The interior and evolution of Enceladus: Current knowledge and future prospects (Invited)

Mathematics – Logic

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[5418] Planetary Sciences: Solid Surface Planets / Heat Flow, [5450] Planetary Sciences: Solid Surface Planets / Orbital And Rotational Dynamics, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution, [6280] Planetary Sciences: Solar System Objects / Saturnian Satellites

Scientific paper

The nature and evolution of the interior of Enceladus, and especially that of any liquid water, is a topic of great astrobiological significance. Cassini observations of salty grains in the E-ring strongly suggest the presence of subsurface liquid [1], though this liquid may only be regional [2,3] rather than global in extent. Here I will focus on three questions of comparable importance. 1. How can its heat budget be explained? The current heat output from the South Polar region [4] greatly exceeds the equilibrium tidal heat production [5]. There are two solutions to this paradox: either Enceladus produces heat intermittently; or it produces heat at a constant rate, but gets rid of that heat intermittently. Enceladus does not undergo episodic tidal heating of the kind which may occur at Io [6], for either a convective [7] or a conductive ice shell. It may undergo episodic heating whenever the growing tidal stresses cross a critical threshold to initiate fault motion [8]. Alternatively, convection on Enceladus driven by constant heating may result in episodic overturn and pulsed heat loss [9]. Either solution implies a duty cycle roughly consistent with observations of 40Ar in the plume [10]. One way of distinguishing between these two solutions is astrometric observations, as at Io [11]. If the current tidal heating rate at Enceladus is that measured by Cassini [4], the eccentricity damping results in a fractional change in mean motion of ~6e-11 /yr, potentially measurable with ground-based observations. 2. Is the ice shell convecting? The ice shell of Enceladus is marginally unstable to convection. A convective shell is dissipative and also results in rapid reduction of ice shell thickness contrasts. Regional topographic anomalies, especially at the South Pole [2,12], are suggestive of shell thickness contrasts and a conductive shell. A conductive shell is also more compatible with the long-term orbital evolution of Enceladus [13] and results in a longer-lived ocean. On the other hand, the tiger stripes may be evidence of mobile-lid convection [9,14], while tidally-driven convection results in preferential survival of subsurface liquid near the poles [15]. 3. Is the ocean a long-lived feature? A global subsurface ocean on Enceladus can survive for only tens of Myr [16], unless the core is strongly heated or dissipation within the ocean itself is important [17]. Regional oceans can survive for longer [15]. Freezing of the ocean, if it occurs, will cause a reduction in dissipation, and increased orbital eccentricity. Subsequent remelting of the ocean is possible - depending on the ocean freezing temperature - and probable if tidal heating is locally concentrated. This style of cyclic behaviour, however, never results in surface heat fluxes in excess of the equilibrium value [5]. [1] Postberg et al. 2009 [2] Collins & Goodman 2007 [3] Tobie et al. 2008 [4] Spencer et al. 2006 [5] Meyer & Wisdom 2007 [6] Ojakangas & Stevenson 1986 [7] Meyer & Wisdom 2008 [8] Stevenson AGU 2008 [9] O'Neill & Nimmo 2010 [10] Waite et al. 2009 [11] Lainey et al. 2009 [12] Schenk & McKinnon 2009 [13] Zhang & Nimmo 2009 [14] Barr 2008 [15] Behounkova et al. in press [16] Roberts & Nimmo 2008 [17] Tyler 2009

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