Free Astronomy Magazine January-February 2016

13 JANUARY-FEBRUARY 2016 PLANETOLOGY Pluto as a result of a very remote impact of the dwarf planet with a foreign object of significant size. Unlike Nix, the other three small satellites have a bi-lobed shape, suggesting that they may be con- glomerates of two or more pre-ex- isting smaller objects; a scenario that suggests the existence in an- cient times of a larger population of satellites. Unlike what happened for the ex- plorations of giant planets, this time the in-situ observation has not produced new discoveries of satellites, but of the ones already known it has however allowed to more accurately calculate their di- ameters (for all 4 by a few tens of km), orbits (travelled between 20 and 38 days periods) and axis-rota- tion periods (from a little more than 10 hours to over 5 days). These latter periods, as well as chaotic, are far from the typical synchronization with the period of revolution around Pluto, as a result of Charon’s de- stabilizing mass. Perhaps due to the simple fact that it was the one clo- sest to New Horizons during its flyby, the most in- teresting of the four small satel- lites appears to be Nix, which we can see on this page from different angles and at different resolutions. The composite image lower down clearly shows an impact crater that has given astronomers much to think about. In fact, its size is very large compared to that of the moon, such that the energy released by the event causing it must have been close to the limits that Nix could withstand. Researchers do not exclude that the small satellite may actually be a fragment of a larger object that was destroyed in ancient times and that this crater was actually formed on its progenitor. Also the crater’s colour appears in- consistent with the rest of the vis- ible surface. Since meteorite im- pacts dig deep into the ground and bring to light the subsurface mate- rial, if the impact had occurred di- rectly on Nix, the little moon would then have an inner composition distinctly different from that of its surface; a quite unusual scenario for such small objects, which do not generally have a differentiated structure. At the time of writing, more than 70% of the data recorded by New Horizons has yet to be download- ed, it is therefore reasonable to expect further surpris- ing discoveries over the coming months and we will not fail to comment on them. N ix, Pluto’s third satellite for size and distance from the planet, was the one more closely photographed by New Horizons. Here it can be seen from different angles and at different resolutions. [NASA/Johns Hopkins University Applied Physics Laborato- ry/Southwest Research Institute] n