The surface compositions of Pluto and Charon

The surface of Pluto as it is understood on the eve of the encounter of the New Horizons spacecraft (mid-2015) consists of a spatially heterogeneous mix of solid N 2 , CH 4 , CO, C 2 H 6 , and an additional component that imparts color, and may not be an ice. The known molecular ices are detected by near-infrared spectroscopy. The N 2 ice occurs in the hexagonal crystalline β-phase, stable at T > 35.6 K. Spectroscopic evidence for wavelength shifts in the CH 4 bands attests to the complex mixing of CH 4 and N 2 in the solid state, in accordance with the phase diagram for N 2 + CH 4 . Spectra obtained at several aspects of Pluto’s surface as the planet rotates over its 6.4-day period show variability in the distribution of CH 4 and N 2 ices, with stronger CH 4 absorption bands associated with regions of higher albedo, in correlation with the visible rotational light curve. CO and N 2 ice absorptions are also strongly modulated by the rotation period; the bands are strongest on the anti-Charon hemisphere of Pluto. Longer term changes in the strengths of Pluto’s absorption bands occur as the viewing geometry changes on seasonal time-scales, although a complete cycle has not been observed. The non-ice component of Pluto’s surface may be a relatively refractory material produced by the UV and cosmic-ray irradiation of the surface ices and gases in the atmosphere, although UV does not generally penetrate the atmospheric CH 4 to interact with the surface. Laboratory simulations indicate that a rich chemistry ensues by the irradiation of mixtures of the ices known to occur on Pluto, but specific compounds have not yet been identified in spectra of the planet. Charon’s surface is characterized by spectral bands of crystalline H 2 O ice, and a band attributed to one or more hydrates of NH 3 . Amorphous H 2 O ice may also be present; the balance between the amorphization and crystallization processes on Charon remains to be clarified. The albedo of Charon and its generally spatially uniform neutral color indicate that a component, not yet identified, is mixed in some way with the H 2 O and NH 3 · n H 2 O ices. Among the many known small bodies in the transneptunian region, several share characteristics with Pluto and Charon, including the presence of CH 4 , N 2 , C 2 H 6 , H 2 O ices, as well as components that yield a wide variety of surface albedo and color. The New Horizons investigation of the Pluto–Charon system will generate new insight into the physical properties of the broader transneptunian population, and eventually to the corresponding bodies expected in the numerous planetary systems currently being discovered elsewhere in the Galaxy.