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Dawn arrives at Vesta: Will scientists find a water-coated protoplanet?

Vesta, the second-largest object in the asteroid belt, may have a thin skin of water molecules like parts of the moon. Dawn, an orbiting spacecraft, arrives today to begin orbiting and observing.

By Staff writer / July 15, 2011

This image of Vesta was captured by Dawn spacecraft on July 9, from a distance of about 62,000 miles (100,000 kilometers) away from the protoplanet. Each pixel in the image corresponds to roughly 5.8 miles.

NASA / AP

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Around 10 p.m. Pacific time tonight, a 1,600-pound spacecraft called Dawn is scheduled to begin orbiting the asteroid Vesta – the second-largest asteroid in a belt of cosmic rubble that orbits the sun between Mars and Jupiter.

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This is the first mission aimed at orbiting an asteroid, as well as the first designed to orbit two objects as part of the same project.

Vesta, thought to have formed within the first 30 million years after the sun ignited some 4.6 billion years ago, represents a case of planetary arrested development, researchers say. Thus, the object, large enough to cover about half the state of California, is expected to open a unique window on processes that led to the formation of the rocky, inner planets – Mars, Earth, Venus, and Mercury.

"We're very excited," says Carle Pieters, a planetary scientist at Brown University in Providence, R.I., and a member of Dawn's science team. "We've been working with the data that's been pouring down" during the spacecraft's final approach to the asteroid, "and we're just thrilled with some of the initial images. Everything is working perfectly."

Although Vesta has long been characterized as an asteroid, over the years, scientists have come to appreciate it as a "protoplanet" – an object that was on its way to joining the ranks of planets until something stunted its growth.

Ground and space-based observations have indicated that Vesta's surface is made up largely of solidified lava, dubbed basalt. This evidence for early volcanic activity on Vesta has led to the notion that early on, it was heated by the radioactive decay of a handful of elements. These radioactive elements, created when a nearby star exploded in a supernova, found their way into the cloud of dust and gas from which the sun and planets formed.

The volcanism stopped because the elements involved – radioactive varieties of aluminum and iron – have relatively short lives in radioactive form, compared with the elements that continue to toast Earth's core, Dr. Pieters explains.

Still, the volcanic activity went on long enough for the material making up Vesta to separate, or differentiate, with the heavier elements gathering at Vesta's center and the lighter ones making their way toward the surface.

Indeed, this differentiation is what has earned Vesta the title "protoplanet."

Data from Dawn will provide a reality check on this scenario.

And while Vesta appears to be essentially solid rock, the object also could host tiny amounts of water – as thin layers of water molecules on rock surfaces.

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