From: Henry Baker <hbaker1@pipeline.com> 'A second surprise was that the dwarf planet’s surface was unmarred by craters. “We have not yet found a single impact crater in this image,” Dr. Spencer said.' http://www.nytimes.com/2015/07/16/science/pluto-flyby-photos-reveal-mountain... I don't see the problem. Most of the chaotic behavior during planet formation occurs in the inner Solar system -- in particular, inside of the large planets -- e.g., Jupiter, Saturn, Uranus, Neptune. Outside of these planets, the density is far, far smaller, and the opportunity for collisions is also far, far smaller. Time -- at least measured by number of impact craters -- runs far, far slower in these outer regions. --I disagree with HB, this is a surprise. Obviously, whenever Pluto formed, stuff was falling in. So there must have been many craters. Why are there none now? There are only 2 options (a) when Pluto formed it was liquid (e.g. "magma ocean") and bombardment stopped before it solidified. But if HB is correct about "time running slower" then Pluto would have had time to solidify before bombardment ended, in which case (a) refuted. (b) Pluto had geological activity such as volcanism and plate tectonics, which erased the craters and built the mountains, this all happening much later. This is surprising. Also, Pluto has many moons, guessed to have formed in a collision between two big objects yielding Pluto and its moons. If so, then there should have been bombardment at that time. Charon appears to have craters. I therefore conclude the correct answer is probably (b), that is Pluto had later geo-activity, and/or melted, while Charon did not. It makes sense that the biggest one (Pluto bigger than all its moons) should be the one to melt (more gravitational energy) -- or to exhibit geo-activity: more radioactive heat per surface for it to leak out of, if the activity is driven by radioactivity.