NASA's first hard look at the protoplanet Vesta has given scientists an unprecedented view of its makeup, terrain and history -- and revealed that major activity on this ancient rock occurred far more recently than researchers had expected.
Images sent back from NASA's trailblazing Dawn spacecraft reveal the full size of a massive crater in the southern hemisphere and indicate that it may have been made just 1 billion years ago, well after Vesta formed more than 4.5 billion years ago, according to one of half a dozen studies published in Friday's edition of the journal Science.
"We have been able to use a time machine and take our thoughts and understandings right back to the beginning," said University of California, Los Angeles geophysicist Christopher Russell, the Dawn mission's principal investigator.
Dawn was launched in September 2007 to investigate Vesta's properties, including its mineral composition, topography and mass. It is equipped with cameras, spectrometers and neutron and gamma-ray detectors, which have been taking measurements since entering the asteroid's orbit in July.
Vesta sits in the middle of the asteroid belt that lies between the orbits of Mars and Jupiter. That ring of rocky debris is thought to be the remnants of a would-be planet that never formed because of the disruptive pull of Jupiter's gravity. The asteroid is the second largest in the belt and one of only three protoplanets in the solar system, a seed around which a full planet could have grown.
Scientists believe that studying the asteroid will give them a sense of what the early Earth and moon might have looked like. The rocks of Vesta are far older than ours, since they haven't been altered by factors such as volcanic activity or the presence of living things.
"Everything we are made of was in the solar system 4.5 billion years ago," said Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology who was not involved in the mission. But on Earth, "it's just been recycled or changed over and over again." On Vesta, he said, "here's a case where the building blocks have never been recycled since their first formation."
The Science papers reveal that Vesta is a large, rocky mass about 178 miles wide, 173 miles long and 139 miles deep. (By way of comparison, the moon is nearly 300 times more massive.) Cameras that have surveyed about 80 percent of the planet's surface reveal a heavily cratered northern hemisphere and a surprisingly smooth southern hemisphere, marred by two giant impact craters partially layered on top of one another.
The younger, larger crater, known as Rheasilvia, stretches about 310 miles across the southern hemisphere. The impact that created it is probably responsible for the southern hemisphere's smooth appearance, the researchers said, since much of the pulverized material settled around the basin.
By counting the number of smaller craters inside Rheasilvia and estimating how often debris rams the protoplanet, the researchers were able to peg the basin's age at about 1 billion years -- surprisingly young, considering that Vesta is thought to have come together just 2 million years after the first solids started forming in the nascent solar system.
The Dawn instruments determined that Vesta's surface is made up of basaltic rock, a relatively airy material. That finding led researchers to suspect that the asteroid was once melted -- and may have experienced enough melting to allow heavier elements such as iron to sink to the center while lighter elements rose to the surface.
This also implies that Vesta must have formed very early in the solar system's history, when short-lived radioactive materials could have trapped heat inside these protoplanetary bodies, allowing some melting to occur.
The scientists said that having a substantial iron core may be partly what allowed Vesta to survive an impact as massive as the one that created Rheasilvia.
The older impact basin, Veneneia, stretches 249 miles and is thought to be at least 2 billion years old.
The pyroxene-rich chemical composition of the asteroid, which was detected by Dawn's spectrometers, firmly links it to a class of common meteorites that have landed on Earth, researchers found.
Those meteorites, called howardite-eucrete-diogenite meteorites, account for about 6 percent of the space rocks that have struck our planet -- and they allowed scientists to be far better prepared for their maiden voyage to Vesta than they ever were when they visited Mars, Venus or Mercury for the first time, the researchers pointed out.
Vesta is just the first stop on Dawn's journey. The next will be Ceres, another protoplanet in the asteroid belt.
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