Thursday 23 January 2014
An international team including researchers from LESIA and IMCCE  discovered transient emissions of water vapour from Ceres, the largest asteroid, using the Herschel space telescope. This result is published in the 23 January issue of Nature.
© IMCCE-Observatoire de Paris / CNRS / Y. Gominet
Asteroid (1) Ceres, now classified, as Pluto, as a dwarf planet, was the first asteroid to be discovered, in 1801, by Piazzi. It contains 1/5 of all the matter comprised between Mars and Jupiter, within the asteroid Main Belt.
Despite this major position among asteroids, the composition of Ceres is still ill known. From its spectral characteristics, however, it should be composed of minerals akin to those of carbonaceous chondrites, which are among the most primitive meteorites.
Since the end of the 1970s, the presence of ice within, or on the surface, of Ceres is a highly debated question. Supporters of the presence of polar caps or of underground ice rely on a spectral signature in the near infrared attributable to ice, as well as on a marginal detection in 1992 around Ceres of the OH radical, a photodissociation product of water. Various internal structures are put forward, including the presence of large ice pockets or even of an internal ocean.
On the other hand, detractors underline the possible link between the infrared signature and some minerals, casting doubt on the presence of ice, which furthermore rapidly sublimates at the distance from the Sun where Ceres is orbiting. In addition, the detection of OH in 1992 could never be confirmed, despite various attempts including the Very Large Telescope (VLT) of the European Southern Observatory (ESO).
Yet, the presence of water on Ceres is crucial for our general concept of the origin of water in the Solar System as well as on the Earth. The traditional view separates the primitive Solar System into a "dry" part and an ice-rich part, the limit being around the distance of Jupiter’s orbit. The presence of water on Ceres would agree with the last evolution models of the Solar System, where the planet migration leads to a mix up between asteroids and planets, leaving the present Main Belt with a multitude of various bodies. This migration placed many water-rich objects on orbits crossing that of the Earth, bringing water to its oceans.
Spectra of the 557 GHz line of water vapour observed at different dates by the HIFI instrument of the Herschel space telescope. The line is observed in absorption against the thermal continuum of Ceres.
In this context, a team of astronomers of the European Space Agency (ESA) and of Observatoire de Paris (LESIA and IMCCE laboratories) used the European Herschel space telescope to study the presence of water on Ceres . After a negative observation in 2011, water vapour was clearly detected several times in 2012 and 2013, putting an end to the controversy. As for comets, the activity of Ceres seems to be linked to the variation of its distance to the Sun along its orbit.
There was still another surprise. The first analysis of the data from the Herschel telescope showed inconsistent results. Guided by the maps of Ceres’surface obtained some years ago with the Hubble space telescope, the W. M. Keck telescope at Hawaii and the VLT, the astronomers could determine that water was ejected from two well-localized sources on the surface of Ceres, like two gigantic geysers. Using a model developed at LESIA for cometary jets, they showed that each source was emitting about 6 kg/s of water, which would correspond to a surface of 0.7 km2 of ice exposed to the Earth.
If the emission of water vapour from Ceres is well explained by a comet-like mechanism, the origin of this water is still an open question. This dwarf planet is supposed to be constituted of a solid nucleus of silicates recovered by a 50-km thick ice mantle, topped by a surface crust of several ten metres. Water vapour could come from the ice layer exposed by impacts, or it could trace the presence of geysers. The Dawn spacecraft, launched by NASA in 2007, is presently en route to Ceres after having studied asteroid (4) Vesta in 2011. Images and high-resolution spectra of the surface of Ceres will allow us to better understand the origin of this water.
The line-over-continuum intensity of the water line on 6 March 2013, as a function of the longitude of the sub-terrestrial point. The intensity is maximum at the longitudes of the dark regions Piazzi and Region A, suggesting that the water emission is coming from these regions. The blue curve is a model assuming that each of these two sources produce 6 kilogrammes of water per second.
 (Observatoire de Paris/CNRS/Université Pierre et Marie Curie/Université Lille 1)
 These observations were part of the MACH-11 ("Measurements of 11 Asteroids and Comets with Herschel") guaranteed time programme (PI. L. O’Rourke, ESAC) which observed 11 asteroids and comets which were or are to be observed by space probes: the targets of missions Rosetta, Deep Impact, Hayabusa 2, Marco-Polo R, OSIRIS-Rex and Dawn. They were complemented by two additional director’s discretionary time observations (PI. M. Küppers, ESAC) for confirming the results.