The mysterious bright spots glowed from Ceres' dark surface like alien headlights, capturing many Earthlings' imaginations. But researchers say they're the result from mineral salts, citing data captured by NASA's Dawn mission to study the dwarf planet.
While the findings will likely dampen speculation over the spots, they also portray Ceres as a complex planet — one that may be holding a reserve of ice.
The spots on Ceres, the largest object in an asteroid belt between Mars and Jupiter, caused a stir in February, after the Dawn spacecraft sent home an image that showed two very bright spots inside a large crater on Ceres, where the surface is otherwise so dark it's often compared to fresh asphalt.
Dispelling theorists' notions of an alien city tucked away in our solar system, researchers say in a study published by the journal Nature, "These unusual areas are consistent with hydrated magnesium sulfates mixed with dark background material, although other compositions are possible."
The salt is called hexahydrite, NASA says, noting "A different type of magnesium sulfate is familiar on Earth as Epsom salt."
A thin haze in the crater also suggests frozen water could be near the surface, according to the study, which was led by Andreas Nathues of the Max Planck Institute for Solar System Research in Germany.
The brightest spot is seen in the Occator Crater, a large area that's featured in a new video released by NASA, showing Ceres' rotation. The video uses "false color" to emphasize differences in the composition of Ceres' surface, as seen from an orbit altitude of about 2,700 miles.
From the editor's summary of the German study:
"Recent reports of water vapor, bound water and OH on Ceres raised the possibility there may be surface water there, and the new images reveal multiple bright spots on the floor of crater Occator that could be from surface ice. The largest of these, corresponding to the crater's central pit, produces haze clouds inside the crater with a diurnal rhythm, a clear indication of possible sublimation of water ice."
The bright spots have been a source of both mystery and wide-ranging theories online, where some observers noted that the earliest images seemed to show the spots maintaining their brightness from many viewing angles. To some, that suggested the light's source is internal, rather than reflected.
Or as the top commenter on a previous Two-Way post wrote, "I'm not saying it's aliens. But it's aliens."
When NASA asked the public to guess what's creating the brightness in an online survey, both salt deposits and ice got more than 10 percent of the vote – but both of them were dwarfed by the somewhat ominous choice "Other," which attracted nearly 40 percent of the votes.
Speculation about the bright spots' significance also spiked when NASA announced it had spotted a "pyramid-shaped peak towering over a relatively flat landscape."
While the new studies might make it harder for people to imagine Ceres as a space colony or way station for an advanced race, one detail from the latest research may help dedicated theorists hold onto their now-tenuous hopes.
After positing that ammonia on the small planet may have reacted with phyllosilicates, the NASA team that works on the Dawn project writes in a study of their own, "This suggests that material from the outer Solar System was incorporated into Ceres, either during its formation at great heliocentric distance or by incorporation of material transported into the main asteroid belt."
The implied mode of transportation, of course, would be from objects tumbling through space; Ceres' surface is pocked by impact craters.
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