The State of the Plasma Sheet and Atmosphere at Europa

Thursday, 18 December 2014: 10:50 AM
Donald E Shemansky1, Yuk L Yung2, Xianming Liu3, Jean Yoshii3, Candice J Hansen4, Amanda Hendrix4 and Larry W Esposito5, (1)Space Environment Technologies, Pacific Palisades, CA, United States, (2)California Institute of Technology, Pasadena, CA, United States, (3)Space Environment Technologies, Hawthorne, CA, United States, (4)Planetary Science Institute Albuquerque, Albuquerque, NM, United States, (5)Univ of Colorado, Boulder, CO, United States
The Hall et al. (1995) report announcing the discovery of atomic oxygen FUV emission from Europa included a conclusion that the atmosphere was dominated by O2. Over the following 20 years publications referencing the atmosphere accepted this conclusion, and calculations of rates, particularly mass loading of the magnetosphere depended on a composition that was of order 90% O2. Analysis of the Europa emission spectrum in the present work, leads to the conclusion that the O I emission properties were misinterpreted. The interpretation of the source process depends on the ratio of the O I 1356 and 1304 A multiplet emissions (R(4:5) = (I(1356)/I(1304)). The value of R(4:5) never reaches the lower limit for electron impact dissociation of O2 for any of the 7 recorded disk averaged measurements between 1994 and 2013. Analysis of the Cassini UVIS exposures show the 1304 A multiplet to be optically thick, and the emissions are modeled as direct electron and solar photon excitation of O I. The result is a model atmosphere dominated by O I and O II, with neutral density a factor of 100 below the original O2 model. Other considerations show incompatibility with an O2 atmosphere. Deep exposures using the Cassini UVIS EUV spectrograph provide the state of the plasma sheet at Europa. The ion species are identified as mainly outwardly diffused mass from the Io plasma torus with a minor contribution from Europa. Plasma time-constants are of the order of 200 days. Neutral species in the plasma sheet are not measureable. The energy flux in the magnetosphere L-shells are mainly responsible for energy deposition maintaining the plasma sheet. The energy content in the Io and Europa L-shells, as measured, is similar, but the mean radiative cooling rate in the Io plasma torus at the time of the Cassini encounter was 565 femtoergs cm-3 s-1, compared to 7.3 at Europa, reflecting the difference between an active and inactive planetary satellite, particularly considering the fact that most of the radiation at the Europa plasma sheet is from ions that originated at the orbit of Io. The stochastic observational evidence in disk averaged Europa oxygen emission obtained over the 1994 to 2012 period shows no indication of transient events. A significant neutral transient injection in the Europa plasma sheet would take of order year time-scales to relax to steady state