And new observations of the sphere of rock and ice circling our Sun between Mars and Jupiter have added to the mystery, researchers said Monday.
Astrophysicists have been looking to a $473-million (446-million-euro) mission to test theories that Ceres is a water-rich planetary "embryo"—a relic from the birth of the Solar System some 4.5 billion years ago.
But an early batch of data from NASA's Dawn probe, unveiled at a conference of the European Geosciences Union (EGU), may have made the Ceres riddle even greater.
In orbit around Ceres since March 6 after a seven-and-a-half-year trek, Dawn peered at two bright spots on its surface deemed to be telltales of its chemical and physical ID.
But instead of explaining the spots, analysis found the two seemed to "behave distinctly differently," said Federico Tosi, who works on Dawn's Visible and Infrared Mapping Spectrometer (VIR).
While Spot 1 is colder than its immediate surroundings, Spot 5 is not.
One picture, as seen by the human eye, shows Ceres as a "dark and brownish" ball with both white spots clearly visible.
But in thermal images, Spot 1 becomes a dark spot on a reddish ball, indicating it was cooler than the rest of the surface,. The "biggest surprise" was that Spot 5 simply disappeared on the thermal image.
"For sure, we have bright spots on the surface of Ceres which, at least from a thermal perspective, seem to behave in different ways." say scientists from the Max Planck Institute. (Dawns first color map of Ceres).