Is Enceladus' Internal Ocean Doomed to Freeze?
Abstract
Enceladus is geologically hyperactive, with plumes of water vapor, other volatiles such as ammonia, and salty particles erupting from its South Polar Terrain. The plumes are spatially associated with a region of greatly increased local heat flux, with a total power output 15.8 × 3.1 GW spread over an area of 70,000 km2 (e.g., Spencer and Nimmo 2013, AREPS 41), corresponding to a regional heat flux of 180-270 mW m-2. Tidal strains of the magnitude only possible in an ice shell that is decoupled from a rocky interior by an internal ocean or regional sea are required to generate this much heat (e.g., Behounkova et al. 2012, Icarus 219). Yet, numerous studies conclude that Enceladus' ocean cannot be in present-day thermodynamic steady state with a conductive or convective ice I shell (e.g., Roberts and Nimmo 2008, Icarus 194; Behounkova et al. 2012). Regardless of where Enceladus' tidal heating is concentrated (i.e., the poles), and regardless of whether its outer ice I shell convects, Enceladus' ocean is predicted to freeze on a geologically rapid time scale, implying that activity on Enceladus is only a transient or episodic phenomenon. These arguments strictly apply only to pure water ice oceans, however. We have previously argued that if the presence of salts or ammonia is allowed for, the ocean may be cooler and can be maintained essentially permanently by tidal heating in the ice above (McKinnon and Barr 2008, LPS XXXIX). Here we elaborate on the conditions under which Enceladus' ocean can be stabilized or even increase in thickness due to present-day tidal heating within the ice shell and the presence of salts and ammonia, which we now know are there (Waite et al. 2009, Nature 460; Postberg et al. 2011, Nature 474). As previous work has found, we cannot explain the present-day heat flow, but there is no fundamental reason that Enceladus' ocean or sea should completely freeze for present-day orbital eccentricities.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.P53E..07M
- Keywords:
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- 6282 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS Enceladus;
- 5430 PLANETARY SCIENCES: SOLID SURFACE PLANETS Interiors;
- 6280 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS Saturnian satellites;
- 5422 PLANETARY SCIENCES: SOLID SURFACE PLANETS Ices