Pluto's Wonders Come into Focus
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Taken together, these features show that more than four billion years after its formation, Pluto still somehow retains enough internal heat to maintain an active geology and, here and there, a very youthful surface replenished by cryovolcanism and the seasonal sublimation and deposition of volatile ices. Deep within the world, Pluto’s heat could be sufficient to sustain an ocean of ammonia-rich water beneath a thick roof of water-ice bedrock. Long linear striations upon parts of Pluto’s surface hint that any subsurface ocean may be slowly freezing, deforming the ground and releasing additional latent heat as it turns to ice.
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New Horizons has revealed Pluto’s tenuous atmosphere of gaseous nitrogen and methane to be colder and more compact than previously thought, and layered with hazes of soot-like hydrocarbon particles produced by ultraviolet light and cosmic rays. The particles are reddish, but at sunrise and sunset when sunlight passes through the thickest hazes, they scatter the light to give Pluto’s sky a blue tint. The particles are also sticky, and grow like snowflakes over tens of thousands of years, until at last they become heavy enough to fall, accumulating as crimson sludge in the world’s most ancient terrains.
The most primordial part of Pluto’s surface may be a hemisphere-spanning splash of red called Cthulhu Regio, a region so thoroughly pulverized by craters it is thought to be some four billion years old. Curiously, it is directly adjacent to the western edge of what could be Pluto’s youngest landform, the fresh, cream-colored ices of Sputnik Planum. Even Sputnik Planum, it turns out, has surprisingly ancient roots: Its youthful ice fills a deep basin that may be the oldest, largest impact crater still in existence on Pluto.
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Besides Cthulhu Regio, Pluto’s most notable other old, reddish, impact-generated feature isn’t actually on the dwarf planet at all—it’s Pluto’s largest moon, Charon. Long thought the product of a cataclysmic impact of the same sort that made Earth’s moon, Charon’s violent origins have been all but confirmed by New Horizons. Most of Charon’s surface is actually grayish bright water ice, with craters indicating it is more than four billion years old—a strong hint it coalesced from shattered and ejected pieces of Pluto’s water-ice crust. Its connection to Pluto hasn’t been completely severed, though: Mordor Macula, a cap of dark red hydrocarbons at its north pole, is likely produced by ultraviolet light reacting with wisps of upper atmosphere that drift away from Pluto’s gravity and freeze onto Charon, building up like layers of red varnish over billions of years.

http://www.scientificamerican.com/article/pluto-s-wonders-come-into-focus/