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Published June 2022 | Published + Submitted
Journal Article Open

Seasonal Variations of Chemical Species and Haze in Titan's Upper Atmosphere


Seasonal variation is significant in Titan's atmosphere owing to the large change of solar insolation resulting from Titan's 26.7° axial tilt relative to the plane of Saturn's orbit. Here we present an investigation of hydrocarbon and nitrile species in Titan's upper atmosphere at 400–1200 km, which includes the mesosphere and the lower thermosphere, over more than one-fourth of Titan's year (2006–2014, L_S = 318°–60°), using 18 stellar occultation observations obtained by Cassini/Ultraviolet Imaging Spectrograph. Vertical profiles of eight chemical species (CH₄, C₂H₂, C₂H₄, C₂H₆, C₄H₂, C₆H₆, HCN, HC₃N) and haze particles are retrieved from these observations using an instrument forward model, which considers the technical issue of pointing motion. The Markov Chain Monte Carlo algorithm is used to obtain the posterior probability distributions of parameters in the retrieval, which inherently tests the extent to which species profiles can be constrained. The results show that no change of the species profiles is noticeable before the equinox, while the decrease of atmospheric temperature and significant upwelling in the summer hemisphere are found five terrestrial years afterward. Altitude of the detached haze layer decreases toward the vernal equinox and then disappears, and no reappearance is identified within the time range of our data, which is consistent with observations from Cassini/Imaging Science Subsystem. This study provides observational constraints on the seasonal change of Titan's upper atmosphere and suggests further investigations of the atmospheric chemistry and dynamics therein.

Additional Information

© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2021 December 17; revised 2022 April 13; accepted 2022 April 14; published 2022 June 2. This research was supported in part by the Cassini/UVIS program via NASA grant JPL.1459109 to the California Institute of Technology and was partially supported by funding from NASA's Astrobiology Institute's proposal "Habitability of Hydrocarbon Worlds: Titan and Beyond" (PI R.M. Lopes). All the data and tools in this work are publicly available. Cassini/UVIS data are available on NASA PDS (pds.nasa.gov). The Python package emcee is available online (dfm.io/emcee/current). We thank Tommi T. Koskinen for sharing Cassini/UVIS results, Jun Cui for sharing Cassini/INMS results and comments, Sandrine Vinatier and Christophe Mathé for sharing Cassini/CIRS results, and Karen Willacy for sharing photochemical model results.

Attached Files

Published - Fan_2022_Planet._Sci._J._3_130.pdf

Submitted - 2204.07259.pdf


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August 22, 2023
October 24, 2023