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Tidal Heating of Ice Shells with Variable Eccentricity

Presentation #205.04 in the session “Tides and Interiors”.

Published onJun 01, 2021
Tidal Heating of Ice Shells with Variable Eccentricity

It has long been understood that eccentricity tides give rise to Europa’s diverse geology and control the long-term processes affecting its icy shell. However, Europa’s eccentricity is not a constant value. Rather, it varies periodically over ~Myr timescales and daily, as captured by the JPL HORIZONS ephemerides. Due to frequency-dependence of tidal heating, a more sophisticated approach is required to model these processes and track the long-term thermal evolution of Europa’s ice shell. We modeled the thermal environment of Europa’s ice shell to find equilibrium in shell thickness and temperature profile. Tidal heating from an eccentric orbit is used as a local source term for conductive heat transfer. The heating of viscoelastic ice is temperature and frequency dependent, which a large temperature gradient across Europa’s ice and a rich tidal spectrum both contribute to. We compare three eccentricity models; one assuming it constant, another using a spectral decomposition of 600yrs of HORIZONS ephemerides output, and a final which considers the very long period (~100Myr) signals in eccentricity history. The results of the constant eccentricity model, at present value, indicate that Europa’s shell is at most 34km thick, however much thinner shells are possible with sufficient heating from the deeper interior. The HORIZONS eccentricity model finds shells that differ from this result very minimally. The long period eccentricity model, however, does result in thinner shells and a detectable difference in the final temperature profiles than both other models. Finally, the diffusion time scale on these processes is on the order of millions of years. This indicates that for very long period eccentricity variations, an instantaneous value can be used in these types of thermal evolution models.

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