Our solar system is a wondrous place. Not much time goes by after one exciting discovery before we come across a new finding that surpasses the previous one in impact factor. That may well be what is in store for NASA’s Dawn mission to the asteroid belt, a space mission to, effectively, study our planetary origins.
Artist rendition of Dawn spacecraft’s journey
The Dawn spacecraft “visited” the third largest member of the asteroid belt, called Vesta, in 2011 and 2012, and is now headed towards an encounter with the considerably larger Ceres in 2015. Despite being traditionally known as asteroids, the relatively large sizes of Vesta and Ceres create a bit of a headache for astronomers as to their exact designation (under the new definitions introduced by the International Astronomical Union that demoted Pluto from the planet club in 2006, Ceres is now termed a “dwarf planet”), and so there doesn’t seem to be an agreement among the astronomical community about what best to call them. Nevertheless, most experts agree that those two huge rocks are protoplanets, that is, precursors of what would otherwise have been one or more planets.
Ceres is an almost spherical body with a diameter of 974 kilometers; that’s about the size of Texas! Its mass alone comprises about one-third of the total mass of bodies in the asteroid belt. With the help of very precise calculations, planetary scientists have concluded that it contains a rocky core at its center, which in turn is surrounded by a mantle of water ice that could be up to 100 km thick. If this is true, then it would mean that Ceres contains more water than the amount of fresh water on Earth!
Size comparison of Earth, the Moon, and Ceres. Image: NASA
Ceres viewed with the Hubble Space Telescope. Image: NASA
In fact, we might already have direct evidence of the presence of water on Ceres. An international team of astronomers recently discovered what could very well be a tenuous cloud of water vapor emanating from distinct sources on the surface of the dwarf planet. The observations were carried out with the Herschel space telescope, and they could point to either cryo-volcanic activity (meaning that volcanoes would erupt water instead of molten rocks) or comet-like sublimation of near-surface ice (sublimation is the transformation of ice directly into vapor). Which of these two mechanisms is the right one is something that Dawn might be able to answer in about a year’s time.
Vesta is slightly more irregular in shape than Ceres, and it has a mean diameter of 525 km. It contains very little water, and its south pole is marked by an enigmatic feature: an enormous crater almost as large as Vesta itself, with a diameter of 460 km and 13 km deep. There is no general consensus yet about what caused it, but some estimates suggest that a 42-km projectile, traveling at a whopping 5 km/s, might have been the culprit. Imagine what it would have been like to witness such an explosive event!
South pole of Vesta, imaged by the Dawn spacecraft. Credit: NASA.
One of the most amazing outcomes of this collision is that it might have been responsible for sending pieces of Vesta our way, in the form of meteorites. This has been inferred from laboratory studies of a particular type of meteorites, called howardite-eucrite-diogenite, or HED for short, whose stony properties seem to match those of Vesta and its “family”, a group of smaller asteroids thought to have been expelled from the giant impact on Vesta’s south pole. I find it mind-blowing that we on Earth are able to hold in our hands fragments of material that was formed billions of years ago, when the solar system was still in its infancy…
One of the last images of Vesta taken by Dawn as it departed towards Ceres. Credit: NASA/JPL-Caltech/ UCAL/MPS/DLR/IDA
Perhaps in the not-too-distant future astronauts will be able to land on, and explore, Ceres and Vesta. The good thing about landing on bodies of that size is that you won’t need as much fuel to take off again, because they have less gravity than, say, the Earth or Mars. Could it be you who will walk on the surface of an asteroid for the first time?