When the Dawn spacecraft finally reaches Ceres early next year, it may find a world much stranger than expected.
Back in January, the European Space Agency's Herschel Space Observatory discovered a cloud of water vapor surrounding the little world. This may be venting from Ceres in much the same way that water vapor is erupting from Saturn's moon, Enceladus, which is only slightly larger than the asteroid. Or it may be possible that this is water vapor subliming from the surface as Ceres is warmed by the sun, in much the same way that the coma surrounding a comet is formed.
The explanation that the cloud of water vapor is the result of cryovolcanoes like those on Enceladus would require a source of internal heat. In the case of Enceladus, this is the result of internal tides created by Saturn. There is no nearby large body to do this for Ceres, so the question of what the source of heat might actually be is open.
"This is the first time water vapor has been unequivocally detected on Ceres," says Michael Küppers of ESA, "or any other object in the asteroid belt and provides proof that Ceres has an icy surface and an atmosphere."
While this sort of activity has not been observed before in an asteroid belt object, it is not entirely unknown. There is the enigmatic Chiron, for instance. Although originally classified as an asteroid, it was later found to have comet-like attributes. Chiron's orbit takes it in a loop between Uranus and Saturn. In 1988, as it was making its closest approach to the sun, Chiron was observed to double in brightness. It eventually grew a comet-like tail. Ices bound within its surface soil were boiling away, blowing ice and dust into a cloud surrounding the body.
Not knowing quite what to call it, astronomers tossed a coin that landed on its edge so, depending on who you are talking to, the object is either the asteroid 2060 Chiron or the comet 95P/Chiron.
By the way, I've been hedging about labeling Ceres as either a planet, asteroid or comet since the IAU, with its usual clarity, recently reclassified Ceres. It had once been the largest asteroid, an honor now claimed by Vesta, but it is now a "dwarf planet" in the same category as Pluto.
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Ceres and Chiron art by Ron Miller