ROBERT SIEGEL, HOST:
When you think of the planet Saturn, you probably picture its rings - the lovely ones that show up in all of the photos. And Saturn also has another ring that we normally don't see. The outermost ring is truly huge. It's the biggest ring in the solar system. NPR's Nell Greenfieldboyce reports that scientists have just gotten a new glimpse of it.
NELL GREENFIELDBOYCE, BYLINE: The first hint that Saturn had a secret ring came over 300 years ago, when the Italian astronomer Cassini looked through a telescope and discovered a strange moon going around the planet. The moon is called Iapetus.
DOUG HAMILTON: One side is black, and one side is white.
GREENFIELDBOYCE: Doug Hamilton showed me a picture of Iapetus he has hanging in his office at the University of Maryland in College Park. It looks like someone splattered it with black paint.
HAMILTON: Why is that? It's so unusual. We see no other satellites like that in the whole other solar system.
GREENFIELDBOYCE: Scientists had long suspected that the answer might be a ring. Maybe there was a hidden ring made of some black material, and Iapetus was just plowing through it.
HAMILTON: It's exactly the same effect you get driving your car down the highway in a rainstorm, so the rain then all hits the windshield.
GREENFIELDBOYCE: Even though a ring of black dust is not exactly easy to see in the blackness of space, Hamilton and some colleagues went looking with an infrared space telescope. And in 2009, they announced their discovery - a huge ring that dwarfs all the other rings of Saturn. It's more than 200 times as big across as Saturn itself.
HAMILTON: So it's absolutely immense - much bigger than any other ring that we know of.
GREENFIELDBOYCE: But if you were flying through it in a spaceship...
HAMILTON: You wouldn't see anything. If you were actually immersed in the ring, yeah, you'd see absolutely nothing.
GREENFIELDBOYCE: Hamilton says that's because in an area of space the size of a mountain around your spaceship, there'd be maybe 20 dust particles.
HAMILTON: That's pretty empty space, but nevertheless, we see it as a ring.
GREENFIELDBOYCE: And it's a really weird one. Besides being big, it's tilted and probably orbits backwards. Now Hamilton and his colleagues have used a different infrared space telescope to get an even better look at this oddity. Their work appears in the journal Nature.
HAMILTON: We now, for the first time, know the full size of the ring.
GREENFIELDBOYCE: It's about 30 percent bigger than they thought, and they also were able to better tell what the ring is made of.
HAMILTON: Most of the light we're seeing is coming from fairly small particles - dust grains.
GREENFIELDBOYCE: All this dust intrigues Matthew Tiscareno. He's a planetary scientist at Cornell University. He says the main rings of Saturn that people know and love aren't very dusty.
MATTHEW TISCARENO: Most of the ring particles are between marble-sized and house-sized.
GREENFIELDBOYCE: He says scientists assumed the black dust in the outermost ring had to be streaming off another moon called Phoebe. But these new observations suggest there's got to be dust coming from somewhere else, too.
TISCARENO: What we're learning today does raise a lot of questions that we don't have good answers to.
GREENFIELDBOYCE: He says one possibility is that hidden moons too small to see are adding dust to this hidden ring. Nell Greenfieldboyce, NPR News.
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