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Pluto may have deep seas, ancient faults

Cool world In July 2015 we get our first close look at the dwarf planet Pluto and its moon, Charon — a fact that has scientists hypothesising more than ever about what we might see there.

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," says Brown University's Amy Barr, co-author of a new paper with Geoffrey Collins in the latest issue of the journal Icarus.

By 'ancient' they mean sometime way back during the first billion years of the solar system's history.

Barr and Collins modelled 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," says Barr. That's because as the ocean freezes, the remaining liquid portion gets enriched with salts and ammonia, which serve as antifreeze.

Locked together

Next comes the part where that ocean could have created icy tectonic plates on Pluto's surface.

"One thing that we know is the angular momentum will be conserved as the system evolved," says Barr.

With that fact, they simulated a bunch of scenarios based on where Charon's orbit was right after the collision, since nobody actually knows where Charon started.

Then in each scenario they saw Charon's orbit gradually migrate outward, just like the Moon's orbit did around Earth.

When Pluto and Charon were closer and still hot from their collision they pulled more forcefully on each other and were more egg-shaped as a result. But as Charon moved away, Pluto became more spherical. But to change shape, the icy surface would have had to crack and create faults — telltale signs of tectonics.

"In the scenarios we see, you generate more than enough stress to create all kinds of tectonic features," says Barr.

Tectonic features

But will New Horizons see those faults? Probably, says Jeffrey Moore, the head of New Horizon's geology and geophysics imaging team at the NASA Ames Research Center.

"It would probably be surprising if we didn't see tectonism," says Moore.

One potential complication is Pluto weather.

Telescopes discovered years ago that Pluto has an atmosphere when it is at its closest approach to the Sun, and then that atmosphere freezes to the surface when Pluto is on the more distant part of its elliptical orbit.

That regular change could be enough to erode the surface of Pluto to the point where it might hide the tectonic features.

"It's not inconceivable that the tectonics are eroded and covered up by sediments," says Moore. But he doubts that will be the case, pointing to examples of worlds with atmospheres that freeze to the surface seasonally — like Jupiter's moon Callisto.

"Callisto has sublimation and deposit of its atmosphere but you can still see the large features," says Moore.

Also, New Horizons will be looking at Pluto with a resolution that will be better than 100 meters per pixel in some places, says Moore. So the chances are pretty good.

And if no tectonic faults are seen?

"We'll just have to go back and revisit this when we get there," says Moore.