October 31, 2012
Protoplanet Vesta Shows Signs Of Space Weathering
Lee Rannals for redOrbit.com - Your Universe Online
Findings described in an upcoming issue of the journal Nature show that data indicates that carbon-rich asteroids have been splattering dark material on Vesta's surface over a long span of the space rock's history.
Soils on the Moon and on asteroids have undergone extensive weathering over time, which includes the accumulation of tiny metallic particles containing iron, dulling the layers of these bodies.
Dawn's visible and infrared mapping spectrometer and framing camera detected no accumulation of these tiny particles on Vesta, making the protoplanet remain bright and pristine.
Early pictures of Vesta showed a variety of dramatic light and dark splotches on its surface. These materials were unexpected and now show that Vesta has a brightness range that is among the largest observed on rocky bodies in our solar system.
Scientists had originally thought the dark material on the protoplanet might come from the shock of high-speed impacts melting and darkening the underlying rocks. However, an analysis of Dawn data revealed that the distribution of dark material is widespread and occurs in both small spots and diffuse deposits.
The likely source of the dark material is now shown to be carbon-rich asteroids, which are 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 believe that in order to get the amount of darkening Dawn observed on Vesta, about 300 dark asteroids with diameters between 0.6 and 6 miles likely hit Vesta during the last 3.5 billion years.
All of these estimated asteroids could have provided enough blanketing to wrap Vesta in about 3 to 7 feet thick of material.
“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.”
Dawn, which launched in 2007 and spent a year observing Vesta, is now on its way to the dwarf planet Ceres, where it will conduct a study of the planet's structure and composition.
Vesta and Ceres are the most massive objects in the main asteroid belt between Mars and Jupiter. Ceres is the largest object in the main belt, and scientists believe it could harbor ice beneath its rock crust.
“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.”