Europa's Optical Aurora

Creators: de Kleer, Katherine; Brown, Michael E.

Abstract

Auroral emissions provide opportunities to study the tenuous atmospheres of solar system satellites, revealing the presence and abundance of molecular and atomic species as well as their spatial and temporal variability. Far-UV aurorae have been used for decades to study the atmospheres of the Galilean satellites. Here we present the first detection of Europa's visible-wavelength atomic oxygen aurora at 6300/6364 Å arising from the metastable O(^1D) state, observed with the Keck I and Hubble Space Telescope while Europa was in eclipse by Jupiter on six occasions in 2018 February–April. The disk-integrated O(^1D) brightness varies from <500 R up to more than 2 kR between dates, a factor of 15 higher than the O I 1356 Å brightness on average. The ratio of emission at 6300/5577 Å is diagnostic of the parent molecule; the 5577 Å emission was not detected in our data set, which favors O2 as the dominant atmospheric constituent and rules out an O/O_2 mixing ratio above 0.35. For an O_2 atmosphere and typical plasma conditions at Europa's orbit, the measured surface brightness range corresponds to column densities of (1–9) × 10^(14) cm^(−2).

Additional Information

© 2018 The American Astronomical Society. Received 2018 June 22; revised 2018 August 1; accepted 2018 August 7; published 2018 September 27. The authors are grateful to Samantha Trumbo for assistance in obtaining the Keck data. K.dK. is supported by a Heising-Simons Foundation 51 Pegasi b postdoctoral fellowship. Support for this work was also provided by NASA through grant No. HST-GO-15425.002-A from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. This work was based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. Some of 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. This work made use of the JPL Solar System Dynamics high-precision ephemerides through the HORIZONS system.

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Published - de_Kleer_2018_AJ_156_167.pdf

Submitted - 1809.04617.pdf

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