What It Is
In March 2015, the Dawn spacecraft arrived at Ceres, the largest object in the asteroid belt. It soon observed about 130 shiny spots that dotted the dwarf planet’s surface. Scientists were unsure what these were; some guesses included volcanic activity, salt, and water ice.
Of course, these weren't the only ideas.
A plethora of enthused individuals suggested that the bright spots may be aliens, or alien technology, or some other strange never-before-encountered-substance (I hesitate to link to any of these sites, lest we give them more traffic, but a Google search yields many such articles).
At the time of the initial detection, scientists were careful to note that these bright spots are very likely a perfectly natural phenomenon, and now we have further proof of this, as detailed in a paper published in Nature,
Slight color variations from Dawn's camera data show that the brightest spots, found in Occator crater, contain magnesium sulfate. Other substances, including water ice, might be present as well. In other word, its a briny mixture of ice and salts that, apparently, reside just beneath portions of the surface.
While this may not be as exciting as an alien spaceport, it is a remarkable find, as it shows that water is sublimating from the surface of an active crater—it means that this frozen wasteland is actually an active world.
The Herschel space telescope observed a haze surrounding the bright spots, indicative of some kind of frozen substance that may be turning to gas when hit by sunlight, and water ice is a logical possibility.
Below, you can see a video of Ceres rotation and the Occator Crater. Here, the dwarf planet is shown in false-color renderings, which highlight differences in surface materials.
What It Means
If water vapor is indeed sublimating from Occator's bright spots, then the spots may actually be some of the youngest features on the planetoid—otherwise the water in the bright areas would have all been spent by now.
"The high reflectance of the bright material in Occator crater… is a strong indication that those deposits are young and indicate active geologic processes at Ceres," geologist Carol Raymond from the Dawn team said in an email with Popular Science. "This is a fascinating possibility and the team is eagerly mapping the geology and morphology of the surface and analyzing the spectral data to characterize these processes."
The paper postulates that, beneath the surface of Ceres, there is a layer of salty water ice, and that when space rocks crash into Ceres, they reveal the layer below. If this hypothesis is correct, it would explain why the entire dwarf planet is dotted with bright spots.