First observed with the Hubble Space Telescope in 2003, the curious bright spots on the dwarf planet Ceres—the largest world in the main asteroid belt between Mars and Jupiter—was brought into exquisite focus with the arrival of NASA’s Dawn spacecraft in 2015. The largest and brightest of these spots—a single 340-meter-high mound named Cerealia Facula and a cluster just to its east named Vinalia Faculae, both of which are located inside the 92-km-wide Occator Crater—were focal points during Dawn’s approach to Ceres. After investigation by the spacecraft after it entered orbit on March 6, 2015, the bright spots were determined to be deposits of salt—specifically sodium carbonate, which had risen to Ceres’s surface from somewhere underground. But when, from where and how far down remained a mystery.
Now, scientists on the Dawn mission have found the answers.
The following is from a news report issued by NASA on August 10, 2020:
Long before Dawn arrived at Ceres in 2015, scientists had noticed diffuse bright regions with telescopes, but their nature was unknown. From its close orbit, Dawn captured images of two distinct, highly reflective areas within Occator Crater, which were subsequently named Cerealia Facula and Vinalia Faculae. (“Faculae” means bright areas.)
Scientists knew that micrometeorites frequently pelt the surface of Ceres, roughing it up and leaving debris. Over time, that sort of action should darken these bright areas. So their brightness indicates that they likely are young. Trying to understand the source of the areas, and how the material could be so new, was a main focus of Dawn’s final extended mission, from 2017 to 2018.
The research not only confirmed that the bright regions are young – some less than 2 million years old; it also found that the geologic activity driving these deposits could be ongoing. This conclusion depended on scientists making a key discovery: salt compounds (sodium chloride chemically bound with water and ammonium chloride) concentrated in Cerealia Facula.
On Ceres’ surface, salts bearing water quickly dehydrate, within hundreds of years. But Dawn’s measurements show they still have water, so the fluids must have reached the surface very recently. This is evidence both for the presence of liquid below the region of Occator Crater and ongoing transfer of material from the deep interior to the surface.
The scientists found two main pathways that allow liquids to reach the surface. “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,” said Dawn Principal Investigator Carol Raymond. “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.”
By studying Ceres’ gravity, scientists learned more about the dwarf planet’s internal structure and were able to determine that the brine reservoir is about 25 miles (40 kilometers) deep and hundreds of miles wide.
“Dawn accomplished far more than we hoped when it embarked on its extraordinary extraterrestrial expedition. These exciting new discoveries from the end of its long and productive mission are a wonderful tribute to this remarkable interplanetary explorer.”
— Marc Rayman, Dawn Mission Director, JPL
These findings also confirm that Ceres, like many of the moons orbiting Jupiter and Saturn and even its distant dwarf planet cousin Pluto, is a world rich in liquid water.