Research Article

The small satellites of Pluto as observed by New Horizons

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Science  18 Mar 2016:
Vol. 351, Issue 6279, aae0030
DOI: 10.1126/science.aae0030
  1. Pluto’s family of satellites.

    NASA’s New Horizons mission has resolved Pluto’s four small moons, shown in order of their orbital distance from Pluto (from left to right). Nix and Hydra have comparable sizes (with equivalent spherical diameters of ~40 km) and are much larger than Styx and Kerberos (both of which have equivalent spherical diameters of ~10 km). All four of these moons are highly elongated and are dwarfed in size by Charon, which is nearly spherical with a diameter of 1210 km. The scale bars apply to all images.

  2. Fig. 1 Best-resolved images of Pluto’s four small moons.

    Celestial north is up; east is to the left. The Styx image is a deconvolved (12) composite of six images from U_TBD_1_02 (Table 1) that has been resampled with pixels one-eighth of the native pixel scale for cosmetic purposes. The Nix image is a deconvolved single image from N_LEISA_LORRI_BEST and is displayed with the native pixels. The Kerberos image is a deconvolved composite of four images from U_TBD_2 and has been resampled with pixels one-eighth of the native pixel scale for cosmetic purposes (12) (fig. S1). The Hydra image is a deconvolved composite of two images from H_LORRI_BEST with pixels one-half of the native scale. Some surface features on Nix and Hydra appear to be impact craters (12).

  3. Fig. 2 Rotational light curves for Pluto’s small moons.

    Systematic measurements of the brightnesses of Pluto’s small satellites were obtained by LORRI during the approach to Pluto from May through early July 2015. Hv refers to the total (i.e., integrated over the entire target) visible magnitude (V band) referenced to a heliocentric distance of 1 AU, a spacecraft-to-target distance of 1 AU, and a solar phase angle of 0° (using a phase law of 0.04 mag/deg). Different colors are used for the seven different observing epochs (12) (table S4); ±1σ error bars are shown for each measurement (some error bars are smaller than the symbols). Three different algorithms were used to search for periodic variations in the data (12). The rotational periods derived from that analysis (Table 2) were then used to phase the brightness data, producing the light curves displayed above. These double-peaked light curves presumably result from the rotation of elongated bodies, with the light-curve amplitude determined by the variation in the cross-sectional area presented to the observer, which depends on the body’s shape and the angle between the rotational pole and the line of sight to the body. The rotational phases for all the resolved observations of the small satellites (Table 1) are indicated by the vertical red lines, although the angle between the observer and the rotational pole may be different for these observations relative to the earlier ones. The dashed curves are sinusoids with the best-matched periods. The amplitudes for Styx, Nix, Kerberos, and Hydra, respectively, are 0.30, 0.20, 0.37, and 0.07 mag. The dashed horizontal lines are the mean Hv values, which are 11.75, 8.28, 11.15, and 7.77 mag for Styx, Nix, Kerberos, and Hydra, respectively.

  4. Fig. 3 Cumulative crater size-frequency distributions for Nix, Hydra, Pluto’s encounter hemisphere (EH), and Charon’s Vulcan Planum (VP).

    The curves for Pluto and Charon are from Moore et al. (21). Nix and Hydra crater sizes (table S2) are scaled downward by a factor of 2.1 (appropriate for porous regolith-type material) to account for the difference in gravity between these small moons and Pluto (12). Standard Poisson statistical errors (Embedded Image) are displayed. The phase angle for the “Nix (LORRI)” data (N_LEISA_LORRI_BEST; phase angle 9.45°) was less ideal for topographic feature identification than the phase angle for the “Nix (MVIC)” observation (N_MPAN_CA; phase angle 85.9°). Thus, the lower crater density for Nix (LORRI) versus Nix (MVIC) may be an artifact of the viewing and lighting geometry. The yellow line indicates the Greenstreet et al. (22) prediction for the cumulative density of craters on Pluto’s surface over a span of 4 billion years for their “knee” model. Although not saturated in appearance, Nix and Hydra both exhibit slightly higher crater densities than Pluto and Charon, implying a surface age of at least 4 billion years (see text).

  5. Fig. 4 Color ratios for the surfaces of Pluto, Charon, Nix, and Hydra.

    Blue/Red and Red/NIR color ratios derived from MVIC images are displayed (Blue = 400 to 550 nm, Red = 540 to 700 nm, NIR = 780 to 975 nm). Gold points are from Pluto’s surface; silver points are from Charon’s surface. Blue contours show the distribution of colors on Nix’s surface; black contours show the distribution of colors on Hydra’s surface. The normalized solar color is denoted by the star (at coordinate [1,1] in the plot); surfaces redder than solar are at the lower left of the star, and regions bluer than solar appear at the upper right. Pluto exhibits a diversity of colors over its surface (24). Charon has less color diversity than Pluto, and the range of its colors follows a mixing line (24). Nix and Hydra have nearly solar colors (i.e., gray color) that are distinct from either Pluto or Charon, with Hydra being slightly bluer than Nix.

  6. Fig. 5 Color of Nix’s surface.

    (A) Panchromatic LORRI image of Nix taken from N_LEISA_LORRI_BEST (Table 1). (B) Enhanced MVIC color image of Nix taken from N_COLOR_BEST. (C) The LORRI image of Nix was colored using the data derived from the MVIC image. Most of Nix’s surface is neutral (i.e., gray) in color, but the region near the largest impact crater is slightly redder than the rest of the surface. Celestial north is up and east is to the left for all images.