25-mile-deep reservoir of salty water found beneath surface of Ceres
Through its 11-year stint studying the asteroid belt between Mars and Jupiter, NASA’s Dawn mission has profoundly changed our understanding of the dwarf planet Ceres. New analysis of data collected during the latter stages of this mission has offered some compelling answers to long-standing questions about the makeup of Ceres, starting with presence of a 25-mile-deep reservoir of brine beneath the surface.
Over the past few years as data from the Dawn mission continued to roll in, scientists have been piecing together a puzzle of peculiar geological activity on Ceres. The nature of unusually bright spots that feature in its Occator Crater posed a mystery, with scientists working to understand how they are formed and what they might represent.
Evidence had suggested that these deposits are formed by salty liquid bubbling up to the surface to leave the highly reflective deposits on the crust, but where could such a liquid come from? While an underwater reservoir of brine was seen as a strong possibility, the discovery of new evidence has now led scientists to conclude not only that this is indeed the case, but that ongoing geological activity is keeping these salty surface features fresh and shining brightly.
The environment of Ceres’ surface is one where salts holding water would be expected to dehydrate in just a matter of centuries. But the team found salt compounds concentrated in one of the better-known bright spots, called Cerealia Facula, that were still carrying water, suggesting that they must have made their way to the surface only recently. According to principal investigator for the Dawn mission, Carola Raymond, the pathways for this process could be traced back to the crater impact millions of years ago.
"For the large deposit at Cerealia Facula, the bulk of the salts were supplied from a slushy area just beneath the surface that was melted by the heat of the impact that formed the crater about 20 million years ago," she explains."The impact heat subsided after a few million years; however, the impact also created large fractures that could reach the deep, long-lived reservoir, allowing brine to continue percolating to the surface."
Adding further support to the idea of liquids on Ceres was the discovery of small hills resembling pingos here on Earth, which are small icy mountains found in the polar regions that are created by pressurized groundwater. These had been seen on Mars before but never on a dwarf planet.
The team also carried out new gravity measurements of Ceres using the latest data from Dawn, revealing that the brine reservoir is around 25 miles (40 km) deep, and hundreds of miles wide. These gravity measurements also offered new insights into the dwarf planet’s interior, finding that its crustal density increases with depth. This increase in density is much greater than could be explained by pressure alone, so the team believes that it is the result of the underground reservoir incorporating salt and mud into the lower crust as it freezes.
"Dawn accomplished far more than we hoped when it embarked on its extraordinary extraterrestrial expedition," says Mission Director Marc Rayman of NASA's Jet Propulsion Laboratory in Southern California. "These exciting new discoveries from the end of its long and productive mission are a wonderful tribute to this remarkable interplanetary explorer."