Protoplanet Vesta: Forever young?
Vesta, as seen by Dawn
This image from NASA's Dawn spacecraft features the distinctive crater Canuleia on the protoplanet Vesta.
Canuleia, about 6 miles (10 kilometers) in diameter, is distinguished by the rays of bright material that streak out from it.
There is also a more subdued unnamed crater of about the same size to the northeast. A comparison of these two craters
illustrates how freshly excavated materials on Vesta appear quite different from background soils. The image was taken
by Dawn's framing camera on Oct. 20, 2011, from an altitude of about 420 miles (680 kilometers).
October 31, 2012
By Jia-Rui C. Cook and Stuart Wolpert
Like a movie star constantly retouching her makeup, the protoplanet Vesta is continually stirring its outermost layer and
presenting a young face.
New data from NASA's Dawn mission show that a common form of weathering that affects many airless bodies like Vesta in the
inner solar system, including the moon, surprisingly doesn't age the protoplanet's outermost layer.
The data also indicate that carbon-rich asteroids have been splattering dark material on Vesta's surface over a long span
of the body's history.
The findings are described in two papers published Nov. 1 in the journal Nature.
Over time, soils on the moon and on asteroids have undergone extensive weathering. Scientists see this in the accumulation of
tiny metallic particles containing iron, which dulls the bright, fluffy outer layers of these bodies. Yet Dawn's visible and
infrared mapping spectrometer (VIR) and framing camera detected no accumulation of these tiny particles on Vesta, and the
protoplanet (sometimes called a giant asteroid) remains bright and pristine.
Still, the bright rays of the youngest features on Vesta are seen to degrade rapidly and disappear into background soil.
Scientists know that frequent, small impacts from asteroids and comets continually mix the fluffy outer layer of broken debris.
Vesta also has unusually steep topography relative to other large bodies in the inner solar system, which leads to landslides
that further mix the surface material.
Early pictures of Vesta showed a variety of dramatic light and dark splotches on Vesta's surface. These light and dark materials
were unexpected and now show that Vesta has a brightness range that is among the largest observed on rocky bodies in our
Scientists initially theorized that the dark material on Vesta might come from the shock of high-speed impacts melting and
darkening the underlying rocks or from recent volcanic activity. An analysis of data from VIR and the framing camera, however,
has revealed that the distribution of dark material is widespread and occurs in both small spots and diffuse deposits, without
correlation to any particular underlying geology. The likely source of the dark material is now shown to be carbon-rich asteroids,
which are also believed to have deposited hydrated minerals on Vesta.
"Ever since Dawn arrived at Vesta [in July 2011] and we saw the bright and dark streaks across the surface, we have wondered
how the zebra got her stripes," said Christopher T. Russell, a professor in UCLA's Department of Earth and Space Sciences and
principal investigator for the Dawn mission. "Now we know that the bright streaks and spots are due to very pure early Vestan
material, and the dark patches are deposits on the surface most probably due to collisions with material from the dark outer
reaches of the asteroid belt."
Scientists estimate that to get the amount of darkening Dawn observed on Vesta, approximately 300 dark asteroids with diameters
between 0.6 and 6 miles (1 and 10 kilometers) likely hit Vesta during the last 3.5 billion years. This would have been enough to
wrap Vesta in a blanket of mixed material about 3 to 7 feet (1 to 2 meters) thick.
Launched Sept. 27, 2007, Dawn spent more than a year investigating Vesta, which is in the doughnut-shaped asteroid belt between
Mars and Jupiter. Dawn orbited Vesta and observed the protoplanet's surface beginning in July 2011. It departed in September 2012.
Studies of meteorites found on Earth that are linked to Vesta suggest that Vesta formed from interstellar gas and dust during the
solar system's first 2 to 5 million years.
"Vesta has been recording the history of the solar system from the beginning — more than 4.5 billion years ago," Russell said.
"We're going back further than ever before on the surface of a body."
Vesta as seen by Dawn
This image from NASA's Dawn spacecraft shows a close up of part of the rim around the crater Canuleia on the protoplanet Vesta.
Canuleia, about 6 miles (10 kilometers) in diameter, is the large crater at the bottom-left of this image. This close-up image
illustrates the structure of the interior of the crater and complex details of the fresh rays across the soil of Vesta.
The image was taken by Dawn's framing camera on Dec. 29, 2011, from an altitude of about 130 miles (210 kilometers).
Dawn has a high-quality camera, along with a back-up; a visible and near-infrared mapping spectrometer to identify minerals on
the surface; and a gamma ray and neutron spectrometer to reveal the abundance of elements such as iron and hydrogen, possibly
from water, in the soil. Dawn also probed Vesta's gravity using extremely precise navigation.
The study of Vesta, however, is only half of Dawn's mission. The spacecraft is now on its way to the dwarf planet Ceres,
where it will conduct a detailed study of Ceres' structure and composition. Vesta and Ceres are the most massive objects in
the main asteroid belt between Mars and Jupiter. Ceres, the largest object in the main belt, could harbor substantial water
or ice beneath its rock crust — and possibly life. The spacecraft will rendezvous with Ceres and begin orbiting in 2015,
conducting studies and observations for at least five months.
"Ceres is the largest asteroid and one of the darker bodies in the belt," Russell said. "We will soon learn more about the
dark materials that have added so many highlights to the face of Vesta."
Dawn, the second scientific mission to be powered by an advanced NASA technology known as ion propulsion,
is the first NASA mission to orbit two solar system targets beyond the moon.
UCLA is in charge of Dawn's science and public outreach. Russell leads the science team that has the lead role in analyzing and
interpreting the data from Dawn.
For more information about Dawn, visit
JPL manages the Dawn mission for NASA's Science Mission Directorate in Washington. Dawn is a project of the directorate's Discovery
Program, managed by NASA's Marshall Space Flight Center in Huntsville, Ala. The University of California at Los Angeles (UCLA) is
responsible for overall Dawn mission science. Orbital Sciences Corp. in Dulles, Va., designed and built the spacecraft.
The German Aerospace Center, the Max Planck Institute for Solar System Research, the Italian Space Agency and the
Italian National Astrophysical Institute are international partners on the mission team. The California Institute of
Technology in Pasadena manages JPL for NASA.
UCLA is California's largest university, with an enrollment of more than 40,000 undergraduate and graduate students.
The UCLA College of Letters and Science and the university's 11 professional schools feature renowned faculty and offer
337 degree programs and majors. UCLA is a national and international leader in the breadth and quality of its academic,
research, health care, cultural, continuing education and athletic programs. Six alumni and six faculty have been awarded
the Nobel Prize.