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The Optical Aurorae of Europa, Ganymede, and Callisto

de Kleer, Katherine and Milby, Zachariah and Schmidt, Carl and Camarca, Maria and Brown, Michael E. (2023) The Optical Aurorae of Europa, Ganymede, and Callisto. Planetary Science Journal, 4 (2). Art. No. 37. ISSN 2632-3338. doi:10.3847/psj/acb53c.

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The tenuous atmospheres of the Galilean satellites are sourced from their surfaces and produced by a combination of plasma-surface interactions and thermal processes. Even though they are thin, these atmospheres can be studied via their auroral emissions, and most work to date has focused on their aurorae at UV wavelengths. Here we present the first detections of the optical aurorae of Ganymede and Callisto, as well as detections of new optical auroral lines at Europa, based on observations of the targets over 10 Jupiter eclipses from 1998 to 2021 with Keck/HIRES. We present measurements of O i emission at 6300/6364, 5577, 7774, and 8446 Å and place upper limits on hydrogen at 6563 Å. These constitute the first detections of emissions at 7774 and 8446 Å at a planetary body other than Earth. The simultaneous measurement of multiple emission lines provides robust constraints on atmospheric composition. We find that the eclipse atmospheres of Europa and Ganymede are composed predominantly of O₂, with average column densities of (4.1 ± 0.1) × 101₂ cm⁻² and (4.7 ± 0.1) × 10₂ cm⁻², respectively. We find weak evidence for H2O in Europa's bulk atmosphere at an H₂O/O₂ ratio of ∼0.25, and place only an upper limit on H₂O in Ganymede's bulk atmosphere, corresponding to H₂O/O₂ < 0.6. The column density of O₂ derived for Callisto is (4.0 ± 0.9) × 10¹⁵ cm⁻² for an assumed electron density of 0.15 cm⁻³, but electron properties at Callisto's orbit are very poorly constrained.

Item Type:Article
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URLURL TypeDescription
de Kleer, Katherine0000-0002-9068-3428
Milby, Zachariah0000-0001-5683-0095
Schmidt, Carl0000-0002-6917-3458
Camarca, Maria0000-0003-3887-4080
Brown, Michael E.0000-0002-8255-0545
Additional Information:© 2023. 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. Support for this work was provided by NASA through grant to program HST-GO-15425 from the Space Telescope Science Institute, which is operated by the Associations of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. We also gratefully acknowledge support from the NASA Solar System Observations program via grants 80NSSC22K0954 and 80NSSC21K1138. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facility: Keck/HIRES - .
Group:Division of Geological and Planetary Sciences, Astronomy Department
Funding AgencyGrant Number
W. M. Keck FoundationUNSPECIFIED
Issue or Number:2
Record Number:CaltechAUTHORS:20230221-191753000.3
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:119325
Deposited By: George Porter
Deposited On:21 Feb 2023 23:46
Last Modified:21 Feb 2023 23:46

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