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don't judge a moon by its cover

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presented by science at nasa

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at first glance saturn's moon phoebe

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doesn't look much like a planet it's

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only 220 kilometers in diameter smaller

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by far than most u.s states

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the moonlit is dark airless and pitted

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with craters nearly as wide as phoebe

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itself

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yet researchers have just announced that

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this small apparently misshapen hunk of

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ice and rock orbiting backwards around

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saturn is more like a planet than anyone

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imagined

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astronomers have long known that phoebe

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wasn't like other moons in the saturn

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system so when nasa's cassini spacecraft

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reached saturn in 2004 visiting phoebe

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was a top priority

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in fact it was the first moon cassini

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encountered on its way into saturn

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data from the flyby have occupied

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mission scientists ever since and they

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released their findings in the journal

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icarus in april 2012

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by combining cassini data with modeling

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techniques previously applied to other

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solar system bodies we've been able to

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go back in time and clarify why phoebe

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is so different from the rest of the

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saturn system says jonathan lanine a

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co-author on the study and a cassini

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team member at cornell university

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first of all phoebe is more spherical

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than it looks grid lines wrapped around

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the moon resemble a well-worn classroom

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globe once round but a bit battered from

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overuse

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cratering has made the body visually

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irregular but in the distant past it was

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clearly much more uniform

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co-author peter thomas a cassini team

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member at cornell explains

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from the shape seen in cassini images

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and modeling the likely cratering

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history we were able to see that phoebe

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started with a nearly spherical shape

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apparently phoebe formed early enough in

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the solar system's history to capture

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radioactive materials in its core

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as in the core of earth radioactive

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decay inside phoebe would produce a

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significant amount of heat warming the

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interior and reshaping the moon into a

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sphere

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in this scenario heavier materials would

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sink to the core and lighter materials

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would float to the top

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indeed cassini data indicate that phoebe

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has denser rock-rich material

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concentrated near its center internal

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heating

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layering a spherical shape

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planets have these same properties

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this doesn't mean though that phoebe is

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a full-fledged planet

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objects like phoebe represent building

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blocks of planets says jpl's julie

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castillo rojas lead author of the study

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they give scientists clues about what

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conditions were like around the time of

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the birth of planets and their moons

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the researchers believe phoebe is

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probably a cousin of pluto born in the

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outer reaches of the solar system

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this would solve a number of mysteries

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while saturn's regular moons orbit in a

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common plane and go around the planet in

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the same direction

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phoebe's orbit is tilted and retrograde

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it actually circles saturn backwards

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at some point in the distant past the

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little planetesimal probably wandered by

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saturn and got caught by the gas giant's

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gravitational field

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given that phoebe's density and

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composition are similar to pluto's it is

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likely that phoebe came from the same

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place pluto lives

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the kuiper belt the region of ancient

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icy rocky bodies beyond neptune's orbit

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remarkably phoebe is just one of more

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than 60 moons in the saturn system each

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with its own unique history

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at least 59 stories remain to be told

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for more news from saturn as cassini