Europa's Lyman-α emissions from HST/STIS observations

An image of Lyman-α (Lyα) emission from Europa obtained with the Hubble Space Telescope's Space Telescope Imaging Spectrograph (HST/STIS) has provided the first evidence of localized water vapor (H₂O) aurora, potentially originating from outgassing. Subsequent STIS observations have revealed the presence of a global atomic hydrogen (H) exosphere at Europa. We present a comprehensive analysis of STIS Lyα observations of Europa acquired in 1999 and between 2012 and 2020 to search for localized auroral emissions and constrain the properties of Europa’s H exosphere. We analyzed the complete dataset of the STIS observations obtained when Europa was sunlit and not transiting Jupiter. We constructed a model that accounts for all known sources of Lyα emission, including resonantly scattered sunlight from Europa’s H exosphere. To identify localized anomalies, such as H₂O aurora, we subtracted the modeled Lyα emission and analyzed the residuals. We detected emission from Europa’s H exosphere at all observing epochs, but we found that it is attenuated by absorption in Earth’s exosphere when Europa’s radial velocity relative to Earth (and, thus, the Doppler shift) is low. From the velocity dependence of this attenuation, we estimated an H-exosphere temperature of ∼1000 K and derived an upper limit of 5100 K. For the best-constrained epoch in 2014/2015, we inferred a vertical H column density of 1. 4 cm 10^ 12 -2 and an H source rate of 1. 1. No localized emission enhancements were detected in any of the observations, including the image previously interpreted as evidence of H₂O aurora near Europa’s south pole. The discrepancy with earlier results arises primarily from differences in the assumed position of Europa’s disk on the detector. The inclusion of an H-exosphere signal in the present analysis also contributes to this difference. When adopting the same disk position as in the previous study and neglecting the H-exosphere signal, the localized emission enhancement was again detected with a similar statistical significance. However, because of the updated approach to disk positioning and the more complete modeling of emission sources, including the H exosphere, we consider the results presented here as the preferred interpretation. s^ -1 We find evidence to support a persistent hydrogen exosphere at Europa, but no evidence of localized water vapor.