two good reads
http://space.io9.com/heres-the-science-so-far-on-plutos-icy-mountains-and-bu-1718618581
http://space.io9.com/how-much-do-we-really-know-about-collisions-in-the-kuip-1720637251?utm_source=taboola
I found this part interesting:
"Impacts on the terrestrial planets and the moons of the giant planets are “hypervelocity” impacts, that is, the impact speeds are far greater than the speed of sound in the planet or moon, so the impacts are essentially explosions. Very roughly, hypervelocity impacts typically produce craters about 10 times bigger than the size of the impactor. Impacts on Pluto and Charon occur at typical speeds of 2 km/s, which is comparable to the speed of sound. Thus the impacts are barely hypervelocity."