Image: Asteroid with frost
Gabriel Perez
Researchers verified that the asteroid 24 Themis - a large rock hurtling through space in the asteroid belt between Mars and Jupiter - is coated in a layer of frost.
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updated 4/28/2010 3:30:38 PM ET 2010-04-28T19:30:38

Scientists have found lots of life-essential water — frozen as ice — in an unexpected place in our solar system: an asteroid between Mars and Jupiter.

The discovery of significant asteroid ice has several consequences. It could help explain where early Earth first got its water. It makes asteroids more attractive to explore, dovetailing with President Barack Obama's announcement earlier this month that astronauts should visit an asteroid. And it even muddies the definition between comets and asteroids, potentially triggering a Pluto-like scientific spat over what to call these solar system bodies.

This asteroid has an extensive but thin frosty coating. It is likely replenished by an extensive reservoir of frozen water deep inside rock once thought to be dry and desolate, scientists report in two studies in Thursday's issue of the journal Nature.

Two teams of scientists used a NASA telescope in Hawaii to look at an asteroid called 24 Themis, one of the bigger rocks in the asteroid belt between Mars and Jupiter. They examined light waves bouncing off the rock and found the distinct chemical signature of ice, said University of Central Florida astronomy professor Humberto Campins, lead author of one of the studies.

Astronomers have long theorized that hydrogen and oxygen and bits of water locked in clay are in asteroids, but this is the first solid evidence. And what they found on 24 Themis, a rock more than 100 miles wide (198 kilometers wide) with temperatures around 100 degrees below zero Fahrenheit (-73 degrees Celsius), was more than they ever expected. About a third of the rock seemed to be covered in frost.

Furthermore, scientists didn't just find ice; they found organic molecules, similar to what may have started life on Earth, Campins said.

"This asteroid holds clues to our past and how the solar system and water on Earth may have originated, and it also has clues to our future with exploration of near-Earth asteroids," Campins told The Associated Press.

"We're showing that they're wetter than we thought," Campins said. "We're showing they have organic molecules that might have been the building blocks of life on Earth."

Below the beltScientists say that Earth was dry when it formed billions of years ago. So where did the water come from? One leading theory is from crashing comets, which are essentially icy snowballs.

But comets come from the outer reaches of the solar system and tend to have more heavy hydrogen than the water in our oceans, said Donald Yeomans, manager of NASA's Near Earth Object Program office. Icy asteroids between Mars and Jupiter might have the right heavy hydrogen ratio to match what's on Earth, said Yeomans, who wasn't involved in the studies.

MIT's Richard Binzel, also praised the studies, calling the findings "one more piece in the puzzle for an abundance of water arriving on Earth and having available the ingredients for life."

Normally, the ice on the asteroid should have escaped Themis as a gas over thousands of years, but it's still there after a billion years or so, Campins said. That means there's likely a supply of ice inside the rock, replenishing the surface, he said.

And if that's the case for other similar asteroids — especially those that come closer to Earth — then it would be a boon for visiting astronauts, Campins and others said. The astronauts could use the water to drink and to help make fuel. The new NASA space plan calls for astronauts to head to a nearby asteroid sometime in about 15 years as a stepping stone to Mars.

The icy asteroid also just makes a mess of the differences between asteroids and their cosmic cousin, the comet. The general definition has been that asteroids are dry rocks and comets icy snowballs.

Now it seems to be more a continuum of dry and icy with not much difference between asteroids and comets, Campins and others said.

And that, said Andrew Rivkin of Johns Hopkins University, co-author of the other study in Nature, could wind up another cosmic controversy like the debate a few years ago about whether Pluto was a planet. Pluto wound up being reclassified by the International Astronomical Union as a dwarf planet.

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