Research Article

Color, composition, and thermal environment of Kuiper Belt object (486958) Arrokoth

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Science  13 Feb 2020:
DOI: 10.1126/science.aay3705
  • Fig. 1 The CA05 color observation of (486958) Arrokoth.

    (A) LORRI panchromatic context image obtained as a rider during the CA05 observation, so the geometry is nearly identical to the MVIC observation, but with a finer spatial scale of 83 m pixel–1. Abbreviations and informal feature nicknames mentioned in the text are (clockwise from the top): SL smaller lobe, MD Maryland, LA Louisiana, dr depressed region, LL larger lobe, bm bright material, ND North Dakota, and sp spots. These features can be seen more clearly in higher resolution LORRI images [(3), their figure 1]. (B) Color observation CA05, at a spatial scale of 340 m pixel–1. The BLUE filter (400-550 nm) is displayed in blue, RED filter (540-700 nm) in green, and NIR filter (780-975 nm) in red. (C) Color slope map obtained by fitting a linear model to the BLUE, RED, and NIR reflectance values.

  • Fig. 2 Principal components analysis of the CA05 color data.

    (A to D) Principal components images (PCs) 1 through 4 above, along with the corresponding eigenvector below. The images show the regional distribution of the spectral characteristic indicated by the eigenvector. For example, the PC2 eigenvector shows a red slope (B). The corresponding image shows high values where Arrokoth is reddest and low values where it is least red. The y axes of the eigenvector plots give the contribution of each filter to that PC. (E) False color image highlighting color contrasts across Arrokoth with PC2, PC3, and PC4 displayed in the red, green, and blue channels, respectively, with shading from the PC1 image.

  • Fig. 3 Comparison of Arrokoth’s color with other populations.

    (A) Histogram of Arrokoth color slopes shown in Fig. 1C. Typical color ranges for other Solar System small body populations are indicated via horizontal bars, e.g., (8, 18, 2629). (B) Color-color plot using Sloan g, r, and z filters, with the additional wavelength providing sensitivity to spectral curvature in addition to slope. In this color-color space, KBOs form three distinct zones (8). The gray tracks are populated by KBOs on dynamically excited (higher inclination and/or eccentricity) orbits while CCKBOs occupy the red zone. Arrokoth lies above the center of the CCKBO clump, because its red slope declines less into the z-band than the average for CCKBOs. The yellow Sun symbol indicates solar color, corresponding to neutral spectral reflectance at visible wavelengths.

  • Fig. 4 The CA04 infrared spectral observation of Arrokoth.

    (A) The LORRI CA04 rider image for context, with a spatial scale of 138 m pixel–1. (B) LEISA image with regions of interest (ROIs) indicated, with a much coarser mean spatial scale of 1.9 km pixel–1. (C) Average and ROI spectra (points) compared with the Hapke model fitted to the average spectrum (black curves). Vertical gray lines indicate the wavelengths of the two strongest CH3OH ice absorptions. Purple arrows mark other possible features discussed in the text. I/F is defined as the ratio of the bidirectional reflectance to that of a perfectly diffusely scattering surface illuminated normally (31).

  • Fig. 5 Principal components analysis of the CA04 infrared spectral imaging data.

    Same as Fig. 2, but for the LEISA observation in Fig. 4. Edge pixels have been removed; the white outlines show the full extent of Arrokoth.

  • Fig. 6 Microwave radiometry of Arrokoth.

    (A) 7.1 GHz microwave sky background based on an all sky radio map (35) sampled at REX’s 4.2 cm wavelength and smoothed to REX’s 1.2° beam width (36), indicating the locations of the CA03 and CA08 observations. Reproduced from (36) with permission. (B) Observed flux during the CA08 scan in green, with the later background scan in blue. Shaded areas were used to calibrate the two observations for background subtraction. The black curve is a model response for a TB = 30 K source with the 414 km2 projected area of Arrokoth. (C) CA07 LORRI image (3) obtained 10 minutes before the mid-time of the REX scan, at nearly identical lighting geometry but about a 10° shift in viewing geometry, showing more of the lit crescent than was visible at the time of the CA08 REX observation.

  • Fig. 7 Models of insolation and temperature across the surface of Arrokoth.

    (A) The Insolation averaged over Arrokoth’s orbit viewed from two different orientations. (B) Re-radiation received from other portions of the shape on the same scale and from the same orientations as (A), also averaged over the orbit. The top color bar applies to (A) and (B). (C) Seasonally averaged additional warming resulting from re-radiation is shown as a temperature increase above that which would have been computed in the absence of self-radiation. (D) Summer day side temperature distribution as seen from New Horizons during LORRI observation CA06 (3). (E) Winter night side temperature distribution as seen from New Horizons during the REX CA08 scan. In all panels, the x, y, and z axes correspond to the principal axes of inertia with the origin at the center of mass (13).

Supplementary Materials

  • Color, composition, and thermal environment of Kuiper Belt object (486958) Arrokoth

    W. M. Grundy, M. K. Bird, D. T. Britt, J. C. Cook, D. P. Cruikshank, C. J. A. Howett, S. Krijt, I. R. Linscott, C. B. Olkin, A. H. Parker, S. Protopapa, M. Ruaud, O. M. Umurhan, L. A. Young, C. M. Dalle Ore, J. J. Kavelaars, J. T. Keane, Y. J. Pendleton, S. B. Porter, F. Scipioni, J. R. Spencer, S. A. Stern, A. J. Verbiscer, H. A. Weaver, R. P. Binzel, M. W. Buie, B. J. Buratti, A. Cheng, A. M. Earle, H. A. Elliott, L. Gabasova, G. R. Gladstone, M. E. Hill, M. Horanyi, D. E. Jennings, A. W. Lunsford, D. J. McComas, W. B. McKinnon, R. L. McNutt Jr., J. M. Moore, J. W. Parker, E. Quirico, D. C. Reuter, P. M. Schenk, B. Schmitt, M. R. Showalter, K. N. Singer, G. E. Weigle II, A. M. Zangari

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Fig. S1
    • Caption for Data S1
    • References
    Data S1
    A low facet count version of the Arrokoth shape model (3) used in our themal models is provided as an ASCII file consisting of 1,039 lines representing vertices starting with the character v, followed by 1,962 lines representing facets starting with the character f. Each vertex line has three white-space separated floating point values representing x, y, z coordinates in units of km. The axes are aligned with the shape�s principal axes of inertia and the origin corresponds to the center of mass assuming a uniform internal density. Each facet line has three white-space separated integers specifying three vertices to define the corners of that triangular facet, in order such that the right hand rule points in the direction of the exterior of the shape. The index of the first vertex line is 1 and the last is 1,039.

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