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Asteroids are often thought of as dangerous space rocks that could smack into Earth and destroy all life. But scientists suspect that in the early days of our planet, asteroid impacts gave us some of the building blocks of life, key ingredients like water. That's one reason scientists are excited by the recent discovery of a frosty layer of water on one asteroid they've been studying.

NPR's Nell Greenfieldboyce has more.

NELL GREENFIELDBOYCE: Out between Mars and Jupiter is a region of space filled with asteroids. In science fiction movies, this kind of place is usually depicted as a minefield of closely packed rocks that a spaceship has to slip through.

But asteroid expert Andrew Rivkin from Johns Hopkins University says it's not really like that.

Mr. ANDREW RIVKIN (Johns Hopkins University): I love "Empire Strikes Back" but the asteroids are very, very far apart from one another. It is nowhere near as dense as what you see in, you know, "Empire" or those sorts of movies.

GREENFIELDBOYCE: He and a colleague used an infrared telescope in Hawaii to study one asteroid named 24 Themis. It's big, about 120 miles across. By studying the light reflected off this asteroid, the researchers saw evidence that its surface is covered in a thin layer of frozen water.

Mr. RIVKIN: And I thought that looks weird. I don't know if I believe that.

GREENFIELDBOYCE: Rivkin says on an asteroid like this, with no atmosphere, you wouldn't expect frozen water to stick around. Any ice should have disappeared long ago.

Another group of researchers was using the same telescope to study the same asteroid, and they saw the same thing.

Dr. HUMBERTO CAMPINS (Planetary Science/Astronomy, University of Central Florida): And I thought, oh my God, this looks like ice.

GREENFIELDBOYCE: Humberto Campins is at the University of Central Florida in Orlando. He says this thin surface layer of ice must be constantly refreshed, but it's not clear where it's coming from.

Dr. CAMPINS: You could have a subsurface layer or a buried ice layer in this asteroid that would have survived the age of the solar system.

GREENFIELDBOYCE: It would be that old because, unlike the planets, asteroids are thought to have existed basically unchanged for billions of years.

Mr. HENRY HSIEH (Researcher, Queen's University): So, the ice that we see there right now is sort of related to the ice that could have come from the main asteroid belt that hit us about four billion years ago.

GREENFIELDBOYCE: Henry Hsieh is a researcher at Queen's University in Belfast. He says the Earth probably formed dry and its water got delivered later, perhaps by impacts from icy comets and asteroids. So, the frozen water on Themis is like a fossil from that period in our history.

Mr. HSIEH: The interesting bit, I think, with this discovery is that it gives us a way to kind of probe the cousins of the asteroids that hit us and probably gave us water, you know, in the earlier stages of the Earth's formation.

GREENFIELDBOYCE: In two reports on this discovery in the journal Nature, the researchers say they saw signs of other interesting things on this asteroid too, like organic compounds. That's another ingredient for life that asteroids might have brought to Earth. But Hsieh says learning more with telescopes will be hard, because asteroids like this one are relatively small and so far away.

Mr. HSIEH: To kind of really, really answer some, you know, some of the real questions that we have probably will require a spacecraft visit.

GREENFIELDBOYCE: And one spacecraft is currently on its way to the asteroid belt. NASA's Dawn Mission will orbit two massive asteroids. It will arrive at the first next year. And President Barack Obama's administration is trying to get support for a new plan for NASA that envisions sending astronauts to a nearby asteroid, possibly as early as 2025.

Nell Greenfieldboyce, NPR News.

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