Vox writes Those mysterious bright spots on Ceres? NASA finally got a close-up look "In the summer of 2015, as NASA's Dawn spacecraft approached the dwarf planet Ceres, scientists kept seeing a pair of bizarre shiny dots peering back."
The crater is 57 miles across and 2.5 miles deep. "The latest images," NASA announced, "taken from 240 miles above the surface of Ceres, reveal a dome — with fractures crisscrossing the top and flanks — in a smooth-walled pit in the bright center of the crater."
So why is the crater shiny? In a paper published last December in Nature, scientists argued that the reflection may come from a magnesium sulfate called hexahydrite. The idea is that Ceres has a salty layer of water ice just beneath its surface. At some point in the past, asteroids pummeled the dwarf planet, bringing that mixture to the surface. The water ice then evaporated away in the sun, leaving only the bright-colored hexahydrite behind. And because the rest of the planet is so dark, those bright spots stick out. Still, this needs further exploration. The existence of subsurface water ice remains one of the central mysteries of Ceres.