NASA's Dawn spacecraft has provided an even closer look at the bright spots on the surface of the dwarf planet Ceres — but the origins of the spots are still subject to debate.
The latest view, released Wednesday, shows the flashes of sunlight reflected by the spots inside a 57-mile-wide (90-kilometer-wide) crater as Dawn flew within 4,500 miles (7,200 kilometers) of Ceres on May 16. There's one big spot with a smattering of smaller spots off to the right. The picture also shows that Ceres' surface is covered with scads of craters and channels.
Ceres has a diameter of 590 miles (950 kilometers), which makes it the largest object in the main asteroid belt as well as the smallest known dwarf planet. Dawn began orbiting Ceres in March after making a three-year trek from the asteroid Vesta, the second most massive asteroid.
Scientists have long suspected that Ceres held reservoirs of water ice — and the leading hypothesis is that the bright spots consist of ice. Last year, scientists associated with the European Space Agency's Herschel mission said they detected a plume of water vapor rising from Ceres' surface.
"We have looked for a plume, and thus far, we have not found it," UCLA astronomer Christopher Russell, the Dawn mission's principal investigator, told NBC News in an email. "But we have these bright spots that have the reflectivity of ice, and whose spectrum of reflected light is similar to that expected from ice. So ice is a good bet."
Dawn's camera will have to get closer for confirmation.
"How is the ice getting to the surface, if it is ice?" Russell asked. "That is why we are awaiting still higher resolution. Is it coming out as a water/ice volcano and making a mountain of ice on the surface, or is there a hole there which has dug down to ice or water? And what is causing all the little bright spots near the big bright spot? We are sure there is some logical explanation for all this, but for now, we are just scratching our heads."
Carol Raymond, who's the mission's deputy principal investigator at NASA's Jet Propulsion Laboratory, said there's still a chance that the reflective material is a type of salt, or perhaps a mixture of ice and salt. Data from Dawn's upcoming survey, due to begin on June 5, "will help us to test and refine hypotheses of the origin, but like a good detective story we will continue to peel this onion as we descend closer to the surface," she wrote in a follow-up email.
Starting in August, Dawn will map Ceres from an altitude of 900 miles (1,450 kilometers). That should provide definitive data about the reflective material, as well as detailed maps of the geological setting for the bright spots, Raymond said.
The mystery should be solved by December, when Dawn settles into its lowest orbit, just 230 miles (375 kilometers) above the surface. Or will it? It's hard to believe that the "alien Death Star" hypothesis will die that easily.