Dawn spacecraft confirms Vesta is a protoplanet

As suspected, Vesta is no mere asteroid. It is a protoplanet, a remnant of the early days of our solar system that never grew up because Jupiter kept stealing all its potential mass. A 10-month study by NASA's Dawn spacecraft mapped the features and probed the composition of the second largest asteroid in the inner solar system. It provided evidence for its protoplanetary status and for the widely accepted notion that it is the source for about 6 percent of the meteorites found on Earth. Vesta, has, as expected, an iron core covered by layers of rock that have been laid bare in huge craters up to eight miles deep, scientists said at a NASA press conference Thursday. The huge space rock, 330 miles in diameter, is believed to have formed the same way the moon, Earth and other rocky planets of our solar system did - growing from the dust and gas left over from formation of the sun and eventually attaining enough mass to heat up and melt into an iron core surrounded by layers of rock. One of the big surprises is that some collisions on Vesta occurred fairly recently, said David O'Brien, of Tucson-based Planetary Science Institute. The two largest craters on Vesta, both in the south polar region, resulted from collisions 1 billion and 2 billion years ago. That contrasts sharply with the known history of Earth's moon, where most craters date back at least 3.8 billion years, a period known as the "late heavy bombardment." The largest impact created the Rheasilvia Basin, excavating 250,000 cubic miles of material, O'Brien said - enough to "fill the Grand Canyon 1,000 times over." Some of the material was strewn across the entire surface of Vesta and some was ejected - "more than enough to explain the meteorites found on Earth," said O'Brien. Dawn has mapped 2,000 craters bigger than 2.5 miles across on Vesta's surface, said O'Brien. "Vesta is special because it survived the intense, collisional atmosphere of the solar system for billions of years," said Carol Raymond, of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "It is the only intact member of a family of similar bodies that have now disappeared." The scientists said the other bodies either became the planets we know today or were blasted into tinier space rocks that populate the asteroid belt where Vesta is located, between the orbits of Mars and Jupiter. In July, the Dawn spacecraft will depart Vesta for the far side of that asteroid belt and an even bigger target - the icy dwarf planet Ceres, 590 miles in diameter. Dawn is expected to reach Ceres in 2015. O'Brien is co-author on four of six papers published this week in the journal Science. Eight other members of the Planetary Science Institute - Robert Gaskell, R. Aileen Yingst, Brent Garry, Thomas Prettyman, Mark Sykes, Pasquale Tricarico, William Feldman and Robert Reedy - contributed to the papers. Sykes, director of the Planetary Science Institute, called the studies "quite an interesting window into the impact system of the solar system and the physics of a nearly planetary body."