Free Astronomy Magazine January-February 2016

12 PLANETOLOGY frared, at 2.2 µm, and it is part of a scan of the surface of Charon that highlights its chemical/mineralogical composition. The two craters are similar in size, shape and depth, but while Skywalker appearance is consistent with the surrounding surface dominated by water ice, Organa does in- stead show an excess of ammonia ice, de- picted in green in the image. Why this difference between two structures that are so similar and close to each other? Researchers speculate that Organa may be a crater relatively recent and that the event that originated it has exposed the ammonia hidden underground; alterna- tively, the excess ammonia may have been brought by the impacting body. Leaving aside this specific case, from the study of impact craters visible on Charon and Pluto, the researchers have estimated that the smaller bodies of the Kuiper belt, with diameter less than 1 km, must be less abun- dant than shown by current models. This means that the larger bodies may not have been formed through the slow aggregation of smaller objects, but rather through a faster process, from spatially more concentrated primordial mate- rial. If this was the case, the next candi- date target of the New Horizons mission, the Kuiper Belt Object (KBO) 2014 MU69, whose diameter should be about 40-50 km, could prove very interesting, since it would allow to study a mine of primordial matter of the solar system, remained un- changed over the past 4 billion years. And what about the small satellites of Pluto? Even they have sprung some sur- prises. Styx, Nix, Kerberos and Hydra (in order of distance from Pluto) are very small compared to Charon, as shown in the comparison scale below, but even in their small way they showed unexpected features. All were probably formed (along with Charon) from material taken from O n the side, an animation of the orbits of Pluto’s system, with a represen- tation of the ro- tatory motions. Below, a scaled photo compari- son of Pluto’s sat- ellites. [NASA/ Johns Hopkins University Ap- plied Physics Lab- oratory/South- west Research Institute]