When NASA's Dawn mission witnessed Ceres' weird bright spots up-close for the first time, planetary scientists were baffled. What material could produce such a bright feature on an otherwise grey surface? Now, a year since the probe arrived in orbit around the dwarf planet, scientists may be closing in on an answer.
However, it's not Dawn that has found the latest clue as to what these bright patches could be; it was a powerful observatory on Earth that noticed very slight changes as Ceres' surface is gently heated by the sun.
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The bright spots in question are mainly clustered inside a large crater called Occator. Now that Dawn is in its lowest mapping orbit around the small world nestled in the Main Asteroid Belt (between the orbits of Mars and Jupiter), zipping across the cratered landscape at an altitude of only 240 miles, it has captured wonderfully detailed observations of the crater's floor. The bright feature is actually a cluster of bright spots with diffuse, almost powder-like material surrounding the brightest patches.
The leading theory, so far, is that it's an icy material such as water ice, but some kind of mineral deposit is also a possibility. Now, with the help of the HARPS spectrograph attached to the ESO 3.6-meter telescope at La Silla, Chile, it seems the ice theory has just become a whole lot stronger.
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"As soon as the Dawn spacecraft revealed the mysterious bright spots on the surface of Ceres, I immediately thought of the possible measurable effects from Earth," said Paolo Molaro, at the INAF–Trieste Astronomical Observatory and lead author of a study published in the journal Monthly Notices of the Royal Astronomical Society. "As Ceres rotates the spots approach the Earth and then recede again, which affects the spectrum of the reflected sunlight arriving at Earth."
As Ceres rotates every 9 hours, HARPS is so sensitive that it can detect the very slight Doppler shift in spectrum frequency as the bright spots rotate toward and away Earth, but during observations for 2 nights in July and August 2015, more changes not related to Ceres' spin were detected.
"The result was a surprise," said co-author Antonino Lanza, also from the INAF–Catania Astrophysical Observatory. "We did find the expected changes to the spectrum from the rotation of Ceres, but with considerable other variations from night to night."
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And it appears that these changes are consistent with some kind of volatile (ice) being exposed to sunlight and venting vapor into space, causing an increase in reflectivity. It seems that when Occator experiences solar heating, plumes are produced and then evaporate, creating a complex spectroscopic signal that evolves during that hemisphere's daytime. This finding appears to be consistent with earlier observations made by Dawn showing a mysterious haze over Occator.
We know that Ceres is rich in water, but whether or not these bright spots are due to water ice erupting to the surface has yet to be proven, though the evidence seems to become more compelling the longer we study the solar system's innermost dwarf planet.