The spires formed over long periods when Searles Lake had high water levels and hot spring water bubbled up, mixed with alkaline waters, and interacted with communities of algae. The otherworldly features have appeared in movies and shows such as Star Trek V, Lost in Space, Planet of the Apes, and Battlestar Galactica.
“The pinnacles look incredible and are made from calcium carbonate, a mineral we found on Bennu,” said McCoy. “But they’re different in many ways,” cautioned Alfonso Davila, an exobiologist at NASA’s Ames Research Center, noting that the samples on Bennu were tiny flakes, not large-scale deposits like those at Searles Lake.
Mars, the dwarf planet Ceres, and Saturn’s moon Titan are other celestial bodies where scientists have discovered large-scale evaporite deposits on the surface, Davila noted. The image above shows a sodium carbonate deposit on Ceres, likely the product of brine spilling across the crater after a meteorite punched a hole into the icy crust.
“Searles Lake offers important clues, but remember that the journey that evaporite minerals on Bennu, Ceres, or Enceladus have gone through was likely quite different in some ways than those at Searles,” Davila said. Earth has an atmosphere, life, liquid oceans, and warm temperatures. “We can’t say all the same things about Bennu.”
NASA Earth Observatory images by Lauren Dauphin, using Landsat data from the U.S. Geological Survey and MODIS data from NASA EOSDIS LANCE and GIBS/Worldview. Bennu OSIRIS-REx photo originally published in Kaplan, et al. courtesy of Hannah Kaplan. Ceres image of Occator Crater from the Dawn mission and courtesy of the NASA Scientific Visualization Studio. Trona Pinnacles photo by Bob Wick for the California Bureau of Land Management. Story by Adam Voiland.