When the Dawn spacecraft left Vesta and began its transit to Ceres, scientists already knew from Hubble and ground-based imagery that there was something odd about Ceres: there was a big bright area on one side. The imagery they had was nowhere near good enough resolution to tell what it was, or even how big it was, but even before Dawn could see Ceres, that bright spot was a major area of interest.
Then, as Dawn approached and entered orbit, it started to return pictures which showed the bright spot — and to everybody’s frustration (coupled with excitement for the unknown!), the pictures were only deepening the mystery. Whatever it was, this surface feature was so bright compared to the rest of Ceres that it was impossible to photograph; it was always massively overexposed, obliterating any details. As Dawn arrived and started moving into closer and closer orbits, the resolution improved, but all that was happening with the bright spot was that it kept getting smaller with each image — whatever it was was *still* so bright it was overwhelming detail immediately around it. It quickly became apparent that it was approximately in the middle of a crater, dubbed Occator Crater, and although planetary scientists were cautious about drawing a link, it was hard to imagine that being coincidence. But they would need to see more. As with Iapetus, they needed to know if this was light stuff on top of darkness, or a scraped out bit of darkness revealing light stuff underneath. As the orbits contracted and Dawn got better imagery, the point eventually split into two, then more, and finally there was enough detail to show that it’s a thin layer of brilliantly white material (which spectroscopic analysis has identified as sodium carbonate) clustered around the central uplift of Occator Crater, and a series of what look like splash points nearby. Perhaps whatever created Occator, or possibly a later impactor, broke through into a layer of briny water; there is evidence Ceres had significant subsurface water in the past and possibly still does. That water would’ve boiled away almost instantly, exposed to sunlight and the vacuum of space, leaving the white precipitate behind. Or maybe the surface was weakened, allowing hydrothermal activity — salty geysers, resulting in a sort of sodium carbonate snow. Other, much smaller white patches have also been found on Ceres, so it isn’t a unique occurrence. Perhaps it’s just the most recent.
Many questions remain about the white spots in Occator Crater. But now that Dawn is in its final orbit (roughly 20-22 miles above the surface of Ceres), it’s returning stunning new images and data that may allow scientists to puzzle out the whole story. In the meantime, here’s one of the closest images taken of Cerealia Facula, the brighter central region in Occator Crator: