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L. S. Bruesch, J. R. Green (Caltech/JPL)
Observations have shown that Europa's atmosphere contains atomic sodium (Brown and Hill, 1996). The origin of this Na is still under investigation. There are two possible sources: (1) exogenic: implantation on Europa's surface from Io, or (2) endogenic: originating from Europa's surface ice, indicating a saline H2O-ice rather than pure H2O-ice surface.
Recent work has demonstrated that the desorption rate of Na from Europa's surface is larger than the rate of implantation of Na from Io (Johnson, 2000); therefore suggesting that the Na source is endogenic.
Experimental work has been conducted showing that endogenic Na desorbs from a surface by one or both of the following processes: (1) heavy ion bombardment (Benninghoven, 1969), and/or (2) UV irradiation (Yakshinskiy and Madey, 1999). However, neither of these processes were directly applied to a Europa simulant or conducted under Europa-like conditions.
We are currently conducting experiments to determine if UV irradiation alone, at levels appropriate to Europa, will produce Na in the atmosphere. We simulate Europa's surface, using H2O-ice doped with salts (McCord, 1998). This simulant is exposed to Europa-like conditions in a high vacuum, cryogenic chamber and then irradiated with a UV light source. We use a quadrupole mass spectrometer to study the resulting atmosphere. Results will be reported at the meeting.
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