The asteroid Ceres has been thought to contain abundant water. Observations acquired with the Herschel Space Observatory now show that this Solar System object is spewing water vapour from its surface. See Letter p.525
Writing in this issue, Küppers et al.1 report that Ceres — a dwarf planet or the largest asteroid in the Solar System, depending on the definition used — is releasing water vapour from its surface at a rate of about 2 × 1026 molecules, or 6 kilograms, per second. The presence and abundance of water in asteroids2,3 are relevant to many areas of research on the Solar System, ranging from the origin of water and life on Earth to the large-scale migration of giant planets such as Jupiter.
Water has been suspected of being a significant component of Ceres for more than 30 years4. But it is only now that observations obtained by Küppers et al., using the European Space Agency's Herschel Space Observatory, have allowed the direct identification of water molecules escaping from two regions on the surface of this object (Fig. 1). The authors' result backs up previous indirect observational evidence5,6 for water in this planetary body, and is particularly timely given that NASA's Dawn spacecraft7 will soon visit Ceres, fresh from its successful mission to another intriguing small world, the asteroid Vesta.
One of the most puzzling questions about the origin and evolution of asteroids is why Vesta and Ceres are so different. They are both located in the main asteroid belt, between the orbits of Mars and Jupiter, and their orbits are quite close to each other: about 2.4 and 2.8 astronomical units from the Sun, respectively (1 astronomical unit is the mean Sun–Earth distance). Yet these objects are opposites in terms of their composition and appearance. Whereas Vesta has experienced extensive heating and volcanic eruptions that covered the entire asteroid, Ceres' surface and interior have not reached temperatures high enough to melt rocks.
Interestingly, a greater abundance of water in Ceres than Vesta may have been a crucial factor in producing the two bodies' radically different final states8. The source of the water vapour observed by Küppers and colleagues may be related to the process of heat dissipation that precluded the melting of rocks in Ceres. More specifically, one of the proposed production mechanisms for the water vapour being released from Ceres involves the melting of subsurface ice that then flows to the surface and evaporates into space. Water vapour has a high capacity to transport heat, and so, during the formation of Ceres about 4.6 billion years ago, the sublimation of water ice might have efficiently dissipated the interior heat into space. This would have stopped Ceres from ending up with an igneous surface like that of Vesta.
If this is indeed what happened during the formation of the two asteroids, one may ask: why did Ceres form with (and why does it still contain) more water than Vesta? It is most likely that Ceres formed in a colder outer region of the nascent Solar System than Vesta, beyond the snow line — the distance from the young Sun at which temperatures were low enough for water to form ice. But this hypothesis raises the question of why Ceres and Vesta are so close to each other now. It has been suggested that, soon after the formation of the asteroids and the planets, mixing of material from the inner and outer regions of the Solar System occurred. Such mixing would have been caused by migration of the orbits of Jupiter and the other giant planets9, and that could have moved Ceres and Vesta from distant formation sites to their current locations.
One of the first clues that giant planets in the Solar System could undergo significant migration came from the discovery10 in 1995 that certain giant exoplanets are closer to their hosts than Mercury is to the Sun — orbiting at distances at which they could not have formed. The best explanation for these 'hot Jupiters' is that they formed far from their host star and that later their orbits reduced dramatically.
Planetary migration has since been used to explain several puzzling observations. For example, the migration of Jupiter may have been responsible for the different compositional groups observed within the asteroid belt9 and for a period of extensive impacts — known as the Late Heavy Bombardment — that occurred about 4 billion years ago11,12,13. According to this scenario, as the giant planets migrated, they disturbed populations of small rocky and icy bodies (asteroids and comets), which hit the early Earth and Moon. These small bodies delivered organic molecules and water to Earth. Hence, early impacts by asteroids and comets might have played a considerable part in the origin and evolution of life on our planet.
Küppers and colleagues' detection of water vapour around Ceres and, more generally, our knowledge of Ceres and Vesta, are consistent with emerging views of how giant-planet migration and other related processes shaped the Solar System's early history. But the pieces of the puzzle of Solar System formation do not fit perfectly, and more is likely to be discovered through further studies of the miniature worlds that we call asteroids.