Space.com has just published an article on the wholly unexpected observation of water on the asteroid Ceres. These observations were made at the ESA Herschel Observatory, and have recognised plumes of water shooting from the asteroid's surface. This water then accretes into thin clouds around the asteroid, which absorb the low amounts of heat that radiate from the world's surface. This heat is then measurable from Earth-based observatories.
To say this was unexpected is understating the matter. While there has been some speculation that the larger bodies in the asteroid belt may contain water, it has generally been thought that the belt is mostly dry, with water ice being present in the outer solar system bodies. To find reasonably large deposits of water on Ceres questions what we thought we knew about how the asteroid belt formed.
There are two possibilities for how the water is being ejected into space from the surface. The more exciting one is in the form of 'ice-canoes,' volcanic eruptions of liquid water from subsurface ice deposits. However, the more likely explanation is that these plumes are caused by sublimation of ice on the surface, when variations in heat cause water molecules to escape from the ice, something that is commonly seen on comets as they approach the sun. Indeed, it is possible that the water on Ceres was accreted from cometary material over time, rather than having been present from its early life.
On the other hand, it is theorised that the slow sublimation of water has kept Ceres' surface at a more stable temperature, explaining why the surface is smoother than the ruined, igneous surface of the asteroid Vesta.
The presence of water in the asteroid belt has important implications for future space travel. The belt is already known to be replete with valuable minerals and has been touted as a potential site for mining bases, providing a stepping stone to the outer solar system. Adding water to that mineral wealth only makes the belt a more enticing resource.
Ceres is the largest of the asteroids and the smallest object currently classified as a dwarf planet. We should find out more about this mysterious rock when NASA's Dawn spaceprobe arrives there next year.