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Photometric Properties of Pluto's Main Surface Units

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Photometric Properties of Pluto's Main Surface Units Pluto System After New Horizons 2019 (LPI Contrib. No. 2133) 7054.pdf PHOTOMETRIC PROPERTIES OF PLUTO’S MAIN SURFACE UNITS. S. Protopapa1, C. Olkin1, W. Grundy2, J.Y. Li3, A. Verbiscer4, D.P. Cruikshank5, C. J.A. Howett1, A. Stern1, H.A. Weaver6, L.A. Young1, the New Horizons Science Team 1Southwest Research Institute, Boulder, CO 80302, USA, 2Lowell Observatory, Flagstaff, AZ 86001, USA; 3Planetary Science Institute, Tucson, AZ 85719, USA; 4University of Virginia, Char- lottesville, VA 22904, USA; 5NASA Ames Research Center, Moffett Field, CA 94035, USA; 6Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA, ([email protected]) Introduction: The chemistry of Pluto’s atmos- high degree of surface variations on Pluto [4,5], this phere and surface has become a key factor in under- approximation is likely incorrect. standing the origin and evolution of this icy dwarf We report here a detailed study of disk-resolved planet, and by extension that of a vast number of photometric properties of Pluto using NH similar sized and smaller bodies in the Kuiper Belt, Ralph/MVIC images in the visible wavelength range beyond the terrestrial and giant planets. 400-910 nm acquired during the Pluto’s flyby [7]. A pixel-by-pixel Hapke radiative transfer model The derivation of a regionally-based photometric has been developed by Protopapa et al. [1] and ap- model permits us to (1) decouple the intrinsic surface plied to two resolved scans of Pluto’s encounter hemi- albedo variability from effects related to the observ- sphere acquired by the New Horizons (NH) infrared ing geometry, and therefore investigate quantitatively imaging spectrometer, LEISA [2]. This model yields the true heterogeneity of Pluto’s surface; (2) combine compositional maps defining the spatial distribution visible (MVIC) and near-IR observations (LEISA) of the abundance and textural properties of the mate- accounting for the different viewing geometries at rials present on the surface of Pluto. This has been by which these data were acquired; (3) model visible and far the most successful approach to date in terms of near-infrared measurements of Pluto to derive quanti- providing quantitative information about the composi- tative information of its surface composition. tion of Pluto. However, this analysis has some limita- Methodology: Photometric models describe the tions, namely: dependence of the radiance factor (RADF, also com- 1) Important compositional information is hidden in monly referred to as I/F) on scattering geometry, the visible wavelength range where low albedo organ- which is defined by the incident angle (i), emergent ic compounds known as tholins present the most di- angle (e) and phase angle (g). We employ the Hapke agnostic spectral signatures [3]. High quality filter radiative transfer model [8] to assess the photometric band imagery in this range exist from the NH properties of Pluto using a single-lobe Henyey- Ralph/MVIC instrument [2], but have yet not been Greenstein phase function [9]. studied with this technique. One of the main findings of the Protopapa et al. study [1] is the correlation be- tween Pluto’s coloration and the abundance of tholins used in the best-fitting models. As an example, the highest concentration of these dark compounds is found in Cthulhu Macula, the lowest albedo and red- dest unit of Pluto’s observed surface [4,5]. On the other hand, Lowell Regio, which displays a golden coloration, was found to be highly depleted in tholins. This result is a clear example of how the true contri- bution of the coloring agents cannot be assessed if the visible spectral domain is disregarded. 2) The estimates of the concentration and particle size Fig. 1: Left panel: “Enhanced” color image of Pluto of each surface compound strongly rely on the choice obtained with MVIC’s BLUE, RED, and NIR filter of Pluto’s photometric properties (Hapke parameters images displayed in blue, green, and red color chan- as the cosine asymmetry factor x, compaction param- nels, respectively. Right panel: The same as on the eter h, amplitude of the opposition effect B0, and left but over-plotted in magenta are the points select- mean roughness slope q). These properties have pre- ed as representative of the Cthulhu Macula region. viously been treated as global quantities, constant across all of Pluto’s terrains [6]. However, given the As a preliminary step in the analysis, we re- project each MVIC color frame to a common perspec- tive view. This enables us to select the same region of Pluto System After New Horizons 2019 (LPI Contrib. No. 2133) 7054.pdf interest across Pluto’s scans acquired at different limited to solve for the mean roughness slope θ. With viewing geometries. Figure 1 shows, as an example, respect to the average single scattering albedo of the region selected as representative of the Cthulhu Pluto, which is on the order of 0.6 (Verbiscer, private Macula, while Fig. 2 shows the photometric data from comm.), Cthulhu Macula displays a systematically this region in the BLUE filter (which has a central lower value of 0.15 at 492 nm. This is consistent with wavelength of 492 nm). This set of measurements a high abundance of tholins in this region. represents an example of data set to be fit with pho- tometric models. Fig. 3: The joint error distribution in the BLUE filter of (w,|x|) and (w,q) on the left and right panel, respectively for Cthulhu Macula. The 1-, 2-, and 3-s contours are shown. The solutions for the Hapke pa- rameters and the correspondent 3-s errors are dis- played in the panels. Summary: We present a multi-wavelength, re- gionally dependent photometric analysis of Pluto’s surface similar to the one shown here for Cthulhu Macula. We will perform a comparative analysis and use these properties to quantitively infer the composi- tion of Pluto’s different terrains and investigate the different coloring agents across Pluto’s surface and discuss this at the meeting. Fig. 2: Photometric data in the BLUE filter (cen- tral wavelength of 492 nm). The three panels show References: [1] Protopapa S. et al. (2017) Icarus, the radiance factor I/F of the Cthulhu Macula region 287, 218–228. [2] Reuter D. C. et al. (2008) Space (see Fig. 1 right panel) plotted with respect to phase Science Reviews, 140, 129-154. [3] Cruikshank D. P. angle (upper), incidence angle (middle), and emission et al. (2005) Advances in Space Research, 36, 178– angle (lower). Different colors correspond to different 183. [4] Grundy W. M. et al. (2016) Science, 351, MVIC scans. aad9189. [5] Olkin C. B. et al. (2017) AJ, 154, 258. [6] Buie M. W. et al. (2010) The Astronomical Jour- Given the limited phase angle coverage, we con- nal, 139, 1117–11127. [7] Stern S. A. et al. (2015) sider the amplitude of the opposition effect as well as Science, 350, aad1815. [8] Hapke B. (2002) Icarus, the compaction parameters constant and equal to the 157, 523–534. [9] Henyey L. G. and Greenstein J. L. values obtained from disk-integrated analysis (Ver- (1941) Astrophysical Journal, 93, 70-83 biscer, private comm.). We solve instead for single Acknowledgments: This work was supported by scattering albedo (w), cosine asymmetry factor ξ, as- the New Horizons project. S. Protopapa gratefully suming backscattering, and mean roughness slope θ. thanks the NASA NFDAP Grant for funding that sup- We used a nonlinear least squares minimization pro- ported this work (grant # 80NSSC19K0821). gram to find the best fit solutions. These parameters, along with the final reduced chi-squared are shown in Fig. 3. We conclude that the phase angle range is too .
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