Asteroid or Mini-Planet? Hubble Maps the Ancient Surface of Vesta
Hubble telescope images of the asteroid Vesta are providing astronomers with a glimpse of the oldest terrain ever seen in the solar system and a peek into a broken-off section of the "mini-planet," which exposes its interior.
Hubble's pictures provide the best view yet of Vesta's complex surface, which has geologic features similar to those of terrestrial worlds such as Earth or Mars. The asteroid's ancient surface, battered by collisions eons ago, allows astronomers to peer below the asteroid's crust and into its past. These images trace the asteroid through a full rotation.
NASA's Hubble Space Telescope images of the asteroid Vesta are providing astronomers with a glimpse of the oldest terrain ever seen in the solar system and a peek into a broken off section of the "mini-planet" that exposes its interior.
Hubble's pictures provide the best view yet of Vesta's complex surface, with a geology similar to that of terrestrial worlds such as Earth or Mars. The asteroid's ancient surface, battered by collisions eons ago, allows astronomers to peer below the asteroid's crust and into the past.
Astronomers also believe that fragments gouged out of Vesta during ancient collisions have fallen to Earth as meteorites, making Vesta only the fourth solar system object, other than Earth, the Moon and Mars, where scientists have a confirmed laboratory sample. (About 50-60 other meteorite types are suspected to have come from asteroids, but positive identifications are more difficult to make.)
"The Hubble observations show that Vesta is far more interesting than simply a chunk of rock in space as most asteroids are," said Ben Zellner of Georgia Southern University. "This qualifies Vesta as the 'sixth' terrestrial planet."
No bigger than the state of Arizona, Vesta offers new clues to the origin of the solar system and the interior makeup of the rocky planets. "Vesta has survived essentially intact since the formation of the planets," Zellner said. "It provides a record of the long and complex evolution of our solar system."
Resolving features down to 50 miles across, Hubble reveals a surprisingly diverse world with an exposed mantle, ancient lava flows and impact basins. Though only 325 miles (525 kilometers) across, it once had a molten interior. This contradicts conventional ideas that asteroids essentially are cold, rocky fragments left behind from the early days of planetary formation.
Besides providing scientists with direct samples, Vesta's chipped surface allows Hubble to study the asteroid's rocky mantle, giving scientists a unique opportunity to see what a planet looks like below the crust. "Our knowledge of the interior composition of the other terrestrial worlds, the Moon, Mars, Venus, Mercury and even Earth, depends heavily on theory and inference," Zellner said. "Vesta allows us to actually see the mantle and study pristine samples in our laboratories."
Before these observations, only the smaller and less geologically diverse asteroids, Ida and Gaspra, have been observed in detail by the Jupiter-bound Galileo spacecraft. Unlike Vesta, these smaller objects are pieces torn off larger bodies by collisions that occurred perhaps only a few hundred million years ago.
Vesta: The Sixth Terrestrial Planet?
Vesta is the most geologically diverse of the large asteroids and the only known one with distinctive light and dark areas – much like the face of our Moon. Previous ground-based spectroscopy of Vesta indicates regions that are basaltic, which means lava flows once occurred on its surface. This is surprising evidence that the asteroid once had a molten interior, like Earth does.
One possibility is that Vesta agglomerated from smaller material that includes radioactive debris (such as the the isotope Aluminum-26) that was incorporated into the core. This radioactive "shrapnel" probably came from a nearby supernova explosion. (In fact a supernova might have triggered the birth of our solar system.) This hot isotope may have melted the core, causing the asteroid to differentiate: heavier, dense material sank to the center while lighter rock rose to the surface. This is a common structure for the terrestrial planets. After Vesta's formation, molten rock flowed onto the asteroid's surface. This happened more than four billion years ago. The surface has remained unchanged since then, except for occasional meteoroid impacts.
One or more large impacts tore away some of the crust exposing a deeper mantle of olivine, which is believed to constitute most of the Earth's mantle. Some of the pieces knocked off Vesta have fallen to Earth as meteorites, which show a similar spectral fingerprint to Vesta's surface composition.
A Piece of Vesta Falls to Earth
In October 1960, two fence workers in Millbillillie, Western Australia, observed a fireball heading toward the ground, and pieces of the fallen meteorite were found ten years later. The fragments stood out from the area's reddish sandy soil because they had a shiny black fusion crust, produced by their fiery entry through Earth's atmosphere.
Unlike most other meteorites, this sample can be traced to its parent body, the asteroid Vesta. The meteorite's chemical identity points to Vesta because it has the same unique pyroxene spectral signature. Pyroxine is common in lava flows, meaning that the meteorite was created in an ancient lava flow on Vesta's surface. The structure of the meteorite's mineral grains also indicates it was molten and then cooled. The isotopes (oxygen atoms with varying number of neutrons) in the specimen are unlike the isotopes found for all other rocks of the Earth, Moon and most other meteorites.
The meteorite also has the same pyroxene signature as other small asteroids, recently discovered near Vesta, that are considered chips blasted off Vesta's surface. This debris extends all the way to an escape hatch region in the asteroid belt called the Kirkwood gap. This region is swept free of asteroids because Jupiter's gravitational pull removes material from the main belt and hurls it onto a new orbit that crosses Earth's path around the Sun.
The Australian meteorite probably followed this route to Earth. It was torn off Vesta's surface as part of a larger fragment. Other collisions broke apart the parent fragment and threw pieces toward the Kirkwood gap, and onto a collision course toward Earth. Meteorites found in other locations on Earth are probably from Vesta too.
Ben Zellner (Georgia Southern University), Alex Storrs (Space Telescope Science Institute Baltimore, MD), Ed Wells (Computer Sciences Corporation, Bethesda, MD), Rudi Albrecht (European Southern Observatory in Garching bei Munchen, Germany) and collaborators used Hubble's Wide Field and Planetary Camera 2 (WFPC 2) to collect images of Vesta in four colors of light between November 28 and December 1, 1994. At the time Vesta was 156 million miles (252 million km) from Earth. In late December 1994, when Vesta was 10 million miles (16 million km) closer to Earth than a month earlier, HST's Faint Object Camera made even higher resolution images. These results are complemented by infrared observations made on December 11, by Olivier Hainaut and colleagues with an adaptive-optics camera on the European Southern Observatory's 3.6-meter telescope in Chile. By combining Hubble and ESO observations astronomers will be able to produce a geochemical map of an asteroid's surface.