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• First detailed analysis of Pluto's H 2 O-ice rich sites. • Evidence for heavy hydrocarbons (i.e., C 3 H 8). • Test for CH 3 OH using new optical constants. On July 14, 2015, the New Horizons spacecraft made its closest approach to Pluto at about 12,000 km from its surface (Stern et al., 2015). Using the LEISA (Linear Etalon Imaging Spectral Array) near-IR imaging spectrometer we obtained two scans across the encounter hemisphere of Pluto at 6–7 km/pixel resolution. By correlating each spectrum with a crystalline H 2 O-ice model, we find several sites on Pluto's surface that exhibit the 1.5, 1.65 and 2.0 µm absorption bands characteristic of H 2 O-ice in the crystalline phase. These sites tend to be isolated and small (≲ 5000 km2 per site). We note a distinct near-IR blue slope over the LEISA wavelength range and asymmetries in the shape of the 2.0 µm H 2 O-ice band in spectra with weak CH 4 -ice bands and strong H 2 O-ice bands. These characteristics are indicative of fine-grain (grain diameters < wavelength or ∼ 1 µm) H 2 O-ice, like that seen in the spectra of Saturnian rings and satellites. However, the best-fit Hapke models require small mass fractions (≲10−3) of fine-grained H 2 O-ice that we can exchange for other refractory materials in the models with little change in χ 2, which may mean that the observed blue slope is possibly not due to a fine-grained material but an unidentified material with a similar spectral characteristic. We use these spectra to test for the presence of amorphous H 2 O-ice and estimate crystalline-to-amorphous H 2 O-ice fractions between 30 and 100%, depending on the location. We also see evidence for heavy hydrocarbons via strong absorption at λ > 2.3 µ m. Such heavy hydrocarbons are much less volatile than N 2 , CH 4 , and CO at Pluto temperatures. We test for CH 3 OH, C 2 H 6 , C 2 H 4 , and C 3 H 8 -ices because they have known optical constants and these ices are likely to arise from UV and energetic particle bombardment of the N 2 , CH 4 , CO-rich surface and atmosphere. Finally, we attempt to estimate the surface temperature using optical constants of pure CH 4 , and H 2 O-ice and best-fit Hapke models. Our standard model gives temperature estimates between 40 and 90 K, while our models including amorphous H 2 O-ice give lower temperature estimates between 30 and 65 K. [ABSTRACT FROM AUTHOR]

Highlights • First spectral analysis of Nix, Hydra and Kerberos. • Crystalline water ice found on all three. • 2.21 µm band seen on Nix and Hydra indicating an ammoniated species. • Disk resolved spectroscopy of Nix. • Temperature and crystalline H2O-ice fraction estimated for Nix and Hydra. Abstract On July 14, 2015, NASA’s New Horizons spacecraft encountered the Pluto-system. Using the near-infrared spectral imager, New Horizons obtained the first spectra of Nix, Hydra, and Kerberos and detected the 1.5 and 2.0 µm bands of H 2 O-ice on all three satellites. On Nix and Hydra, New Horizons also detected bands at 1.65 and 2.21 µm that indicate crystalline H 2 O-ice and an ammoniated species, respectively. A similar band linked to NH 3 -hydrate has been detected on Charon previously. However, we do not detect the 1.99 µm band of NH 3 -hydrate. We consider NH 4 Cl (ammonium chloride), NH 4 NO 3 (ammonium nitrate) and (NH 4) 2 CO 3 (ammonium carbonate) as potential candidates, but lack sufficient laboratory measurements of these and other ammoniated species to make a definitive conclusion. We use the observations of Nix and Hydra to estimate the surface temperature and crystalline H 2 O-ice fraction. We find surface temperatures < 20 K (<70 K with 1- σ error) and 23 K (< 150 K with 1- σ error) for Nix and Hydra, respectively. We find crystalline H 2 O-ice fractions of 78 − 22 + 12 % and > 30% for Nix an Hydra, respectively. New Horizons observed Nix and Hydra twice, about 2–3 hours apart, or 5 and 25% of their respective rotation periods. We find no evidence for rotational differences in the disk-averaged spectra between the two observations of Nix or Hydra. We perform a pixel-by-pixel analysis of Nix’s disk-resolved spectra and find that the surface is consistent with a uniform crystalline H 2 O-ice fraction, and a ∼ 50% variation in the normalized band area of the 2.21 µm band with a minimum associated with the red blotch seen in color images of Nix. Finally, we find evidence for bands on Nix and Hydra at 2.42 and possibly 2.45 µm, which we cannot identify, and, if real, do not appear to be associated with the ammoniated species. We do not detect other ices, such as CO 2 , CH 3 OH and HCN. [ABSTRACT FROM AUTHOR]