With a radius of 470 kilometers, Ceres is the largest of the 700 000 or so bodies in the main asteroid belt, which lies between Mars and Jupiter. This body has intrigued scientists since a long time: its mass corresponds to a quarter of the total mass of all the asteroids in the belt.
In 2005, the Hubble space telescope captured a fuzzy image of its surface, and its overall shape could be assessed.
Since March 6th NASA’s space probe Dawn, launched in 2007, has been in orbit around Ceres: this is the first time that a space probe has been orbited around a dwarf planet.
After having gathered data for a year, Dawn is now able to unveil the interior of this body, using the thousands of pictures taken of its surface by the cameras, and the determination of its gravitational field.
The gravitational field is deduced from an exact determination of the path of the probe around Ceres, using the radio signals emitted by the probe and received by the large ground based antennas of the « DeepSpace Network ».
A partially differentiated body
The data analysis (topography and gravity) was led by NASA’s Jet Propulsion Laboratory, within the framework of an international scientific collaboration in which participated Nicolas Rambaux, a Pierre et Marie University lecturer at the Paris Observatory (Institut de mécanique céleste et de calcul des éphémérides – IMCCE : Observatoire de Paris / CNRS / UPMC / université Lille 1).
It has thus been possible to show that Ceres is «partially differentiated» with a mantle rich in volatile elements and a rocky core containing traces of volatile elements.
The mean density of Ceres is 2162 kg/m3, which turns out to be much smaller than the mean density of other rocky bodies in the Solar System such as the Moon or Vesta.
A comparison of the topographic and gravity data leads to the conclusion that the topography is compensated by isostasy, with a deep not very viscous layer of material.
Finally, the density profile deduced from the new gravity data as well as the study of the interaction of water with the rocks within Ceres suggests that the planet has had a complex thermal history.