Enormous asteroid Vesta is the second-largest body in the asteroid belt. It’s so big, in fact, that many consider it to be more of a planet than just another rocky lump. There are good reasons to consider it planet-like: But for two massive impacts that nearly blew the thing apart (we’ll come back to that later), Vesta would be roughly spherical; and among other things, its innards are differentiated – it has a core, mantle, and crust. Just like Earth and Mars and Venus.
If Jupiter hadn’t formed, and its massive gravity hadn’t stirred the asteroid belt into perpetual crumbliness, it’s possible Vesta may have grown up and become a real planet of its own. But Jupiter did come along, and Vesta froze in a sort of embryonic planet-seed stage. It’s a relic from the beginning of the solar system, and a valuable target for scientists seeking to learn more about how our planetary neighborhood took shape.
Until recently, though, we didn’t know very much about Vesta except that it was big, appeared to be missing a chunk from its south pole, and had donated bits and pieces of itself to the good planet Earth (something like one-sixth of the meteorites that have fallen on Earth are fragments of Vesta).
It wasn’t until 2011 that the full image of the protoplanet began to emerge (even the best Hubble images were blurry collections of pixels). That was when NASA’s intrepid Dawn spacecraft, tasked with exploring two of the asteroid belt’s worlds, zoomed in for a close look. Dawn spent more than a year orbiting the 525-kilometer wide protoplanet. It mapped Vesta’s surface, measured its gravity field, and took detailed images before heading for its next target.
Almost as soon as the spacecraft arrived at Vesta, it relayed images to Earth that puzzled scientists. Running along Vesta’s equator were humongous troughs comparable in size to the Grand Canyon. It looked as if something had grabbed the rock, put one hand on the north pole and the other on the south, and pressed the protoplanet between its palms. Later, the Dawn team would learn that a massive impact had reverberated so mightily through Vesta that it was indeed deformed, and now wears the scars along its equator.
That impact, which occurred more than a billion years ago, was the second of two cataclysmic collisions at Vesta’s south pole. The first, about two billion years ago, created the Veneneia basin, which measures about 400 kilometers across. The second obliterated that bruised crater, carving the 500-kilometer wide Rheasilvia impact basin into the first.
When the dust from the collisions had settled, Vesta’s smashed up south pole had grown an enormous mountain. Stretching 180 kilometers across, and rising 25 kilometers from the base of the crater, the Vestal peak is truly huge.
How Vesta survived such violence isn’t clear. But the scars of the impacts are all over it – including those equatorial fractures – and here on Earth, in the form of fragments that have fallen as meteorites.
Smaller, less intrusive impacts created pockmarks on much of Vesta’s surface, which is remarkably varied in shade and texture. Coal-dark spots parked next to bright white areas have intrigued scientists, who wondered how the asteroid came to be painted with such variegated shades. And how did that mysterious, dark material end up on lighter-colored Vesta anyway?
Turns out, it was left there by other asteroids – dark asteroids known as carbonaceous chondrites. Scientists solved this riddle recently by characterizing the minerals present in Vesta’s dark splotches (part of this work involves generating images like the one above, where the colors correspond to different chemical compositions). In those dark splotches, they detected the mineral serpentine, which only forms under specific conditions. Things like volcanic eruptions and the heating, melting, and recongealing of Vesta as it formed would have destroyed serpentine – but an asteroid impact would not. Furthermore, the team suspects that most of Vesta’s dark marks originated from the asteroid that created the Veneneia crater (this work confirms an earlier hypothesis implicating asteroids).
There are other perplexing features on Vesta’s surface, including small gullies that may have been carved by water.
While scientists work on solving these remaining mysteries, Dawn is busy speeding toward its next target, Ceres. The largest of all the worlds in the asteroid belt, Ceres is a bona fide dwarf planet, an icy chunk that’s very different from dry, dusty Vesta. When Dawn arrives in spring 2015, it will be the first spacecraft sent to orbit two distinct bodies in the solar system – and the first to peer at Ceres.
“After more than two centuries of telescopic study, the largest body between the Sun and Pluto not yet visited by a spacecraft is about to be unveiled,” says Dawn chief engineer and mission director Marc Rayman of the Jet Propulsion Laboratory.
Dawn will map Ceres’ surface and search for clues about how this icy world formed, before ending its mission in 2016.
The spacecraft’s final act won’t be a dramatic plunge to the surface of its target planet, as some of Earth’s other spacecraft have done. Ceres is potentially a wet, mineral-rich world – one that could, in theory, support life. Contaminating Ceres with anything from Earth would be exceptionally irresponsible.
So, when Dawn’s fuel runs out and its messages to Earth cease, it will forever stay in orbit around Ceres.
“The spacecraft will remain a silent celestial monument to human curiosity, creativity, ingenuity, and passion for adventure and knowledge,“ Rayman says. “It will stay in orbit around Ceres as surely as the moon stays in orbit around Earth or Earth stays in orbit around the sun.”