Pluto’s intriguing moons

Roaming around the complicated gravitational field of Pluto and its large moon, Charon, are four smaller satellites—Styx, Nix, Kerberos, and Hydra—all of which were discovered only within the past decade. Now, based on an analysis of Hubble Space Telescope images, Mark Showalter (SETI Institute) and Douglas Hamilton (University of Maryland) have deduced several novel properties of the Plutonian system. The two researchers found, for example, that the orbital periods of Styx, Nix, and Hydra—but not Kerberos—are in simple, whole-number ratios. An analysis of the four moons’ light curves (reflected light versus time) yielded other surprises. The data for Nix and Hydra are incompatible with rotation about a fixed axis. Instead, Showalter and Hamilton posit, the oddly shaped moons—and possibly Kerberos and Styx too—tumble chaotically, as illustrated in the accompanying simulated images of Nix, courtesy of Showalter and Greg Bacon (Space Telescope Science Institute). The light curves also indicate that Kerberos is much darker than its three companions—so much so that it appears to be made of different material. A unique composition would be a mystery because current theory holds that Pluto’s moons were formed when a large object crashed into the dwarf planet: Why should one ejected fragment have a special composition? In mid July, NASA’s New Horizons spacecraft cruised past Pluto. Its observations will test some of the assumptions Showalter and Hamilton used in their orbital and illumination modeling and should definitively establish whether Kerberos is the darkest of Pluto’s moons. (M. R. Showalter, D. P. Hamilton, Nature 522, 45, 2015, doi:10.1038/nature14469 .)