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High signal-to-noise ratio spectra of Europa and Callisto's trailing side show a 0.3% deep 5771 Å absorption band due to condensed O 2 at the same wavelength as a stronger band previously identified on Ganymede. Excellent longitudinal coverage for Europa shows that unlike Ganymede, where the band is much stronger on the trailing side, Europa shows no significant longitudinal variation in the O 2 band strength.
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Spencer et al. (2002) studied this question.
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