Plumes on Enceladus: Lessons for Europa?

Abstract:

The possible detection of a water vapour plume on Europa [1] suggests resemblances to Enceladus, a cryovolcanically active satellite [2]. How does this activity work, and what lesson does Enceladus have for plumes on Europa?

The inferred vapour column densities of the Europa [1] and Enceladus [3] 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 [1]. Follow-up observations at the same orbital phase did not detect a plume [9], 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) [9]. 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 [5]. 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 [10] and surface texture [2] of near-polar regions. Less frequent plume activity would produce subtler effects; whether the sparse available imagery at Europa [11] contains any similar evidence is yet to be determined. Forthcoming Hubble observations may also provide additional constraints.

[1] Roth et al. 2012 [2] Porco et al. 2006 [3] Hansen et al. 2011 [4] Hedman et al. 2013 [5] Nimmo et al. 2014 [6] Hurford et al. 2007 [7] Porco et al. 2014 [8] Manga and Wang 2007 [9] Roth et al., submitted [10] Schenk et al. 2011. [11] Phillips et al. 2000