After becoming the first probe to enter orbit around an object in the main asteroid belt between Mars and Jupiter in July 2011, NASA’s Dawn spacecraft has spent the last 10 months orbiting said object - the giant asteroid Vesta. During that period it has captured more than 20,000 images of Vesta and a multitude of data from different wavelengths of radiation. What it reveals is an asteroid that in many ways shares more in common with a small planet or Earth’s moon than it does with another asteroid.
With a mean diameter of around 326 miles (525 km), Vesta is one of the largest asteroids in the Solar System and the second most massive after Ceres. Formed in a similar way to the terrestrial planets and Earth’s moon, Vesta boasts a geologic complexity that scientists attribute to a process that separated it into a crust, mantle and iron core with a radius of around 68 miles (110 km) some 4.56 billion years ago.
"Vesta has been recording the history of the solar system from the beginning," said Christopher T. Russell, a professor in UCLA's Department of Earth and Space Sciences and the Dawn mission's principal investigator. "We are going back to the beginning of the solar system - more than 4.5 billion years ago. We're going back further than ever before on the surface of a body."
Deep gashes in Vesta’s surface observed by Dawn reveal a pattern of minerals that NASA says may suggest the asteroid was once molten inside and had a subsurface magma ocean, which occurs when a body undergoes almost complete melting and leads to layered building blocks that can form planets. Vesta’s iron core would have formed during this molten period at the dawn of the Solar System.
Data collected by Dawn also reveal that Vesta is the source of a distinct group of meteorites found on Earth. These meteorites, with signatures of an iron- and magnesium-rich mineral known as pyroxene, account for about six percent of all meteorites falling to Earth, making Vesta one of the largest single sources of Earth’s meteorites. Dawn’s mission also marks the first time a spacecraft has visited the source of samples after they were identified on Earth.
Vesta has also been found to have a topography that is quite steep and varied, and includes large mountains formed by a major impact on the asteroid’s surface – the largest of which is more than twice the size of Mount Everest. While scientists had thought that, outside the south polar region, Vesta’s surface may be flat like the moon, some of the craters outside this region formed on very steep slopes and have nearly identical sides, with landslides often occurring.
NASA scientists were also surprised to discover that Vesta’s central peak in the Rheasilvia basin in the southern hemisphere is much higher and wider, relative to its crater size, than the central peaks of craters on bodies like our moon.
There are also similarities with other low-gravity worlds, such as Saturn’s small icy moons, and the light and dark markings on its surface don’t match the predictable patterns seen on Earth’s moon. While Vesta’s surface contains bright spots of various sizes, there are also some areas that are dark as coal, with the light and dark markings forming intricate patterns that suggest the dominance of impact processes in creating mixed layers in Vesta’s regolith.
"We know a lot about the moon and we're only coming up to speed now on Vesta," said Vishnu Reddy, a framing camera team member at the Max Planck Institute for Solar System Research in Germany and the University of North Dakota in Grand Forks. "Comparing the two gives us two storylines for how these fraternal twins evolved in the early solar system."
Dawn will continue to examine Vesta until it departs the asteroid on August 26 headed for a 2015 study date with Ceres, the Solar System’s largest asteroid and only dwarf planet in the inner solar system.