Icy Worlds

The Optical Aurorae of Europa, Ganymede and Callisto

By Keith Cowing
Status Report
February 17, 2023
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The Optical Aurorae of Europa, Ganymede and Callisto
Detections of 7774 and 8446 Å O I emission at Ganymede. For each emission, the lower panel shows a calibrated, background-subtracted image of the spectrum so it can be seen that the emission is localized along the slit (vertical axis), while the upper panel shows the spectrum summed across spatial bins containing the localized emission. The vertical ticks between the panels indicate the locations of the Doppler-shifted emission wavelengths for each transition. Each spectrum has been smoothed with a Gaussian kernel to enhance the features relative to the background. — astro-ph.EP

The tenuous atmospheres of the Galilean satellites are sourced from their surfaces and produced by a combination of plasma-surface interactions and thermal processes. Though thin, these atmospheres can be studied via their auroral emissions, and most work to date has focused on their aurora at UV wavelengths.

Here we present the first detections of Ganymede’s and Callisto’s optical aurorae, as well detections of new optical auroral lines at Europa, based on observations of the targets over ten Jupiter eclipses from 1998 to 2021 with Keck/HIRES. We present measurements of OI emission at 6300/6364, 5577, 7774, and 8446 A and place upper limits on hydrogen at 6563 A. These constitute the first detections of emissions at 7774 and 8446 A 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 \pm 0.1) x 10^{14} cm^{-2} and (4.7 \pm 0.1) x 10^{14} 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 \pm 0.9 x 10^{15} cm^{-2} for an assumed electron density of 0.15 cm^{-3}, but electron properties at Callisto’s orbit are very poorly constrained.

Katherine de Kleer, Zachariah Milby, Carl Schmidt, Maria Camarca, Michael E. Brown

Comments: Published in Planetary Science Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2302.08487 [astro-ph.EP] (or arXiv:2302.08487v1 [astro-ph.EP] for this version)
Journal reference: PSJ 4, 37 (2023)
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Submission history
From: Katherine de Kleer
[v1] Thu, 16 Feb 2023 18:47:46 UTC (3,825 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻