The Great, Wet, Cosmic Rock

The asteroid Vesta turns out to have had a very watery past

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NASA / JPL-Caltech

This NASA image released Sept. 5, 2012 shows the shadowy outlines of the terrain in Vesta's northern region in this image from NASA's Dawn spacecraft.

Asteroids always get left out of the party. Send a spacecraft to Jupiter or Saturn or one of the solar system’s other glamor spots and the press will be all over you. But asteroids? Not so much. They’re cosmic side trips — the turnpike rest stops you speed by on the way to someplace better. But the fact is, asteroids are some of the coolest bodies out there — and some equally cool spacecraft have been visiting them of late. That seems truer than ever this week, with news announced in two papers in the journal Science that the Dawn spacecraft has discovered evidence of water — or at least a watery past — on the asteroid Vesta, long thought to be a wholly dry, wholly barren rock. That finding provides new insight into the formation of the solar system and the possibility that life as we know it could be lurking out there somewhere.

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Dawn was destined to make history no matter what kind of science it uncovered, simply because it’s aiming to do what no spacecraft has done before: orbit two different non-earthly bodies on one mission. Launched in 2007, it arrived at Vesta — the second largest object in the asteroid belt, at 326 miles wide (525 km) — in July 2011 and settled into orbit. Earlier this month, it peeled off and headed for the even larger Ceres — 590 miles across (950 km) — and is set to arrive there in July 2015.

During its time at Vesta, Dawn drew as close as 130 miles (210 km) to the surface, studying the asteroid with nearly two dozen instruments. Both Vesta and Ceres deserve close examination for a couple of reasons. For one thing, asteroids are some of the most ancient artifacts of the early solar system and as such can provide a good look at a long-ago epoch frozen on geological time. Also, the two miniworlds are vastly different. Vesta, though smaller, is more Earth-like, with a layered interior and a surface defined by valleys, craters, ridges and even mountains. Ceres, which is big enough to have been awarded the honorific “dwarf planet” (like the recently demoted Pluto), has an older, less weathered surface with water-bearing minerals, and may even have a very tenuous atmosphere.

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It was the water that really distinguished the two — at least until Dawn took a look. The key instrument that made the discovery was the Gamma Ray and Neutron Detector (GRaND), which scoured Vesta looking for the atomic signature of the different elements that make up — or are dusted across — the surface. It looked especially closely at three formations: the 43-mile-long (70 km) Marcia crater, the 9-mile-long (15 km) Cornelia crater and the 15-mile-long (23 km) Licinia crater. All three sites are, by definition, scenes of ancient violence, where incoming rocks crashed into Vesta, unleashing an explosion that cooked the chemistry of the two bodies together in the heat of the blast. In all three craters, visual cameras spotted extensive pitting — almost as if something had bubbled out of the ground in the moments after the impact. Throughout those pits, the GRaND found something intriguing: hydrogen, and lots of it.

On a rock like Vesta, the best explanation for so much of the element scattered on the surface is that it’s probably part of a hydroxyl molecule, which is a hydrogen-oxygen mix that is often a residue of water. There is almost no chance that liquid water could survive on an airless world like Vesta — at least not in the sunny equatorial regions where the craters are found. Instead, the researchers concluded, the hydroxyl groups must be the remains of water that was imported by the meteorites or icy comets and then instantly boiled away, accounting for the percolated look of the pits. The comets or meteors had to have been moving slowly — by space-collision standards, at least — or even the hydroxyls wouldn’t have survived the blast. But the low gravity of an asteroid tugs only lightly on a piece of approaching rubble, and that keeps the speed relatively low.

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“The source of the hydrogen,” said Thomas Prettyman of the Planetary Science Institute in Tucson, Ariz., and the lead author of one of the papers, “appears to be hydrated minerals delivered by carbon-rich space rocks that collided with Vesta at speeds slow enough to preserve their volatile content.”

What makes this news is not the fact that an asteroid got clobbered by a meteor or that the water aboard the rock boiled away under high temperatures but that it was carrying water at all. Findings over the past several years have shown the solar system to be a far wetter place than we ever knew. There is water ice in the poles of the moon and scattered throughout the lunar dust; hydroxyls and other artifacts of water have been found all over Mars, and last year, the Mars Reconnaissance Orbiter returned evidence of seasonally flowing water on a Martian cliff face; and new findings in the past year have added credence to the theory that even the Earth’s great oceans originated in deep space, with water being delivered in vast quantities by incoming comets. The discovery that even the asteroids have come in for a good dousing over the eons paints a wetter picture still.

“The pits [on Vesta] look just like features found on Mars,” says Brett Denevi of the Johns Hopkins University Applied Physics laboratory, who is the lead author of the second Science paper. “It was totally unexpected on Vesta in these high abundances.”

What Dawn will discover about Vesta’s big sister Ceres is impossible to know for a few more years, but this more-complex, more-ancient world could have an even richer story to tell. Neither body may ever generate the headlines that the glittery planets and their colorful moons receive, but they’ll keep quietly churning out the science all the same.

MORE: Dawn Probe Reaches Vesta: A First Look at a Distant World

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