NASA's Dawn spacecraft has provided scientists with evidence that the giant asteroid Vesta was formed in a similar way to the inner rocky planets such as Earth and Mars.
Launched in 2007, Dawn's mission is to study Vesta and the dwarf planet Ceres with the aim of improving our understanding of the evolution of the early Solar System. Vesta was chosen as an example of a rocky world in the inner Solar System whilst Ceres was chosen as an example of an icy body in the outer Solar System.
Dawn reached Vesta in July 2011 and scientists have been studying the data captured, including a geological map of the asteroid which reveals that Vesta has many planet-like properties.
“It is a transitional body between asteroids and full-sized planets,” said Dawn scientist O’Brien. “It is similar to many of the small planetesimals that were the building blocks of the planets, and at the same time has many features of a small planet itself, having melted and formed a core and crust, and having a diverse range of surface features and compositions.”
Dawn has found evidence that Vesta contains an iron core, which has a diameter of around 220 kilometres. This shows that Vesta is differentiated, meaning that it formed layers as it cooled billions of years ago.
Other data captured by Dawn has given more insights into Vesta.
It was already known that a substantial cater, which is 500 kilometres in diameter, existed near the south pole. The crater is called Rheasilvia, and Dawn has discovered that it was created only one billion years ago.
The age of the crater was determined by counting the number of smaller craters that are sprinkled on top. This youthful crater is much younger than the three billion year old large impact craters that are found on the Moon.
“Vesta is a window back to the earliest days of Solar System history. Understanding the history of impacts in the Solar System is important for understanding the evolution of the Moon and planets, including how life evolved on Earth,” said O’Brien. “By studying the impact history of Vesta recorded by its impact craters, we can better understand the history of impacts on the early Earth.”
By calculating the age of large craters on Vesta, scientists can confirm that certain meteorites found on Earth originated from the asteroid. They are also able to discern the locations where the meteorites were blasted away from their parent body.
Artist's concept of NASA's Dawn spacecraft orbiting the asteroid Vesta. Credit: NASA/JPL-Caltech
A surprising discovery on Vesta is the lack of volcanic features. The Vesta meteorites recovered on Earth point to an era of volcanism on the asteroid, however an over-abundance of impact craters has destroyed any evidence of volcanism on the asteroid’s surface. This indicates that volcanism was probably only active during the early history of Vesta, and only for a short period of time.
Vesta measures about 578 by 560 by 458 kilometers (359 by 348 by 285 miles).
Dawn is due to depart Vesta on 26 August, as it heads off to study its next target, the dwarf planet Ceres.
Ceres was chosen as a very different world to rocky Vesta as it formed much further away from the Sun. Ceres appears to have similarities to the large icy moons of the outer Solar System.
Dawn will arrive at Ceres in 2015.