Pluto System Surface Composition Results

This talk will present an overview of surface composition discoveries from New Horizons' exploration of the Pluto system. The emphasis will be on results that could only have been obtained thanks to the uniquely high spatial resolution provided by a spacecraft visit. The Ralph instrument is New Horizons' primary tool for investigating surface compositions in the Pluto system. Ralph consists of a near-infrared spectral imager sharing a 75 mm aperture telescope assembly with a color CCD camera system. The Linear Etalon Imaging Spectral Array (LEISA) component of Ralph provides spectral coverage from 1.25 - 2.5 µm, at a resolving power (λ/Δλ) of 240. Ices such as CH₄, N₂, CO, CO₂, C₂H₆, NH₃, and H₂O have uniquely diagnostic absorption bands in this wavelength region. The Multi-spectral Visible Imaging Camera (MVIC) has 7 CCD arrays of which 4 have interference filters affixed directly on the focal plane. The filters pass wavelengths ranging from 400 through 975 nm, sensitive to coloration by tholin-type materials as well as a weak CH4 ice absorption band at 890 nm. Both Ralph components are usually operated in a scanning mode, rotating the spacecraft about its Z axis to sweep Ralph's field of view across the scene such that each point in the scene is eventually imaged at each wavelength. The width of the scanned region is 0.9 degrees divided into 256 spatial pixels for LEISA and 5.7 degrees spanned by 5000 pixels for MVIC. Over the course of the summer 2015 flyby, numerous Ralph observations targeted the various bodies in the Pluto system. As of late 2015, transmission of the data to Earth continues, but already a number of spectacular data sets are available for analysis, including LEISA scans of Pluto at 6 to 7 km/pixel and of Charon at 3 km/pixel, as well as MVIC scans of Pluto at 700 m/pixel and of Charon at 5 km/pixel. This work was supported by the NASA New Horizons Project.