Geological evidence for solid-state convection in Europa's ice shell

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Abstract

The ice-rich surface of the jovian satellite Europa is sparsely cratered, suggesting that this moon might be geologically active today1. Moreover, models of the satellite's interior indicate that tidal interactions with Jupiter might produce enough heat to maintain a subsurface liquid water layer2,3,4,5. But the mechanisms of interior heat loss and resurfacing are currently unclear, as is the question of whether Europa has (or had at one time) a liquid water ocean6,7. Here we report on the morphology and geological interpretation of distinct surface features—pits, domes and spots—discovered in high-resolution images of Europa obtained by the Galileo spacecraft. The features are interpreted as the surface manifestation of diapirs, relatively warm localized ice masses that have risen buoyantly through the subsurface. We find that the formation of the features can be explained by thermally induced solid-state convection within an ice shell, possibly overlying a liquid water layer. Our results are consistent with the possibility that Europa has a liquid water ocean beneath a surface layer of ice, but further tests and observations are needed to demonstrate this conclusively.

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Figure 1: Pits, domes and spots of various specific morphologies (shown to a common scale) on the surface of Europa.
Figure 2: Effective viscosity ηeff of ice as a function of temperature T, strain rate ε̇,and grain size d. for conditions relevant to the subsurface of Europa.

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Acknowledgements

We thank W. McKinnon and N. Sleep for reviews; N. Sherman, L. Prockter and G. Collins for their contributions; and J. Kaufman, K. Magee and K. Klaasen for their efforts in acquisition of the E6 Europa imaging data. This work was supported by NASA's Galileo Project.

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Correspondence to R. T. Pappalardo.

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