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AAA  Jul. 16, 2015 3:17 PM ET
Scientists are speculating that Charon's dark spot is ice from Pluto
  Robert Gebelhoff
The Washington Post News Service
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As scientists wait for a tidal wave of data from Pluto and its moons, they are peering through the existing photos and information to see what hypotheses they could test with the data.

One debate that's happening right now is about the dark spot on Pluto's moon-partner, Charon, which has been speculated to be the result of gases leaving Pluto and depositing onto the moon's surface. So in essence, they're sharing ice, and that's pretty cool.

But the theory is just that — a theory.

"We've been arguing," said William Grundy, of the New Horizons composition team. "The counter-proposal is that [the dark spot] is just an impact basin. That's an alternative explanation."

Scientists are also puzzled about the diversity of Pluto's surface. The area of its polar ice cap looks pretty similar to the left side of Pluto's heart, and they're not sure why. It could be due to altitude, or the ancient history of the planet, something coming from the interior.

"Naively, you would expect differentiation by latitude," Grundy said. "That doesn't explain the heterogeneity in Pluto. We don't know; we need to figure that out."

They're looking for what could be Pluto's bedrock, or the material underneath the ice. They're looking for water-ice, but it could also just be a bunch of rock. (By the way, at the temperatures of Pluto, water is basically rock, because it doesn't sublimate like gases.)

These are the mysteries they will be grappling with as they sift through new data over the coming months, looking at the photos and heightening the color to include things like infrared. Different molecules shoot back different colors, so theoretically, a high-definition photo with lots of pixels could tell where different kinds of ices — such as nitrogen, carbon monoxide and methane — are.

New Horizons has already revealed that Pluto is a bit bigger across than previously thought — by about a few miles. That might not seem to be a big difference, but scientists have already calculated its mass, so the discovery suggests the planet is less dense than thought. That could mean more ice and less rock.

The high-resolution photos have also shown that the surface of the icy world seems to be much smoother than its moon-partner, Charon, which seems to have far more impact craters.

Mission scientists suggest that means there is something at play — such as internal heat in the planet that is keeping the rock and ice of the body soft, or atmospheric processes like snow covering up the geologic features and making it smoother.

"I expected that it would be complicated and fascinating, but I had no idea that it would be this complicated and this fascinating," Grundy said.

bc-pluto (TPN)

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