For decades, rockhounds gathered each year at Searles Lake in the Mojave Desert of southeastern California. Wielding crowbars and pickaxes, they hunted for showy deposits of minerals such as halite, trona, calcite, and dolomite. Planetary scientists from NASA and several other institutions are finding many of the same water-soluble minerals, though their tools and destinations are worlds apart.
In September 2016, NASA launched a van-sized spacecraft, OSIRIS-REx, that traveled billions of miles to reach a small diamond-shaped asteroid named Bennu. Scientists think that this “rubble pile” asteroid, which crosses Earth’s orbit every 6 years, formed from fragments of a larger asteroid that broke apart between 1 and 2 billion years ago after a cataclysmic collision.
In September 2020, OSIRIS-REx briefly touched down on Bennu’s surface, collected 4.3 ounces of crumbly regolith, and stowed it safely before returning to Earth. The video below shows what the lumpy asteroid looked like as OSIRIS-REx approached it in December 2018.
In September 2023, the spacecraft released a mini-fridge-sized capsule that parachuted into Utah with the sample. Since then, scientists have been running bits of the dark, powdery sample through a gauntlet of microscopes, mass spectrometers, and CT scanners.
Their findings, described in a new report in Nature, reveal evidence of sodium- and carbonate-rich salts called evaporites. These minerals likely formed when a briny liquid gradually evaporated in the subsurface of a wet, muddy protoplanet about 4.5 billion years ago in the early days of the solar system.
Few places on Earth produce combinations of evaporites similar to those found in the Bennu sample. “One of the best studied is Searles Lake,” said Tim McCoy, a curator of meteorites at the Smithsonian’s National Museum of Natural History and co-lead of the study.
Although the minerals discovered on Bennu predate those at Searles Lake, they formed in similarly salty, wet conditions. The OLI (Operational Land Imager) on Landsat 8 captured the image at the top of this page on August 15, 2024, showing the mostly dry playa of the lake. The lake is named after John Wemple Searles, a prospector and borax miner who began hauling the white mineral from the lake in 1873 using mules.
A century and a half later, signs of mining still abound. A pit mine and various industrial facilities are visible in the northwestern corner of the image around Trona, a company town established in 1913 and a major source of potash used in gunpowder during World War I. Today, the colorful rectangular evaporation ponds still concentrate valuable minerals like soda ash, salt, and lithium from salt-rich brines.
“Soda lakes” like Searles Lake intrigue planetary geologists in part because they contain highly alkaline waters. With a mixture of sodium, carbonate, and chloride ions, their brines are rich in phosphates—a substance key to the development of organic molecules, including some involved in the formation of genetic material, cell membranes, and other components thought to be critical to the formation of life.
NASA scientists recently announced the discovery of organic molecules—including several that are among the ingredients needed for life—in the Bennu sample as well (see video above).
Water is a remarkably good solvent, meaning common salts like sodium chloride (NaCl) or calcium carbonate (CaCO3) dissolve easily in it. At the molecular level, salts break into positively or negatively charged ions, such as sodium (Na+), chloride (Cl-), or calcium (Ca+), and are surrounded by molecules of water when they dissolve. The opposite occurs when water is removed from brines rich with ions, which allows ions to bond and crystallize into salts.
At Searles Lake, minerals typically develop in a predictable sequence in zones akin to bathtub rings, with calcium carbonate and magnesium carbonate forming first, followed by calcium sulfate and sodium carbonate in a middle ring. The most concentrated brines deposit chlorides and fluorides of sodium and potassium in the center of the lake. Minerals can also precipitate directly from fluids without evaporation if the ion concentration is high enough, as probably happened early in the sequence.
“Precipitation occurred in both ways on Bennu’s parent body,” McCoy said. Researchers identified 11 minerals in the asteroid sample that likely formed in liquid water. Six of these—trona, halite, pirssonite, thénardite, calcite, and dolomite—are also found at Searles Lake. The scanning electron microscope image above shows a close-up of trona, a sodium carbonate mineral, found on Bennu. Each needle is under 1 micrometer wide and 5 to 10 micrometers long; a human hair is about 100 micrometers wide. At Searles Lake, trona is often processed to produce soda ash.
Editor’s Note: Read more about Searles Lake and Bennu tomorrow.
NASA Earth Observatory images by Lauren Dauphin, using Landsat data from the U.S. Geological Survey. Bennu mineral sample photograph by Rob Wardell and Tim McCoy (Smithsonian Institution). Story by Adam Voiland.
