Two weeks after leaving asteroid Vesta behind, NASA’s Dawn spacecraft has revealed the presence of hydrated minerals on the surface of the tiny, pockmarked world. Although the space agency has been quick to stress that Dawn did not find actual water ice, the discovery suggests that easily evaporated ‘volatiles’ were most likely delivered to Vesta through fierce meteoroid bombardment. As well as providing the first direct measurements of the elemental ratios of iron-to-oxygen and iron-to-silicon in the asteroid’s surface, this new data solidly confirms a connection between Vesta and a class of meteorites found on Earth, known as the Howardites, Eucrites and Diogenites, which have similar ratios.
In the journal Science, Thomas Prettyman, lead scientist for Dawn’s Gamma Ray and Neutron Detector (GRaND) at the Planetary Science Institute in Tucson, Arizona, explained that the instrument had found signatures of hydrogen, probably in the form of hydroxyl or water, bound to minerals within the Vestan surface. These signatures manifested themselves as pothole-like features, where the volatiles boiled off. “The source of the hydrogen within Vesta’s surface appears to be hydrated minerals delivered by carbon-rich space rocks that collided with Vesta at speeds slow enough to preserve their volatile content,” said Prettyman.
Concurrent research by Brett Denevi, a participating scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, likened the ‘potholes’ to surface features previously seen on Mars…with a fundamental difference. “While water was common on Mars, it was totally unexpected on Vesta in these high abundances,” explained Denevi. “These results provide evidence that not only were hydrated materials present, but they played an important role in shaping the asteroid’s geology and the surface we see today.”
Unlike the Moon, Vesta has no permanently shadowed polar regions in which water ice might survive. According to the GRaND results, the strongest spectral signatures for hydrogen emerged from the asteroid’s equatorial band, where water ice is not stable. The heat produced by meteoroid and other collisions converted hydrogen bound to the minerals into water, which evaporated and in doing so left behind holes as wide as half a mile across and up to 700 feet deep.
The findings come shortly after Dawn’s 5 September departure from Vesta, en-route to its next celestial target, the dwarf world Ceres, with which the spacecraft is expected to rendezvous in February 2015. Dawn spent a little more than a year in orbit around Vesta, typically passing as close as just 130 miles and revealing the tortured surface of the tiny world in unprecedented detail. In view of the fact that the mission came perilously close to cancellation by NASA – and actually was cancelled, for a short time, in early 2006 – its enormous success stands as testimony to the ingenuity of the engineers and scientists who turned this ambitious venture from blueprints into hardware. It also lends weight to ongoing calls for the passing of a Space Leadership Act, one of whose objectives is to avoid the costly and wasteful cancellation of important flagship science missions.
Powered by a trio of xenon ion thrusters – which makes it NASA’s first purely exploration mission to be run wholly by ion propulsion – Dawn will hopefully become the first spacecraft to enter orbit around two extraterrestrial bodies, Vesta and Ceres. Originally cancelled in December 2003 and reinstated in February 2004, it met with significant delay and was scrubbed a second time by NASA in March 2006. Fortunately, Dawn’s prime contractor, Orbital Sciences Corporation, appealed the cancellation and offered to build the spacecraft at cost, forgoing profits in order to gain experience in a new field. Within weeks, NASA agreed to restore the mission and in September 2007 it was successfully launched atop a Delta II booster from Cape Canaveral Air Force Station.
It had been planned for Dawn to visit Vesta and Ceres, both of which reside within the so-called ‘asteroid belt’ betwixt Mars and Jupiter, and whose chemical compositions were believed to be respectively ‘evolved and rocky’ and ‘primitive and icy’ in nature. By the time Dawn was launched, the International Astronomical Union (IAU) had introduced the new nomenclature of ‘dwarf planet’ (of which category Ceres is presently the inner Solar System’s only member). Consequently, when the spacecraft reaches Ceres in early 2015 it will become humanity’s first robotic emissary to directly study a dwarf planet, five months ahead of the arrival of New Horizons at the Pluto-Charon system.
After leaving Earth, Dawn received a gravity assisted boost from Mars in February 2009 and reached Vesta in July 2011. Following a little more than 13 months in orbit, it utilised its ion thrusters to break orbit from Vesta and chart a course to arrive at Ceres in a little over two years’ time.