The distribution of H2O, CH3OH, and hydrocarbon-ices on Pluto: Analysis of New Horizons spectral images

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2019-10, Vol.331, p.148-169
• Main Authors: Cook, Jason C., Dalle Ore, Cristina M., Protopapa, Silvia, Binzel, Richard P., Cruikshank, Dale P., Earle, Alissa, Grundy, William M., Ennico, Kimberly, Howett, Carly, Jennings, Donald E., Lunsford, Allen W., Olkin, Catherine B., Parker, Alex H., Philippe, Sylvain, Reuter, Dennis, Schmitt, Bernard, Singer, Kelsi, Stansberry, John A., Stern, S. Alan, Verbiscer, Anne, Weaver, Harold A., Young, Leslie A., Hanley, Jennifer, Alketbi, Fatima, Thompson, Garrett L., Pearce, Logan A., Lindberg, Gerrick E., Tegler, Stephen C.
• Format: Article
• Language: English
• Subjects: Astrophysics; Composition; Earth and Planetary Astrophysics; Infrared observations; Pluto; Sciences of the Universe; Spectroscopy; Surface; Surfaces
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2018.09.012

•First detailed analysis of Pluto’s H2O-ice rich sites.•Evidence for heavy hydrocarbons (i.e., C3H8).•Test for CH3OH using new optical constants. On July 14, 2015, the New Horizons spacecraft made its closest approach to Pluto at about 12,000 km from its surface (Stern et al., 2015). Using the LEISA (Linear Etalon Imaging Spectral Array) near-IR imaging spectrometer we obtained two scans across the encounter hemisphere of Pluto at 6–7 km/pixel resolution. By correlating each spectrum with a crystalline H2O-ice model, we find several sites on Pluto’s surface that exhibit the 1.5, 1.65 and 2.0 µm absorption bands characteristic of H2O-ice in the crystalline phase. These sites tend to be isolated and small ( ≲  5000 km2 per site). We note a distinct near-IR blue slope over the LEISA wavelength range and asymmetries in the shape of the 2.0 µm H2O-ice band in spectra with weak CH4-ice bands and strong H2O-ice bands. These characteristics are indicative of fine-grain (grain diameters   2.3  µm. Such heavy hydrocarbons are much less volatile than N2, CH4, and CO at Pluto temperatures. We test for CH3OH, C2H6, C2H4, and C3H8-ices because they have known optical constants and these ices are likely to arise from UV and energetic particle bombardment of the N2, CH4, CO-rich surface and atmosphere. Finally, we attempt to estimate the surface temperature using optical constants of pure CH4, and H2O-ice and best-fit Hapke models. Our standard model gives temperature estimates between 40 and 90 K, while our models including amorphous H2O-ice give lower temperature estimates between 30 and 65 K.