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Constraints on Pluto’s H and CH₄ profiles from New Horizons Alice Lyα observations

Gladstone, G. Randall and Kammer, Joshua A. and Adams, Danica J. and Yung, Yuk L. and Pryor, Wayne R. and Strobel, Darrell F. and Young, Leslie A. and Parker, Joel Wm. and Stern, S. Alan (2021) Constraints on Pluto’s H and CH₄ profiles from New Horizons Alice Lyα observations. Icarus, 356 . Art. No. 113973. ISSN 0019-1035. doi:10.1016/j.icarus.2020.113973.

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The Alice spectrograph on New Horizons performed several far-ultraviolet (FUV) airglow observations during the July 2015 flyby of Pluto. One of these observations, named PColor2, was a short (226 s) scan across the dayside disk of Pluto from a range of ∼34,000 km, at about 40 minutes prior to closest approach. The brightest observed FUV airglow signal at Pluto is the Lyman alpha (Lyα) emission line of atomic hydrogen, which arises primarily through the resonant scattering of solar Lyα by H atoms in the upper atmosphere, with a brightness of about 30 Rayleighs. Pluto appears dark against the much brighter (∼100 Rayleigh) sky background; this sky background is likewise the result of resonantly scattered solar Lyα, in this case by H atoms in the interplanetary medium (IPM). Here we use an updated photochemical model and a resonance line radiative transfer model to perform detailed simulations of the Lyα emissions observed in the Alice PColor2 scan. The photochemical models show that H and CH₄ abundances in Pluto’s upper atmosphere are a very strong function of the near-surface mixing ratio of CH₄, and could provide a useful way to remotely monitor seasonal climate variations in Pluto’s lower atmosphere. The morphology of the PColor2 Lyα emissions provides constraints on the current abundance profiles of H atoms and CH₄ molecules in Pluto’s atmosphere, and indicate that the globally averaged near-surface mixing ratio of CH₄ is currently close to 0.4%. This new result thus provides independent confirmation of one of the primary results from the solar occultation, also observed with the New Horizons Alice ultraviolet spectrograph.

Item Type:Article
Related URLs:
URLURL TypeDescription
Gladstone, G. Randall0000-0003-0060-072X
Kammer, Joshua A.0000-0002-3441-3757
Adams, Danica J.0000-0001-9897-9680
Yung, Yuk L.0000-0002-4263-2562
Pryor, Wayne R.0000-0001-8112-8783
Strobel, Darrell F.0000-0002-0944-8675
Young, Leslie A.0000-0002-7547-3967
Parker, Joel Wm.0000-0002-3672-0603
Additional Information:© 2020 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license. Received 4 April 2020, Revised 25 June 2020, Accepted 5 July 2020, Available online 11 July 2020. We thank Tanguy Bertrand and Francois Forget for useful discussions, and the reviewers for useful comments. This work was supported by NASA through contract NASW02008 to SwRI.
Funding AgencyGrant Number
Subject Keywords:Pluto; Atmosphere; Aeronomy; Ultraviolet observations
Record Number:CaltechAUTHORS:20200713-123919818
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Official Citation:G. Randall Gladstone, Joshua A. Kammer, Danica J. Adams, Yuk L. Yung, Wayne R. Pryor, Darrell F. Strobel, Leslie A. Young, Joel Wm. Parker, S. Alan Stern, Constraints on Pluto’s H and CH4 profiles from New Horizons Alice Lyα observations, Icarus, Volume 356, 2021, 113973, ISSN 0019-1035, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:104356
Deposited By: Tony Diaz
Deposited On:13 Jul 2020 20:06
Last Modified:16 Nov 2021 18:30

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