Those bright spots on Ceres refuse to stop being interesting. While the Dawn spacecraft continues to orbit the dwarf planet, bringing us closer to understanding them, scientists have used a telescope on Earth to spot something strange.
Using the HARPS spectrograph at the European Southern Observatory’s (ESO) La Silla Observatory in Chile, astronomers saw a brightening of some of the spots during the daytime in July and August 2015. This suggests the spots contain some material that evaporates in sunlight, possibly ice. Previous research had suggested hydrated magnesium sulfate, or salt, was their primary constituent.
In a paper published in the Monthly Notices of the Royal Astronomical Society, the team described how they were able to observe the bright spots from Earth thanks to the nine-hour rotation of the dwarf planet. This creates a Doppler effect as the spots move toward and away from us, also known as the radial velocity method.
"The result was a surprise," said study co-author Antonino Lanza of the INAF-Catania Astrophysical Observatory in Italy, in a statement. "We did find the expected changes to the spectrum from the rotation of Ceres, but with considerable other variations from night to night."
The spots themselves are surprising, being considerably brighter than the darker surface surrounding them. Scientists have struggled to explain how they formed, with salt flats being the preferred theory at the moment. But ice was also thought to play a part, considering that Ceres likely has a large amount of water under its surface, and these latest observations seem to support that theory.
Above is an animation of the bright spots changing in brightness. ESO/L.Calçada/NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/Steve Albers
The largest of the bright spots is found in the Occator crater, with the Dawn spacecraft in orbit around Ceres being used to get a closer glimpse of some of these regions. It is now in its lowest orbit around the dwarf planet, and hopefully it can help clear up the mystery of what forms the bright spots.
From this latest observation, the researchers suggest that in sunlight the spots form “plumes” of material, which reflect sunlight very effectively. But these plumes evaporate quickly, and towards the end of the day they lose their reflectivity. Each day and night cycle is different, though, giving a random element to how bright the spots can be.
This continues to paint a picture of Ceres being a rather interesting body, especially compared to other objects in the asteroid belt like Vesta, which was visited by Dawn before this mission. In particular, the spots suggest Ceres has some sort of internal activity, which is surprising considering it has no moons or relatively near giant planets (namely Jupiter and Saturn) to cause any tidal effects.
Scientists still aren’t entirely sure what effect is replenishing the bright spots with material, though. "This [research] implies a leakage from the interior and therefore a source of internal heating, which is not easy to find for an isolated body," lead author Paolo Molaro told Gizmodo.
In their paper, the researchers said that a "detailed analysis of Dawn’s images" would help reveal more information about the bright spots. For now, speculation will continue.
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