Research Article

Surface compositions across Pluto and Charon

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Science  18 Mar 2016:
Vol. 351, Issue 6279, aad9189
DOI: 10.1126/science.aad9189
  1. Enhanced color view of Pluto’s surface diversity

    This mosaic was created by merging Multispectral Visible Imaging Camera color imagery (650 m per pixel) with Long Range Reconnaissance Imager panchromatic imagery (230 m per pixel). At lower right, ancient, heavily cratered terrain is coated with dark, reddish tholins. At upper right, volatile ices filling the informally named Sputnik Planum have modified the surface, creating a chaos-like array of blocky mountains. Volatile ice occupies a few nearby deep craters, and in some areas the volatile ice is pocked with arrays of small sublimation pits. At left, and across the bottom of the scene, gray-white CH4 ice deposits modify tectonic ridges, the rims of craters, and north-facing slopes.

  2. Fig. 1 LEISA maps of Pluto’s volatile ices CH4, N2, and CO.

    For each species, the top panel shows the LEISA map, with brighter colors corresponding to greater absorption; the bottom panel shows the same data overlaid on a base map made from LORRI images reprojected to the geometry of the LEISA observation. (A) The CH4 absorption map shows the equivalent width of the 1.3- to 1.4-μm band complex. (B) The N2 absorption map is a ratio of the average over the band center (2.14 to 2.16 μm) to that of adjacent wavelengths (2.12 to 2.14 μm and 2.16 to 2.18 μm). (C) The CO absorption map is a ratio of the average over the band center (1.56 to 1.58 μm) to that of adjacent wavelengths ( 1.55 to 1.56 μm and 1.58 to 1.59 μm). Latitude and longitude grids at 30° intervals [shown in (C)] apply to all maps.

  3. Fig. 2 LEISA map of Pluto’s nonvolatile H2O ice.

    Left: Map showing the correlation coefficient between each LEISA spectrum and a template Charon-like H2O ice spectrum [e.g., (47, 49)], highlighting where H2O absorption is least contaminated by other spectral features. Right: LEISA map superposed on the reprojected LORRI base map.

  4. Fig. 3 LEISA spectra of Pluto.

    (A) Context map produced by averaging the red, green, and blue values from each of the colored maps across the bottom row in Fig. 1 and the right panel in Fig. 2. (B) Specific intensity (I/F) spectra averaged over regions in blue boxes, with envelopes indicating the standard deviations within the boxes. These regions were selected to highlight Pluto’s spectral diversity. Vertical offsets for spectra “a” through “f” are +2.3, +1.7, +1.5, +1.2, +0.45, and 0, respectively. Pluto’s north pole (“a”) shows strong absorptions by CH4 ice. Spectrum “b” is a region characterized by a strong N2 ice absorption at 2.15 μm and weak H2O ice bands at 1.5 and 2 μm. Spectrum “c” is al-Idrisi Montes, very similar to “b” except without the N2 absorption. The area around Pulfrich crater (“d”) has H2O ice absorptions at 1.5, 1.65, and 2 μm and comparatively weak CH4 ice absorptions. Spectrum “e” is the center of Sputnik Planum, with strong CH4 bands, N2 ice absorption at 2.15 μm, and CO ice absorption at 1.58 μm. Spectrum “f” is eastern Cthulhu Regio, with weak H2O ice absorptions at 1.5 and 2 μm and a feature attributed to heavier hydrocarbons at 2.3 μm.

  5. Fig. 4 MVIC colors of Pluto.

    (A) “Enhanced” color with MVIC’s BLUE, RED, and NIR filter images displayed in blue, green, and red color channels, respectively. Geometry is indicated by the wire grid. (B) Distribution of NIR/RED and RED/BLUE color ratios, excluding regions where the incidence angle from the Sun or the emission angle to the spacecraft exceeds 70° from the zenith. The Sun symbol indicates neutral colors; redder colors extend up and to the right. (C to F) Principal component images and eigenvectors for PC1 to PC4, respectively. (G) False-color view with shading from PC1 and the hue set by PC2, PC3, and PC4 displayed in red, green, and blue channels, respectively.

  6. Fig. 5 Pluto MVIC CH4 absorption map.

    The equivalent width of absorption in the MVIC CH4 filter is computed by comparison with the NIR and RED filter images (see supplementary text for details). This filter is centered on a weaker CH4 ice absorption than the one mapped with LEISA data in Fig. 1. Brighter shades correspond to stronger CH4 ice absorption. Differences between the maps are discussed in the text. Except for a sliver of poorly illuminated terrain along the terminator, where geometric effects become extreme, most of the contrast in this map corresponds to regional variations in CH4 ice absorption.

  7. Fig. 6 LORRI albedos on Pluto.

    (A) Normal albedo across Pluto from LORRI panchromatic images sensitive to wavelengths from 350 to 850 nm (10). (B) Histogram of albedo values. (C) Decline of Pluto’s disk-integrated brightness with phase angle, compared with the same data for a Lambertian sphere, the icy satellite Triton, and Earth’s Moon. The Triton curve is based on Voyager green filter and ground-based observations with an effective wavelength of 550 nm (40). The lunar curve is for a Johnson V filter wavelength of 550 nm. The Pluto data points show LORRI brightness after correcting for light-curve variability, relative to zero-phase data from ground-based monitoring (43) (Bessel R filter, 630 nm).

  8. Fig. 7 MVIC colors of Charon.

    (A) Enhanced color, with MVIC’s BLUE, RED, and NIR filter images displayed in blue, green, and red color channels, respectively. Geometry is indicated by the associated wire grid. (B) MVIC NIR/BLUE and NIR/RED color ratio means and standard deviations averaged over 5° latitude annuli, excluding points near the limb with emission angles greater than 75°. (C) Scatterplot of NIR/RED and RED/BLUE color ratios, excluding incidence and emission angles greater than 70°. Most pixels cluster near solar colors, with a mixing line extending toward redder colors at upper right. (D to G) Principal component images and eigenvectors for principal components 1, 2, 3, and 4, respectively.

  9. Fig. 8 LEISA spectra of Charon.

    (A) LORRI composite base map. Regions where I/F spectra were averaged for plotting in (B) are indicated by blue boxes. (B) Vertical offsets for spectra “a” through “d” are +0.7, +0.4, +0.2, and 0, respectively. All four spectra show the characteristic absorption bands of cold, crystalline H2O ice at 1.5, 1.65, and 2 μm. Charon’s north pole (“a”) shows a little more continuum absorption toward short wavelengths, but no other obvious differences, relative to spectra from lower latitudes on Charon. Spectrum “b” is a region around Organa crater showing NH3 absorption at 2.22 μm. (C) Close-up of the region indicated by the green box in (A), with 2.22-μm absorption mapped in green to show the spatial distribution of NH3-rich material (fig. S6 shows the full map). Spectra “c” and “d” compare plains units above and below the tectonic belt.

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