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In 1977, while Apple II and Atari computers were being sold, a tiny dot was observed in an inconvenient orbit. The minor body 1977 UB, to be named (2060) Chiron, with an orbit between Saturn and Uranus, became the first Centaur, a new class of minor bodies orbiting roughly between Jupiter and Neptune. The observed overabundance of short-period comets lead to the downfall of the Oort Cloud as exclusive source of comets and to the rise of the need for a Trans-Neptunian comet belt. Centaurs were rapidly seen as the transition phase between Kuiper Belt Objects (KBOs), also known as Trans-Neptunian Objects (TNOs) and the Jupiter-Family Comets (JFCs). Since then, a lot more has been discovered about Centaurs: they can have cometary activity and outbursts, satellites, and even rings. Over the past four decades since the discovery of the first Centaur, rotation periods, surface colors, reflectivity spectra and albedos have been measured and analyzed. However, despite such a large number of studies and complementary techniques, the Centaur population remains a mystery as they are in so many ways different from the TNOs and even more so from the JFCs. Facultad de Ciencias Astronómicas y Geofísicas Instituto de Astrofísica de La Plata

International audience; 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 km 2 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 µm. 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.

We present high signal precision optical reflectance spectra of 2005 FY9 taken with the Red Channel Spectrograph and the 6.5-m MMT telescope on 2006 March 4 UT (5000 - 9500 A; 6.33 A pixel-1) and 2007 February 12 UT (6600 - 8500 A; 1.93 A pixel-1). From cross correlation experiments between the 2006 March 4 spectrum and a pure CH4-ice Hapke model, we find the CH4-ice bands in the MMT spectrum are blueshifted by 3 +/- 4 A relative to bands in the pure CH4-ice Hapke spectrum. The higher resolution MMT spectrum of 2007 February 12 UT enabled us to measure shifts of individual CH4-ice bands. We find the 7296 A, 7862 A, and 7993 A CH4-ice bands are blueshifted by 4 +/- 2 A, 4 +/- 4 A, and 6 +/- 5 A. From four measurements we report here and one of our previously published measurements, we find the CH4-ice bands are shifted by 4 +/- 1 A. This small shift is important because it suggest the presence of another ice component on the surface of 2005 FY9. Laboratory experiments show that CH4-ice bands in spectra of CH4 mixed with other ices are blueshifted relative to bands in spectra of pure CH4-ice. A likely candidate for the other component is N2-ice because its weak 2.15 micron band and blueshifted CH4 bands are seen in spectra of Triton and Pluto. Assuming the shift is due to the presence of N2, spectra taken on two consecutive nights show no difference in CH4/N2. In addition, we find no measureable difference in CH4/N2 at different depths into the surface of 2005 FY9., The paper will appear in Icarus. It has 33 pages, 2 tables, and 7 figures. Replaced version fixed typo in abstract