Astronomers analyzing data from the now defunct Herschel infrared space observatory have made a huge discovery deep inside the asteroid belt. Dwarf planet Ceres, the largest body in the region, is generating plumes of water vapor.
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This exciting discovery by the European mission proves that the small world possesses significant quantities of water ice on its surface (or, indeed, below its surface) that gets heated by solar energy, causing it to erupt into space as a vapor. Ceres now joins a fascinating group of little worlds in the solar system that not only hold onto vast quantities of water ice, but also blasts water vapor into space. For example, water vapor plumes have recently been discovered around Jupiter's moon Europa and Saturn's moon Enceladus is famous for its water geysers.
"This is the first time water vapor has been unequivocally detected on Ceres or any other object in the asteroid belt and provides proof that Ceres has an icy surface and an atmosphere," said Michael Küppers of the European Space Agency in Spain and lead author of a paper published today (Jan. 22) in the journal Nature.
Ceres has long fascinated astronomers since its discovery in 1801 by Italian priest, mathematician and astronomer Giuseppe Piazzi, and has been the target of various ground and space telescope campaigns.
Although a general idea as to the little world's surface features and composition are known through fuzzy imagery, this is the first conclusive evidence of water vapor being emitted from Ceres. As the most massive asteroid in the asteroid belt that lies between Mars and Jupiter, the 590 mile (950 kilometer) wide object was promoted to dwarf planet status (the smallest dwarf planet in the solar system and closest dwarf planet to Earth) by the International Astronomical Union (IAU) in 2006. At the same time, Pluto was also reclassified - often considered a demotion - to a dwarf planet.
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Famous to astronomers it may be, but you can't beat actually sending a probe to get a closeup look of Ceres before we fully understand its true nature. And it just so happens that we're in luck.
"We've got a spacecraft on the way to Ceres, so we don't have to wait long before getting more context on this intriguing result, right from the source itself," said Carol Raymond, deputy principal investigator for NASA's Dawn spacecraft at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. "Dawn will map the geology and chemistry of the surface in high-resolution, revealing the processes that drive the outgassing activity."
Dawn will arrive in Ceres orbit in February 2015, the second massive asteroid on its tour of the inner solar system. Dawn left the orbit of protoplanet Vesta in September 2012 after orbiting that 525 kilometer (326 mile) wide object for a little over a year. And with this new historic discovery of water plumes around Ceres, Dawn has an exciting time ahead of it, observing not a static and barren world, but a dynamic and rich planetary body.
But before we can get up close and personal, do we know how Ceres is generating this water vapor?
Interestingly, of the four occasions that Herschel studied Ceres, one of those times the signature of water was not detected. This suggests that there may be some seasonal effect influencing the plume's presence. Astronomers think that as Ceres reaches the closest part in its orbit to the sun, the more intense sunlight causes its icy surface to sublimate (i.e. turn straight from ice to vapor without transitioning through a liquid phase) at a rate of around 6 kilograms (13 pounds) per second. As the dwarf planet drifts further away, little vapor is generated due to a reduction in solar energy.
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Herschel also noticed a short term variation in the water plume signal. As Ceres rotates, the water signal rotates with it, suggesting there are discrete regions on the world's surface generating the plumes. In fact, the astronomers have been able to link the water plumes with two darker regions on the surface that were first observed by Hubble. The regions, which are 5 percent darker than the Ceres average, could be absorbing more sunlight, causing the ice to heat up faster and generating the water vapor plumes more readily.
Another idea proposed by planetary scientists is that the vapor could be produced by cryovolcanoes or geysers from cracks int he surface, liberating reserves of water ice from below the surface. Fortunately, we just have to wait for a year until we find out from Dawn.
This discovery has a far-reaching impact. For starters, scientists are finding that the definition of a comet and an asteroid is becoming more and more blurred with the discoveries of "hybrid" objects that possess qualities of both. Perhaps Ceres, a huge asteroid, also has an aptitude for presenting comet-like features. But above all, this finding adds another fascinating twist to our understanding of the prevalence of water throughout our solar system, possibly shedding more light on how Earth acquired so much of the wet stuff.
"Herschel's discovery of water vapor outgassing from Ceres gives us new information on how water is distributed in the solar system," said Göran Pilbratt, ESA's Herschel Project Scientist. "Since Ceres constitutes about one fifth of the total mass of asteroid belt, this finding is important not only for the study of small solar system bodies in general, but also for learning more about the origin of water on Earth."
Publication: Localized sources of water vapour on the dwarf planet (1) Ceres, Küppers et al., Nature, 2014. doi:10.1038/nature12918