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M. A. McGrath (STScI), P. D. Feldman, D. F. Strobel, K. Retherford, B. Wolven, H. W. Moos (JHU)
Observations made with the Hubble Space Telescope on October 5, 1999 have provided the first far-ultraviolet images of Europa. Using the Space Telescope Imaging Spectrograph (STIS) with a 2'' wide slit gives objective spectrum images covering the range 1160--1720\,Å. We detect line emission from O\,{\sc i}\,\lambda1304 and \lambda1356, confirming earlier HST spectroscopic observations by Hall et al.~(1996, 1998). Solar photons reflected from the Europa disk are also detected and modeled to give an estimate of the UV albedo of Europa, which agrees well with the earlier determination of Hall et al. Unlike Io and Ganymede, Europa does not exhibit obvious concentrations of emission in either its equatorial or polar regions. Although both the brightness and morphology of the emission vary over the 6.5 hour span of the observations, they are not obviously correlated with the orientation of the Jovian magnetic field relative to Europa. The observed line brightnesses are somewhat higher than those observed by Hall et al., while the O\,{\sc i} \lambda1356/O\,{\sc i} \lambda1304 ratio is comparable, lending support to the identification of the excitation mechanism as electron impact on O2. The \lambda1356 emission appears to be limb-brightened, as expected from an exospheric atmosphere.
Hall, D. T., D. F. Strobel, P. D. Feldman, M. A. McGrath, and H. A. Weaver 1996, Nature, 373, 677.
Hall, D. T., P. D. Feldman, M. A. McGrath, and D. F. Strobel 1998, Ap.~J., 499, 475.
This work is supported by NASA through grants GO-08224.01-97A to STScI and JHU.