The arrival of NASA’s Dawn mission at the huge asteroid “1 Ceres” in early 2015 has turned out to have been well worth waiting for. This dwarf planet is the largest body in the asteroid belt between Mars and Jupiter and was the first to be discovered. But, until recently, we have only had information from ground and space-based telescopes, which have given us tantalising glimpses of a dark, possibly water-rich object.
Now the Dawn space probe has sent back a bumper harvest of findings, summarised in six new research papers published in a special issue of the journal Science. We now have a map of Ceres that reveals unusual minerals, a surface peppered with craters, and water in the form of ice and possibly an outer atmosphere of vapour. There’s also enough uncertainty in the results to sow the seeds for future research.
The data provides a global geological map of the asteroid showing that its entire surface appears to be covered in phyllosilicates, an important group of clay minerals. Two specific clays are identified: one that is magnesium-rich, the second an ammonium-rich species. There seems to be little or no pattern to the distribution of the two minerals – they are both almost everywhere.
This ubiquity is what is important. The minerals could not have been formed in a local event, such as an impact into an ice-filled crater. They must have been produced by planet-wide alteration, presumably implying there must have been volumes of water. It is clear that enormous quantities of liquid water are not present on Ceres now. But the signal of water-ice has been detected in at least one crater.
Because the temperature of Ceres is relatively warm (between -93℃ and -33℃), water-ice exposed at the surface would rapidly convert into a gas in such a low-pressure environment. So the discovered traces of water-ice suggest some underground ice was recently exposed and that there must be some mechanism to explain how the surface was disturbed in this way. Some researchers think that the answer is cryovolcanism, where subsurface layers of mixed ice and minerals percolate slowly to the surface through cracks and fractures, or more swiftly following an impact. If the minerals are chlorides, then a low temperature brine can keep the subsurface layer mobile.
As well as a geological map of Ceres, we also have a picture of Ceres’ global geomorphology (its surface features). This shows that the surface of Ceres is peppered with impact craters, although the craters are not distributed evenly over the surface. Much more interesting are the three distinct types of mineral flow across the landscape, produced by the movement of ice-rich material, landslides or blankets of ejected particles following impact into ice-rich material. The distribution of the flow types varies with latitude – and the researchers think this means different surface layers of the asteroid contain different amounts of ice.
One of the most remarkable results is the detection of a sudden burst of highly energetic electrons over a period of around a week in June 2015, coinciding with a solar proton storm. The researchers think the protons fired out by the sun interacted with particles in Ceres' weak atmosphere, creating a shock wave that accelerated the electrons. Based on observations by the Hubble Space Telescope, Ceres is believed to have a tenuous exosphere (outer atmosphere) of water vapour. The results from Dawn suggest that this may indeed be the case.
Together, this new set of information shows that Ceres is a world that has been shaped by a series of events, with a strong crust of magnesium- and ammonium-bearing phyllosilicates overlying an interior of briny ice and hydrated minerals. What other hidden secrets will be revealed as research continues on the trove of data from Ceres? Questions still remain about the variety of mineral deposits, the depth of the subsurface ice-rock layer, and, of course, the potential for organic material on the minor planet. The harvest from Ceres so far has been rich and promises to keep us busy for years to come.