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Title: A NEW UNDERSTANDING OF THE EUROPA ATMOSPHERE AND LIMITS ON GEOPHYSICAL ACTIVITY

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 persistentmore » 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 × 10{sup 25} atoms s{sup –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{sub 2}O 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.« less
Authors:
; ;  [1] ;  [2] ;  [3] ;  [4] ;  [5]
  1. Planetary and Space Science Division, Space Environment Technologies, Pasadena, CA 91107 (United States)
  2. Division of Geological and Planetary Sciences, California Institute of Technology, CA 91125 (United States)
  3. Planetary Science Institute, 109 S. Puerto Drive, Ivins, UT 84738 (United States)
  4. Planetary Science Institute, Los Angeles, CA 91001 (United States)
  5. Laboratory for Atmospheric and Space Physics, University of Colorado, CO 80303 (United States)
Publication Date:
OSTI Identifier:
22364863
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 797; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTROPHYSICS; ATOMS; CAMERAS; ELECTRON DENSITY; ELECTRON TEMPERATURE; EXTREME ULTRAVIOLET RADIATION; FAR ULTRAVIOLET RADIATION; LOADING RATE; MASS; NEUTRAL PARTICLES; ORBITS; PLASMA; PLASMA SHEET; PROTONS; SATELLITE ATMOSPHERES; SATURN PLANET