A Glimmer in the Asteroid Belt
Against the dark, asphalt-like surface of dwarf planet Ceres, brilliant white patches gleam. First seen as a mysterious glimmer by telescopes, these features were brought into sharp focus by NASA's Dawn spacecraft, which began orbiting Ceres in 2015. The largest of these "bright spots," or faculae, are located inside Occator crater, a 92-kilometer (57-mile) wide impact basin in Ceres's northern hemisphere.
The crater itself is a complex structure, estimated to be about 22 million years old. Inside, a central pit about 11 kilometers (7 miles) across contains the brightest and largest deposit, named Cerealia Facula. A smaller, more diffuse collection of spots, the Vinalia Faculae, is scattered across the eastern crater floor. These patches are extraordinarily reflective, with an albedo of about 40%, making them four times brighter than the average Cerean surface. Early observations even detected a periodic haze that filled the crater around noon, suggesting some form of ongoing outgassing or sublimation.
Cryovolcanoes of Salt
The bright material is not water ice, as initially speculated, but a concentration of salts—primarily sodium carbonate (Na₂CO₃), a compound of sodium, carbon, and oxygen. This composition shows a geological process driven by a deep, subsurface reservoir of brine.
The prevailing model suggests the impact that created Occator crater about 20 million years ago also generated significant heat, melting a portion of Ceres's water-rich mantle and creating a slushy chamber just beneath the surface. This heat-driven activity, combined with large fractures from the impact that reached a deeper, long-lived brine reservoir, allowed salty liquids to percolate upward.
As this brine reached the vacuum of the surface, the water flashed into vapor and sublimated, leaving behind the highly reflective salt crust. The central deposit, Cerealia Facula, is a dome of this material, suggesting multiple eruptive events. Analysis of the deposits shows they are remarkably young. The central dome of Cerealia Facula may be only 2 million years old, indicating that this cryovolcanic activity continued long after the initial impact and could even be ongoing. The discovery of hydrated sodium chloride, which dehydrates in hundreds of years under surface conditions, further supports the idea of very recent brine exposure.