Plumes on Enceladus: Lessons for Europa?
Abstract:The possible detection of a water vapour plume on Europa  suggests resemblances to Enceladus, a cryovolcanically active satellite . How does this activity work, and what lesson does Enceladus have for plumes on Europa?
The inferred vapour column densities of the Europa  and Enceladus  plumes are similar, but the inferred velocity and mass flux of the former are higher.
At Enceladus, the inferred plume strength is modulated by its orbital position [4,5], suggesting that tides opening and closing cracks control the eruption behaviour [6,7]. An additional source of stress potentially driving eruptions is the effect of slow freezing of the ice shell above
[7,8]. The original detection of the Europa plume was close to apocentre, when polar fractures are expected to be in tension . Follow-up observations at the same orbital phase did not detect a plume , although the Galileo E12 magnetometer data may provide evidence for an earlier plume [Khurana, pers. comm.]. One possible explanation for the plume's disappearance is that longer-period tidal effects are playing a role; there are hints of similar secular changes in the Enceladus data [4,5]. Another is that detectability of the Europa plume
in the aurora observations also depends on variations in electron density (which affects the UV emission flux) . Or it may simply be that eruptive activity on Europa is highly time-variable, as on Io.
At Enceladus, the plume scale height is independent of orbital position and plume brightness . This suggests that the vapour velocity does not depend on crack width, consistent with supersonic flow through a near-surface throat. The large scale height inferred for the Europa plume likewise suggests supersonic behaviour. Continuous fallback of solid plume material at Enceladus affects both the colour  and surface texture  of near-polar regions. Less frequent plume activity would produce subtler effects; whether the sparse available imagery at Europa  contains any similar evidence is yet to be determined. Forthcoming Hubble observations may also provide additional constraints.
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