The Asteroid That Thought It Was a Comet

The difference between an asteroid and a comet was always thought fixed—but now a new paper says the two celestial bodies may be more similar than we thought

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MCCE-Observatoire de Paris / CNRS / Y.Gominet, B. Carry

New research shows that the asteroid Ceres vents water vapor, as seen in this artist's impression

Astronomy buffs know very well that asteroids and comets are entirely different animals. The former are made mostly of rock and metal, and live in a kind of no-man’s land between Mars and Jupiter. The latter are mostly ice, with a bit of rock and dust thrown in. They hang out on the outer fringes of the Solar System, beyond Pluto (although the odd comet does wander in toward the Sun every so often, which is why they were actually discovered before asteroids).

But a new paper in Nature is making the comet-asteroid distinction a little more difficult to maintain. Along with several colleagues, Michael Küppers of the European Space Agency, has found evidence of water vapor spewing from the surface of Ceres—a powerful hint that the largest asteroid in the solar system, at 580 miles (933 km) across, has a suspiciously comet-like store of subsurface ice. “This is very significant,” says University of Central Florida planetary scientist Humberto Campins, who wrote a commentary on the results, also in Nature. “It’s fascinating.”

This isn’t the first hint that Ceres is what Küppers calls “a very peculiar beast of an asteroid.” As far back as the late 1970s, ground-based telescopes looking at sunlight reflected from the asteroid’s surface saw evidence of water-bearing minerals. In the 1990s, observers saw a hint of hydroxyl molecules—essentially H2O minus one hydrogen atom— in the vanishingly thin atmosphere surrounding the asteroid.

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More recently, says Campins, careful measurements of Ceres’ shape revealed it to be too perfectly spherical. Thanks to its spin, a rocky body would have assumed a (very) slightly flattened shape as it cooled from its original molten state. A body with significant amounts of ice inside, on the other hand, would remain flexible, allowing gravity to pull it into a perfect sphere.

All of this evidence was circumstantial, however: the new observations, made with the Herschel Space Telescope, show water vapor coming off the asteroid. It’s coming from two distinct regions on the asteroid’s surface, and could be happening for one of two distinct reasons. The first is that ice lies just below the surface and turns to vapor when heated by the Sun, which is what happens to comets. The second: the ice is deeper, gets heated by radioactive decay in Ceres’ core and erupts through the surface in the form of ice volcanoes. “Personally,” says Küppers, “I lean toward the cometary explanation.”

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The fact that Ceres has enough ice left to be generating water vapor at all, more than four billion years after the asteroid originally formed, suggests that there’s an awful lot of it. That would be strange enough in itself, but it’s completely at odds with the composition of Vesta, the second-largest known asteroid.

That body, 326 miles across, is covered with lava flows, evidence of a mostly rocky and metallic interior. In theory, the larger Ceres should have had similar flows, since it has plenty of rock as well. It doesn’t, though—possibly because Ceres was literally able to let off steam as it cooled, where Vesta couldn’t, trapping heat inside until the molten rock burst free. The Dawn space probe did find evidence of water-bearing minerals on Vesta’s surface during a 2011-2012 encounter, but scientist believe these were created after Vesta formed, when icy comets splattered onto the asteroid.

So why is one asteroid rich in water while its slightly smaller cousin isn’t? Most likely because Ceres formed further out in the solar system. “The further out you form,” explains, Campins, “the more likely you are to condense ice.” Far-off, icy Pluto, which is now classified along with Ceres as a “dwarf planet,” is a case in point. Planetary scientists are increasingly convinced that the giant planets—Jupiter, Saturn, Uranus and Neptune, along with a nameless fifth, long since escaped into deep space—moved around quite a bit during the solar system’s early days. All that movement would have reshuffled the asteroids and comets as well, plausibly explaining why Ceres ended up in a less icy neighborhood than where it started out.

It’s a plausible story, but things will become a lot clearer over a year from now. That’s when the Dawn probe arrives at Ceres for its second major close encounter. Given its impressive performance at Vesta, scientists are practically salivating over what they might see in February, 2015. “We’re living,” says an eager Campins, “in very exciting times.”

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