Posted on behalf of Ron Cowen.
After just 10 weeks orbiting the giant asteroid Vesta, NASA’s Dawn spacecraft has uncovered several surprises about the 530-kilometre-wide rock. It has the Solar System’s second-tallest mountain; a surface divided into a more crater-pocked and considerably older northern half and a smoother, younger southern half; and unidentified dark streaks adjacent to unusually bright regions.
Those were among the discoveries that the Dawn team chose to report on 3 October at a joint meeting of the European Planetary Science Conference and the American Astronomical Society’s Division for Planetary Sciences in Nantes, France. But other, more tentative findings, deemed not quite ready for prime time by the Dawn team, could provide some of the first clues about what the asteroid belt was like during its earliest days, more than 4 billion years ago.
Some of the findings can be seen in a publicly available image of Vesta’s south polar region. The Dawn team has widely publicized the most obvious and riveting feature in the image — a giant, central crater, first glimpsed at low resolution by the Hubble Space Telescope, that scientists suspect is the origin of a common class of meteorites that have fallen to Earth. But at the bottom of the image, Dawn researchers see the faint outline of what they believe to be a second, older crater, says team leader Chris Russell of the University of California, Los Angeles. Half the rim of this apparent crater has been eradicated by the later, bigger impact (see the craters outlined in the picture).
If the presence of this second, more ancient depression is confirmed, it’s likely to reveal secrets about a turbulent time in the early history of the inner Solar System, says Russell. During that era, which began roughly 4.1 billion to 4.2 billion years ago, the asteroid belt was a much more crowded and rowdy place, with collisions between the space rocks much more likely.
If the larger, more obvious crater dates from the tail end of the Late Heavy Bombardment era, some 3.8 billion years ago, the smaller depression might hail from the middle of that highly active era, around 200 million years earlier, says Russell. If so, the diameter, depth and other properties of the depression — the data are still being deciphered — could provide a first indication of just how crowded and rowdy the belt was, says Dawn researcher Paul Schenk of the Lunar and Planetary Institute in Houston, Texas. Schenk and other members of the Dawn team plan to report some of their preliminary conclusions on 12 October at the annual meeting of the Geological Society of America in Minneapolis, Minnesota.
There is other evidence that Vesta was walloped early on by several sizeable chunks of space debris. A 2003 study that used radioactive dating to study meteorites believed to have been chipped off of Vesta revealed that materials in the rocks were heated to high temperatures in separate cataclysmic events 4.48 billion, 4.0 billion, 3.8 billion and 3.5 billion years ago. Study co-author Donald Bogard of the Lunar and Planetary Institute proposes that each of these events corresponds to the formation times of some of the larger craters on Vesta.
The possibility of a second large, elderly crater may also tie together models about the structure of the early asteroid belt and the inner Solar System. At the Nantes meeting on 5 October, William Bottke of the Southwest Research Institute will unveil a model of the asteroid belt in which this reservoir of space rocks, now sandwiched between the orbits of Mars and Jupiter, originally extended closer to the Sun, to about 1.8 times Earth’s distance from the star.
The theory itself is an extension of the ‘Nice’ model that relies on the movement of the giant, outer planets to explain the early activity of the inner Solar System. The model proposes that the outer planets didn’t form where they now reside, but in a game of planetary billiards, Jupiter moved inwards and Saturn and Uranus got pushed outwards due to gravitational interactions with an outer reservoir of space debris. The inward migration of Jupiter, the big bully of the Solar System, stirred up the rocks in the asteroid belt, making them more likely to collide with each other and triggering the Late Heavy Bombardment era. The giant planet’s movement would also have cut the belt to its current width.
This model correctly predicts the early cratering history of the Moon, but the oldest gouges on Vesta could provide an additional test of the theory. A high-resolution gravity map of Vesta, based on the motion of the Dawn craft as it flies over different parts of the asteroid, could provide the ultimate proof of whether one of these extremely ancient craters has already been found, says Russell.