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A New Understanding of the Europa Atmosphere and Limits on Geophysical Activity

Shemansky, D. E. and Yung, Y. L. and Liu, X. and Yoshii, J. and Hansen, C. J. and Hendrix, A. R. and Esposito, L. W. (2014) A New Understanding of the Europa Atmosphere and Limits on Geophysical Activity. Astrophysical Journal, 797 (2). Art. No. 84. ISSN 0004-637X.

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Deep extreme ultraviolet spectrograph exposures of the plasma sheet at the orbit of Europa, obtained in 2001 using the Cassini Ultraviolet Imaging Spectrograph experiment, have been analyzed to determine the state of the gas. The results are in basic agreement with earlier results, in particular with Voyager encounter measurements of electron density and temperature. Mass loading rates and lack of detectable neutrals in the plasma sheet, however, are in conflict with earlier determinations of atmospheric composition and density at Europa. A substantial fraction of the plasma species at the Europa orbit are long-lived sulfur ions originating at Io, with ~25% derived from Europa. During the outward radial diffusion process to the Europa orbit, heat deposition forces a significant rise in plasma electron temperature and latitudinal size accompanied with conversion to higher order ions, a clear indication that mass loading from Europa is very low. Analysis of far ultraviolet spectra from exposures on Europa leads to the conclusion that earlier reported atmospheric measurements have been misinterpreted. The results in the present work are also in conflict with a report that energetic neutral particles imaged by the Cassini ion and neutral camera experiment originate at the Europa orbit. An interpretation of persistent energetic proton pitch angle distributions near the Europa orbit as an effect of a significant population of neutral gas is also in conflict with the results of the present work. The general conclusion drawn here is that Europa is geophysically far less active than inferred in previous research, with mass loading of the plasma sheet ≤4.5 x 10^(25) atoms s^(-1) two orders of magnitude below earlier published calculations. Temporal variability in the region joining the Io and Europa orbits, based on the accumulated evidence, is forced by the response of the system to geophysical activity at Io. No evidence for the direct injection of H_2O into the Europa atmosphere or from Europa into the magnetosphere system, as has been observed at Enceladus in the Saturn system, is obtained in the present investigation.

Item Type:Article
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Shemansky, D. E.0000-0001-7168-871X
Yung, Y. L.0000-0002-4263-2562
Additional Information:© 2014 American Astronomical Society. Received 2014 August 28; accepted 2014 October 31; published 2014 December 2. The authors thank Professor Wing-Huen Ip for critical comments on the Galileo mission results at Jupiter. D.E.S., X.L., J.Y. and were partially supported by the Cassini UVIS program through contract to SET. Y.L.Y. was supported in part by the Cassini UVIS program via NASA grant JPL.1459109 to the California Institute of Technology. C.J.H. and A.R.H. were partially supported by the Cassini UVIS program through contract to PSI. L.W.E. was partially supported by the Cassini UVIS contract to LASP.
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Subject Keywords:atomic processes; molecular processes; planetary systems; plasmas
Issue or Number:2
Record Number:CaltechAUTHORS:20150115-103643143
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Official Citation:A New Understanding of the Europa Atmosphere and Limits on Geophysical Activity D. E. Shemansky et al. 2014 ApJ 797 84
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:53775
Deposited By: Ruth Sustaita
Deposited On:15 Jan 2015 20:07
Last Modified:25 Jun 2020 18:05

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