NASA’s Dawn spacecraft has been taking images of Ceres, the largest body in the asteroid belt, since December 2014. As Dawn’s orbit brought it closer to the dwarf planet’s surface, a mystery appeared: Ceres has bright spots. Over 130 separate areas on Ceres, reaching up to 6 miles wide, shine out from their dark surroundings. After months of study and speculation, scientists have confirmed that the brightest spots are made of sodium carbonate, making Ceres’ Occator Crater the largest deposit of this sort of mineral ever found beyond Earth.
Since images of the bright spots were released, scientists and the public alike have speculated about what they could be: NASA even created a poll to gather opinions. At first, the leading theory suggested that the bright spots were expanses of water ice, either brought to the surface by volcanic activity or revealed by an impact. Even as evidence mounted that the spots were made of some sort of mineral salt, more and more ideas, ranging from the plausible theory that Ceres was covered in an ice mantle just below the surface to the fanciful notion that the spots could be a shining alien city.
Until June, no clear answer revealed itself. According to Andreas Nathues of the Max Planck Institute in Germany, “The main problem we had [analyzing the bright spots] during approach phase was that our spatial resolution was too low.” Because Dawn’s instruments were calibrated based on Ceres’ darker surface, the images didn’t capture any detail in the bright areas. “In June, we divided our exposure times for the bright spots and the dark surface. Then the spatial resolution started to become sufficient to identify what the spots are,” Nathues said.
As Dawn and its newly-calibrated cameras approached Ceres, details of the bright spots came into focus. “Dawn has transformed what was so recently a few bright dots into a complex and beautiful, gleaming landscape,” said Marc Rayman, Dawn’s chief engineer and mission director. Now, two new studies, published in Nature and Nature Geoscience, have concluded that Ceres’ bright spots are sodium carbonate and that Ceres is much drier than previous observations led scientists to expect.
On Earth, sodium carbonate is primarily found in hydrothermal environments, where hot water leaks from the planet’s crust. Similarly, the sodium carbonate on Ceres appears to
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