HST Lyman-α observations of Europa from 1999 to 2020: Constraints on the H exosphere

Jupiter moon Europa possesses a tenuous atmosphere, constantly replenished from weathering of the icy surface (McGrath et al. 2009, Johnson et al. 2009). The atmosphere is likely dominated by molecular oxygen (O₂) near the surface and hydrogen (H₂) at higher altitudes (e.g., Smyth & Marconi 2006). Surface ice sublimation can be a source for H₂O molecules in the atmosphere, in particular in the warmest subsolar region. Further water group species in the atmosphere like OH, O and H are produced by dissociation of the primary molecular atmosphere and possibly also directly through radiolysis of the ice. 

While molecular hydrogen (H₂) has not been detected yet with remote sensing observations, a widely extended atomic hydrogen (H) exosphere was measured through attenuation at the Lyman-α line (1216 Å) in HST/STIS observations of Europa in transit of Jupiter (Roth et al. 2017). A comparison with HST observations of Ganymede in and out of transit (Roth et al. 2023) suggests that the H exosphere at Europa might actually be denser than derived from the transit observations.    

We have analyzed a set of Hubble Space Telescope images of Europa's ultraviolet emissions (out of transit) taken in October 1999 and on 21 occasions between 2012 and 2020. The hydrogen Lyman-α emission (1216 Å) in the data confirms the presence of an extended H corona around Europa (Figure 1), with similar densities at all orbital longitudes. The inferred densities are about 10 times higher than the previously inferred values. We find that the H exosphere signal observed by HST is extinct by the geocorona whenever Europa had a relatively low radial velocity to Earth and thus the Doppler shift of the source signal was small. The transition in the observed exosphere brightness from low Doppler shift (attenuated) to high Doppler shift (unattenuated) allows us to constrain the temperature of H in Europa's exosphere. The derived value of T_H = 1500 (+/- 800) K is the first constraint on atmospheric temperature at Europa. We discuss implications from this temperature about the production of H in the exosphere.       
Finally, we compare the Lyman-α surface reflectance to previous results to further constrain the reflectance inversion from the visible to the far-UV.   

Figure 1: Lyman-α emission profile along detector y axis of HST/STIS, revealing contributions from the background (green), the H exosphere (blue) and surface reflections (red).  

 

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