This image of Pluto and its largest moon, Charon, was taken by the Ralph color imager aboard New Horizons on April 9, 2015, from a distance of about 71 million miles (115 million kilometers). It is the first color image ever made of the Pluto system by a spacecraft on approach.

(via 20150414_First_Color_Image_Ralph.png (200×200))

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Gregory Benford’s Guide to Terraforming the Moon


When it comes to remaking a celestial body in Earth’s image—“terraforming” it—the moon has clear advantages: It gets twice the sunlight of Mars. It’s a three-day trip with current technology, while getting people to Mars would take six months. Furthermore, the moon is dead and it’s small, so it…

Stick with me kids and you could have a job in the Kuiper Belt firing comets into the Moon, making it rain there for ten thousand years. In your new space-hardened, posthuman body. Would you like to know more?

MORE – Gregory Benford’s Guide to Terraforming the Moon

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One of the latest ideas put forward is that perhaps the collision that likely formed Pluto and Charon heated the interior of Pluto enough to give it an internal liquid water ocean, which also gave the small world a short-lived plate tectonics system, like that of Earth.

“We predict that when New Horizons gets there it will see evidence of ancient tectonism,” said Brown University’s Amy Barr, coauthor of a new paper with Geoffrey Collins in the latest issue of the journal Icarus. By “ancient,” Barr means sometime way back during the first billion years of the solar system’s history.

Barr and Collins modeled the Pluto-Charon system based on the idea that the initial collision of the two bodies would have generated enough heat to melt the interior of Pluto creating ocean that would have survived for quite a while under an icy crust.

“Once you create an ocean on an icy body, it’s hard to get rid of it,” said Barr. That’s because as the ocean freezes, the remaining liquid portion gets enriched with salts and ammonia – which serve as antifreeze.

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Cracks in Pluto’s Moon Could Indicate it Once Had an Underground Ocean

In Charon’s case, this study finds that a past high eccentricity could have generated large tides, causing friction and surface fractures. The moon is unusually massive compared to its planet, about one-eighth of Pluto’s mass, a solar system record. It is thought to have formed much closer to Pluto, after a giant impact ejected material off the planet’s surface. The material went into orbit around Pluto and coalesced under its own gravity to form Charon and several smaller moons.
Initially, there would have been strong tides on both worlds as gravity between Pluto and Charon caused their surfaces to bulge toward each other, generating friction in their interiors. This friction would have also caused the tides to slightly lag behind their orbital positions. The lag would act like a brake on Pluto, causing its rotation to slow while transferring that rotational energy to Charon, making it speed up and move farther away from Pluto.

“Depending on exactly how Charon’s orbit evolved, particularly if it went through a high-eccentricity phase, there may have been enough heat from tidal deformation to maintain liquid water beneath the surface of Charon for some time,” said Rhoden.
“Using plausible interior structure models that include an ocean, we found it wouldn’t have taken much eccentricity (less than 0.01) to generate surface fractures like we are seeing on Europa.”

“Since it’s so easy to get fractures, if we get to Charon and there are none, it puts a very strong constraint on how high the eccentricity could have been and how warm the interior ever could have been,” adds Rhoden. “This research gives us a head start on the New Horizons arrival – what should we look for and what can we learn from it. We’re going to Pluto and Pluto is fascinating, but Charon is also going to be fascinating.”

Based on observations from telescopes, Charon’s orbit is now in a stable end state: a circular orbit with the rotation of both Pluto and Charon slowed to the point where they always show the same side to each other. Its current orbit is not expected to generate significant tides, so any ancient underground ocean may be frozen by now, according to Rhoden.

Cracks in Pluto’s Moon Could Indicate it Once Had an Underground Ocean

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