Pluto's internal heat source 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?
 A: Radioactivity.
Best supported, as pointed out by Emilio Pisanty, on §2.2 of Icarus 216, 426 (2011) (eprint).
First, Pluto having an internal source of heat is indeed the most likely scenario (if Pluto and Charon are as old as expected), not only given its apparent youth, but also due to the absence of compressional features on its surface, according to recent paper,


*

*Recent tectonic activity on Pluto driven by phase changes in the ice shell. N.P. Hammond et al. Geophys. Res. Lett. 43, 6775 (2016).


which Ars Technica nicely summarizes here.
The same paper indicates through simulations that radioactive heating is a viable scenario:

The core is heated by long lived radioisotopes $^{238}$U, $^{235}$U, $^{232}$Th and $^{40}$K. We assume an initial abundance of parent isotopes appropriate for CI chondrites.

Also, according to this old NewScientist article:

If the core contains potassium at a concentration of 75 parts per billion, its decay could produce enough heat to melt some of the overlying ice, which is made of a mixture of nitrogen and water. It should have at least that much potassium and probably more, says William McKinnon at Washington University in St Louis, Missouri.

The same article points out that the existence of an ocean today depends on "the sloshiness of the ice that covers it" as well, which should be strong enough to decrease convection and, thus, loss of heat to space.
A: The New Horizons spacecraft has an internal heat source. It is a radioactive material - plutonium. The best known celestial body is Earth. Its internal heat source are radioactive elements. Pluto is not as large and not as dense as Earth. But it also has a lot of rocks. Those rocks contain radioactive elements which generate heat.    
