Before Pluto's fall from planetary grace, there was Ceres. Depending on your definition, it's either the largest asteroid or the smallest dwarf planet -- but for a few glorious decades in the 1800s, the rocky sphere was a full planet in the solar system's pantheon.
Now, astronomers have discovered water vapor steaming off this mysterious little planetoid –- and the discovery, published in the journal Nature, could have fascinating implications for the evolution of our solar system.
"Now we have really for the first time discovered water in the asteroid belt," said lead author Michael Küppers, a planetary scientist based in Spain with the European Space Agency.
Ceres sits in the asteroid belt between Mars and Jupiter –- and it's the only dwarf planet in the inner solar system. It's 590 miles wide and roughly spherical, which is part of why it's considered somewhat planet-like –- like Pluto, it's massive enough for its own gravity to crush it into a more or less spherical shape.
But Ceres soon lost its title as full planet when astronomers realized that its rocky body wasn't alone: It was sitting in a vast field of rocky bodies, or asteroids. Ceres was named the first and the largest among them.
Küppers had been looking to do a little reconnaissance work on Ceres before NASA's Dawn mission visits the planetoid next year. The major question hovering around the dwarf planet: Was it rich in water, or not?
Ceres' relatively low density told astronomers that it could potentially have a high amount of water ice stored away. Astronomers in the 1990s picked up the chemical fingerprint of hydroxyl -– a fragment of the water molecule -– in the light coming from Ceres, but a study in 2011 with more sensitive instruments could not back that claim up.
But using the European Space Agency's powerful Herschel Space Observatory, Küppers and his team were able to look for the chemical fingerprint of full water molecules, which gave them a much stronger signal. They spotted clear signs of water coming from two separate dark spots, on roughly opposite sides of the little world.
Water was coming off Ceres at a rate of 6 kilograms, or about 13 pounds, per second -– and the scientists think there could be so much ice packed in the dwarf planet's mantle that its melted contents would add up to more fresh water than we have on Earth.
The scientists aren't exactly sure how the ice is stored on Ceres or how it's escaping as vapor. It could be that some residual internal heat is causing the water to rise up and explode into geyser-like blasts of water vapor -- not liquid, as liquid water requires a thicker atmosphere (like Earth's) to remain stable. It could also simply be that exposed ice on the surface in these two areas is sublimating from a solid to a gas when the sun hits it.
Whatever the mechanism, the larger question remains: Why is Ceres so wet?
"One of the most puzzling questions about the origin and evolution of asteroids is why Vesta and Ceres are so different," Humberto Campins and Christine Comfort of the University of Central Florida wrote in a commentary on the Nature paper.
After all, Ceres is roughly the same distance from the sun as the lumpy asteroid Vesta, which is volcanic and bone dry. So how did Ceres hold onto this water when Vesta did not?
It turns out Ceres may not be native to this part of the solar system. It probably originated somewhere beyond the 'snow line' -- that imaginary boundary in the solar system beyond which water ice can exist in space, largely out of the reach of the sun's rays.
Ceres may have been one of the wanderers set loose in a major migration in the solar system's history. Massive Jupiter, for example, is thought to have traveled both closer in and then farther out from the sun than where it started -– and its gravitational influence yanked the other planets around the solar system and left scars in the asteroid belt.
The icy asteroid also supports the idea that asteroids aren't as dry as expected, Küppers said. Icy comets are the usual suspect for having brought water to Earth, but it could just as well have been an asteroid, he said.
Answering more questions about Ceres, and what it can tell planetary scientists about the solar system's history, will have to wait. The Herschel Space Telescope ran out of the coolant needed to keep its instruments working last year. But researchers won't have to wait too long: NASA's Dawn spacecraft, now that it's done exploring Vesta, will reach Ceres in spring 2015 and give scientists a close look at this strange, distant world.
"I'm excited to see what Dawn is going to find out," said Küppers, who works on the space agency's comet-hunting Rosetta mission.