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Structure and composition of Pluto's atmosphere from the New Horizons solar ultraviolet occultation

Young, Leslie A. and Gao, Peter and Wong, Michael L. and Yung, Yuk L. (2018) Structure and composition of Pluto's atmosphere from the New Horizons solar ultraviolet occultation. Icarus, 300 . pp. 174-199. ISSN 0019-1035. doi:10.1016/j.icarus.2017.09.006.

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The Alice instrument on NASA's New Horizons spacecraft observed an ultraviolet solar occultation by Pluto's atmosphere on 2015 July 14. The transmission vs. altitude was sensitive to the presence of N_2, CH_4, C_2H_2, C_2H_4, C_2H_6, and haze. We derived line-of-sight abundances and local number densities for the 5 molecular species, and line-of-sight optical depth and extinction coefficients for the haze. We found the following major conclusions: (1) We confirmed temperatures in Pluto's upper atmosphere that were colder than expected before the New Horizons flyby, with upper atmospheric temperatures near 65–68 K. The inferred enhanced Jeans escape rates were (3–7) × 10^(22) N_2 s^(−1) and (4–8) × 10^(25) CH_4 s^(−1) at the exobase (at a radius of ∼ 2900 km, or an altitude of ∼1710 km). (2) We measured CH_4 abundances from 80 to 1200 km above the surface. A joint analysis of the Alice CH_4 and Alice and REX N_2 measurements implied a very stable lower atmosphere with a small eddy diffusion coefficient, most likely between 550 and 4000 cm^2 s^(−1). Such a small eddy diffusion coefficient placed the homopause within 12 km of the surface, giving Pluto a small planetary boundary layer. The inferred CH_4 surface mixing ratio was ∼ 0.28–0.35%. (3) The abundance profiles of the “C_2H_x hydrocarbons” (C_2H_2, C_2H_4, C_2H_6) were not simply exponential with altitude. We detected local maxima in line-of-sight abundance near 410 km altitude for C_2H_4, near 320 km for C_2H_2, and an inflection point or the suggestion of a local maximum at 260 km for C_2H_6. We also detected local minima near 200 km altitude for C_2H_4, near 170 km for C_2H_2, and an inflection point or minimum near 170–200 km for C_2H_6. These compared favorably with models for hydrocarbon production near 300–400 km and haze condensation near 200 km, especially for C_2H_2 and C_2H_4 (Wong et al., 2017). (4) We found haze that had an extinction coefficient approximately proportional to N_2 density.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Young, Leslie A.0000-0002-7547-3967
Gao, Peter0000-0002-8518-9601
Yung, Yuk L.0000-0002-4263-2562
Additional Information:© 2017 Elsevier Inc. Received 5 April 2017, Revised 28 August 2017, Accepted 5 September 2017, Available online 8 September 2017. This work was supported, in part, by funding from NASA's New Horizons mission to the Pluto system. The New Horizons Mission Design and Navigation teams enabled us to watch this glorious sunset and sunrise. Werner Curdt provided the high spectral-resolution solar models. We gratefully acknowledge the publicly available solar data and spectroscopic data: LISIRD Lyman-alpha data from, GOES15 soft X-ray flux from; and the Titan spectroscopic database at Thanks go out to Julie Moses for providing an electronic version of the C_2H_2 cross sections from Wu et al. (2001).
Group:Astronomy Department
Funding AgencyGrant Number
Subject Keywords:Pluto, atmosphere; Atmospheres, structure; Occultations; Ultraviolet observations; Triton, atmosphere; KBO atmospheres
Record Number:CaltechAUTHORS:20171127-075151635
Persistent URL:
Official Citation:Leslie A. Young, Joshua A. Kammer, Andrew J. Steffl, G. Randall Gladstone, Michael E. Summers, Darrell F. Strobel, David P. Hinson, S. Alan Stern, Harold A. Weaver, Catherine B. Olkin, Kimberly Ennico, David J. McComas, Andrew F. Cheng, Peter Gao, Panayotis Lavvas, Ivan R. Linscott, Michael L. Wong, Yuk L. Yung, Nathanial Cunningham, Michael Davis, Joel Wm. Parker, Eric Schindhelm, Oswald H.W. Siegmund, John Stone, Kurt Retherford, Maarten Versteeg, Structure and composition of Pluto's atmosphere from the New Horizons solar ultraviolet occultation, In Icarus, Volume 300, 2018, Pages 174-199, ISSN 0019-1035, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:83435
Deposited By: Tony Diaz
Deposited On:27 Nov 2017 17:12
Last Modified:15 Nov 2021 19:58

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