The distribution of H2O, CH3OH, and hydrocarbon-ices on Pluto

Analysis of New Horizons spectral images

Jason C. Cook, Cristina M. Dalle Ore, Silvia Protopapa, Richard P. Binzel, Dale P. Cruikshank, Alissa Earle, William M. Grundy, Kimberly Ennico, Carly Howett, Donald E. Jennings, Allen W. Lunsford, Catherine B. Olkin, Alex H. Parker, Sylvain Philippe, Dennis Reuter, Bernard Schmitt, Kelsi Singer, John A. Stansberry, S. Alan Stern, Anne Verbiscer & 8 others Harold A. Weaver, Leslie A. Young, Jennifer Hanley, Fatima Alketbi, Garrett L. Thompson, Logan A. Pearce, Gerrick E. Lindberg, Stephen C Tegler

Research output: Contribution to journalArticle

1 Citation (Scopus)

Abstract

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 < wavelength or ∼ 1 µm)H2O-ice, like that seen in the spectra of Saturnian rings and satellites. However, the best-fit Hapke models require small mass fractions (≲10−3)of fine-grained H2O-ice that we can exchange for other refractory materials in the models with little change in χ2, which may mean that the observed blue slope is possibly not due to a fine-grained material but an unidentified material with a similar spectral characteristic. We use these spectra to test for the presence of amorphous H2O-ice and estimate crystalline-to-amorphous H2O-ice fractions between 30 and 100%, depending on the location. We also see evidence for heavy hydrocarbons via strong absorption at λ > 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.

Original languageEnglish (US)
Pages (from-to)148-169
Number of pages22
JournalIcarus
Volume331
DOIs
StatePublished - Oct 1 2019

Fingerprint

Pluto (planet)
Pluto
horizon
ice
hydrocarbons
hydrocarbon
estimates
analysis
distribution
imaging spectrometers
energetic particles
hemispheres
encounters
surface temperature
bombardment
asymmetry
pixel
spacecraft
spectrometer
energetics

Keywords

  • Composition
  • Infrared observations
  • Pluto
  • Spectroscopy
  • Surface
  • Surfaces

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Cook, J. C., Dalle Ore, C. M., Protopapa, S., Binzel, R. P., Cruikshank, D. P., Earle, A., ... Tegler, S. C. (2019). The distribution of H2O, CH3OH, and hydrocarbon-ices on Pluto: Analysis of New Horizons spectral images. Icarus, 331, 148-169. https://doi.org/10.1016/j.icarus.2018.09.012

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

In: Icarus, Vol. 331, 01.10.2019, p. 148-169.

Research output: Contribution to journalArticle

Cook, JC, Dalle Ore, CM, Protopapa, S, Binzel, RP, Cruikshank, DP, Earle, A, Grundy, WM, Ennico, K, Howett, C, Jennings, DE, Lunsford, AW, Olkin, CB, Parker, AH, Philippe, S, Reuter, D, Schmitt, B, Singer, K, Stansberry, JA, Stern, SA, Verbiscer, A, Weaver, HA, Young, LA, Hanley, J, Alketbi, F, Thompson, GL, Pearce, LA, Lindberg, GE & Tegler, SC 2019, 'The distribution of H2O, CH3OH, and hydrocarbon-ices on Pluto: Analysis of New Horizons spectral images', Icarus, vol. 331, pp. 148-169. https://doi.org/10.1016/j.icarus.2018.09.012
Cook JC, Dalle Ore CM, Protopapa S, Binzel RP, Cruikshank DP, Earle A et al. The distribution of H2O, CH3OH, and hydrocarbon-ices on Pluto: Analysis of New Horizons spectral images. Icarus. 2019 Oct 1;331:148-169. https://doi.org/10.1016/j.icarus.2018.09.012
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. / The distribution of H2O, CH3OH, and hydrocarbon-ices on Pluto : Analysis of New Horizons spectral images. In: Icarus. 2019 ; Vol. 331. pp. 148-169.
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TY - JOUR

T1 - The distribution of H2O, CH3OH, and hydrocarbon-ices on Pluto

T2 - Analysis of New Horizons spectral images

AU - Cook, Jason C.

AU - Dalle Ore, Cristina M.

AU - Protopapa, Silvia

AU - Binzel, Richard P.

AU - Cruikshank, Dale P.

AU - Earle, Alissa

AU - Grundy, William M.

AU - Ennico, Kimberly

AU - Howett, Carly

AU - Jennings, Donald E.

AU - Lunsford, Allen W.

AU - Olkin, Catherine B.

AU - Parker, Alex H.

AU - Philippe, Sylvain

AU - Reuter, Dennis

AU - Schmitt, Bernard

AU - Singer, Kelsi

AU - Stansberry, John A.

AU - Stern, S. Alan

AU - Verbiscer, Anne

AU - Weaver, Harold A.

AU - Young, Leslie A.

AU - Hanley, Jennifer

AU - Alketbi, Fatima

AU - Thompson, Garrett L.

AU - Pearce, Logan A.

AU - Lindberg, Gerrick E.

AU - Tegler, Stephen C

PY - 2019/10/1

Y1 - 2019/10/1

N2 - 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 < wavelength or ∼ 1 µm)H2O-ice, like that seen in the spectra of Saturnian rings and satellites. However, the best-fit Hapke models require small mass fractions (≲10−3)of fine-grained H2O-ice that we can exchange for other refractory materials in the models with little change in χ2, which may mean that the observed blue slope is possibly not due to a fine-grained material but an unidentified material with a similar spectral characteristic. We use these spectra to test for the presence of amorphous H2O-ice and estimate crystalline-to-amorphous H2O-ice fractions between 30 and 100%, depending on the location. We also see evidence for heavy hydrocarbons via strong absorption at λ > 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.

AB - 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 < wavelength or ∼ 1 µm)H2O-ice, like that seen in the spectra of Saturnian rings and satellites. However, the best-fit Hapke models require small mass fractions (≲10−3)of fine-grained H2O-ice that we can exchange for other refractory materials in the models with little change in χ2, which may mean that the observed blue slope is possibly not due to a fine-grained material but an unidentified material with a similar spectral characteristic. We use these spectra to test for the presence of amorphous H2O-ice and estimate crystalline-to-amorphous H2O-ice fractions between 30 and 100%, depending on the location. We also see evidence for heavy hydrocarbons via strong absorption at λ > 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.

KW - Composition

KW - Infrared observations

KW - Pluto

KW - Spectroscopy

KW - Surface

KW - Surfaces

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