Independent constraint of Enceladus’ ice shell thickness using thermal observations

Enceladus maintains its global, unconsolidated ocean around its rocky, porous core by tidal dissipation with Saturn and torque from its resonance with Dione [1].  The active South Polar Terrain (SPT) region is associated with intense concentrations of endogenic heat, but it is the significantly lower-power conductive heat flow that dominates global heat loss as it occurs over the entire surface.  If Enceladus’ global ocean is to be sustained over a significant fraction of its existence, heating rates would have to be balanced endogenic heat loss. 

Estimates of heating rates from models vary from 1.5-150 GW [2].  The large range results from uncertainty in both the structure of the bodies’ interiors and their evolution.  Ice shell thickness/shape models, which interpret gravity, libration and topographic data, produce global conductive heat loss estimates of around 18-35 GW [3,4,5].

Endogenic heat loss from the SPT has been estimated using thermal observations from Cassini Composite Infrared Spectrometer (CIRS) to be between 5-19 GW [6,7,8], resulting in a conventional, combined heat loss estimate of around 50 GW [9].

Detecting endogenic heat loss using thermal observations presents a significant challenge, principally relating to limited data coverage and uncertainty about the surface thermal properties but is possible under some circumstances [10].

We use thermal observations CIRS to identify endogenic heat at the north pole of Enceladus in the form of conductive heat flow.   From this estimate we can infer global average heat loss.  We are then able to invoke the same mechanisms used to estimate the global average heat loss from ice shell thickness models [5, 9] to characterize the first north polar and global average ice shell thicknesses independently derived from thermal observations.

Acknowledgments: This work was made possible through NASA’s support of Cassini Data Analysis Program Grant Number 80NSSC20K0477.

 

References

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