When the first images were announced from the New Horizons probe back in July, there was a lot of fanfare about how the surface is young and geologically active, thus requiring an internal heat source. This was presented as a great mystery - the other icy worlds we've seen are moons of gas giants, so their internal heat source was assumed to be tidal heating, but Pluto has no source of tidal friction and thus should have cooled through long ago.
This seemed like an interesting story, but all I can find on it is the press material from July. Is there any post-Horizons literature on Pluto's internal heat source? What is it currently thought to be?