On the origin of the Pluto-Charon binary

The normalized angular momentum density of Pluto-Charon (0.45) exceeds the critical value of 0.39 above which no stably rotating single object exists, suggesting a collisional origin for this binary. The effects of viscosity on Pluto's rotational stability and on the density of Charon are considered. Both a more or less dense Charon would be consistent with a collisional origin if one (the least massive) or both protoobjects were differentiated. It is noted that the angular momentum of the system requires the protoobjects to be comparably (if not equally) sized if off-center impact velocities vary between escape (about 1.3 km/s) and somewhat greater values (about 2.5 km/s) appropriate to Pluto's eccentric and inclined solar orbit.