Astronomers working with the European Space Agency’s (ESA) Herschel space telescope have found yet another place in our Solar System that has water: the asteroid Ceres. They believe that the water comes from icy patches near the surface melting under the Sun’s warmth, but have not yet discounted the possibility of icy cryovolcanoes binging up water from deep within the asteroid’s body.
Back in 1801 when Ceres was first discovered, it was hailed as a new planet. Astronomers of the era had noticed the conspicuous gap between Mars and Jupiter and after William Herschel’s astonishing discovery of a new planet (Uranus) a few years earlier, were convinced that there were more worlds waiting to be found. Over time, as more and more objects were found orbiting in the same region of space, all of them small enough to look just like stars through a telescope, it became clear that these things were related but were not planets, and so a new word was invented: Asteroid. After less than sixty years, Ceres was demoted. 150 years later, as part of the controversial decision that demoted Pluto, Ceres was reclassified again, this time to Dwarf Planet.
It has long been expected that Ceres should have water ice as part of its make-up. At 950 kilometers across, it is large enough to have a complex structure and spherical shape, and it is far enough from the Sun to retain the water that it was created with. But nobody ever expected to see that water melting and jetting off into space. That’s what comets do, after all, and Ceres was always an asteroid. Still, observations of recent years have begun to blur that particular distinction, with an entire new class of asteroids that look like comets having been recently discovered. So for two years, from 2011 to 2013, Herschel was directed to begin a series of infra-red observations of Ceres.
Herschell could not resolve Ceres directly but was able to record the first ever detection of the distinct spectrographic signature of water vapour. However, the recordings varied – some measurements showed more water than others, and occasionally there was no water at all. Plainly the water vapour comes and goes, but why? The most likely ideas is that Ceres has icy patches on its surface, which are normally frozen solid, but occasionally begin melting when conditions are right. Like everything else in the Solar System, its orbit is not perfectly circular, so its distance from the Sun varies periodically. It also rotates and has seasons, and the Hubble Space Telescope has managed to map our lighter and darker regions as they move from day to night. The timing of the appearance of water vapour seems to match up with when the darker regions are exposed to the Sun, and since we expect darker regions to heat up more rapidly than light regions, it makes sense that more ice would melt from there than from any other place.
But there is an alternative idea: Ceres is big enough that it is probably built in layers, with a core, mantle and crust. Planetary scientists suspect that it might have vast reservoirs of fresh water ice – more than the Earth’s entire reserves – frozen deep beneath the surface. If this ice receives enough seasonal heat, it could also melt, and force liquid water up through cracks in the crust to erupt in a geyser. Because this mirrors what happens on Earth with molten rock, we call these geysers “cryovolcanoes”, and they have already been discovered on at least one of Saturn’s moons. The evidence doesn’t yet match up as well for this ideas as it does for the surface ice theory, but it is still too soon to say for certain.
The only way to explain where the water comes from for certain is to get a closer look, and fortunately that is exactly what we’ll be getting next year. NASA’s Dawn mission is a robotic spacecraft which is travelling the asteroid belt and its next target is Ceres. It will arrive in roughly a year’s time, and be able to study Ceres in detail. Part of its mission will be to film the dwarf planet with high resolution cameras, creating a map of its surface and allowing astronomers to see exactly what is happening at Hubble’s dark regions. It will also easily spot any geysers erupting from cryovolcanoes. And as history teaches us, we are almost certain to find things that nobody expected, raising new questions about Ceres and other dwarf planets, and advancing our knowledge of the universe even further. But until then we wait!
The Herschell observations were summarised in a paper by by M. Küppers et al. whch was published in Nature 23 January 2014. It is titled “Localised sources of water vapour on dwarf planet (1) Ceres” and can be read here: arXiv:1403.4579 [physics.gen-ph]