Thar she blows!

Water plumes from the dwarf planet Ceres make an exciting find for astronomers.

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 vapour 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 Kuppers, 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 950km (590 miles) wide and roughly spherical, which is part of why it was once considered planet-like. And like Pluto, it was massive enough for its own gravity to crush it into a more or less spherical shape. But Ceres soon lost its title when astronomers realised 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.

Kuppers 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.

Using the ESA’s powerful Herschel Space Observatory, Kuppers 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 6kg per second, and the scientists think there could be so much ice packed in its 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 vapour. It could be that some residual internal heat is causing the water to rise up and explode into geyser-like blasts of water vapour – 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 sunlight 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 ... Ceres (is) 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.

Researchers won’t have to wait too long to get more answers from Ceres. Nasa’s Dawn spacecraft, now that it’s done exploring Vesta, will reach Ceres early next year and give scientists a close look at this strange, distant world.

“I’m excited to see what Dawn is going to find out,” said Kuppers, who works on the space agency’s comet-hunting Rosetta mission. – Los Angeles Times / McClatchy-Tribune Information Services

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