ESA's Herschel space observatory has discovered water vapour around Ceres, the first unambiguous detection of water vapour around an object in the asteroid belt.
With a diameter of 950 km, Ceres is the largest object in the asteroid belt, which lies between the orbits of Mars and Jupiter. But unlike most asteroids, Ceres is almost spherical and belongs to the category of "dwarf planets", which also includes Pluto.
It is thought that Ceres is layered, perhaps with a rocky core and an icy outer mantle. This is important, because the water-ice content of the asteroid belt has significant implications for our understanding of the evolution of the Solar System.
When the Solar System formed 4.6 billion years ago, it was too hot in its central regions for water to have condensed at the locations of the innermost planets, Mercury, Venus, Earth and Mars. Instead, it is thought that water was delivered to these planets later during a prolonged period of intense asteroid and comet impacts around 3.9 billion years ago.
While comets are well known to contain water ice, what about asteroids? Water in the asteroid belt has been hinted at through the observation of comet-like activity around some asteroids - the so-called Main Belt Comet family - but no definitive detection of water vapour has ever been made.
Now, using the HIFI instrument on Herschel to study Ceres, scientists have collected data that point to water vapour being emitted from the icy world's surface.
Although Herschel was not able to make a resolved image of Ceres, the astronomers were able to derive the distribution of water sources on the surface by observing variations in the water signal during the dwarf planet's 9-hour rotation period. Almost all of the water vapour was seen to be coming from just two spots on the surface.
Variability in intensity of the water absorption signal detected at Ceres by ESA's Herschel space observatory on 6 March 2013. The most intense readings correspond to two dark regions on the surface known as Piazzi and Region A, identified in the ground-based image of Ceres by the Keck II Observatory. The two data points at 110° longitude were taken in a time interval of about 9 hours - equal to the Ceres rotation period - showing that variability in the water vapour production is possible even on short periods. - © Adapted from Küppers et al.
"Herschel's discovery of water vapour outgassing from Ceres gives us new information on how water is distributed in the Solar System. 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," says Göran Pilbratt, ESA's Herschel Project Scientist.
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