Ridged Plains on Europa Reveal a Compressive Past

Post Contributed by Dr. Erin Leonard, Postdoctoral Fellow at the Jet Propulsion Laboratory, California Institute of Technology

Jupiter’s icy moon Europa has a geologically young surface (60-100 million years old), as evidenced by the sparsity of large impact craters. Studying the surface features on Europa allows insight into how resurfacing may have given it a youthful appearance. The majority of Europa’s young surface is made up of Ridged Plains terrain. This terrain has not been extensively studied before because it appears as a smooth and relatively bland in the global-scale images. However, in the few high-resolution images returned by the Galileo mission in the early 2000s, the Ridged Plains are revealed to consist of numerous ridges and troughs that have a range of morphologies—from crisscrossing each other in various directions to orderly sets of parallel structures (Image 1). But how did these ridges and troughs form?

LeonardFigure1

Image 1: A variety of examples of ridged plains on Europa. Note the linear to curvilinear systematic ridge traces in all examples: (A) observation E4ESDRKMAT02 at 26 m/pixel, (B) observation 19ESRHADAM01 at 66 m/pixel, (C) observation 12ESWEDGE_02 at 29 m/pixel, and (D) observation 12ESMOTTLE02 at 16 m/pixel.

In order to investigate the formation mechanism for the Ridged Plains, I have used two-layer sand and putty experiments to simulate tectonic deformation on Europa: the underlying ductile putty layer simulates lower-crustal ductile deformation and the overlying sand layer simulates upper-crustal brittle deformation. I conducted a variety of experiments by varying the thickness of the two layers and the deformation mechanism (extension or compression), in order to compare observations of ridged plains on Europa to the surface structures generated by each analogue experiment. The results show that the ridge-trough systems in the compressional experiments, which generate surface folds, best match observed cross-sectional shape and map-view patterns of Europa’s Ridged Plains (Image 2).

LeonardFigure2

Image 2: Experiment results compared to ridged plains terrain in cross-section view. The top panel (A) is part of Frame 1989 from the 12ESMOTTLE Galileo SSI data, illustrating the subparallel ridge-and-trough morphology of the ridged plains cut by a scarp, so that we see the ridges in cross-section. The middle panels (B) and (C) show the cross-section of a compression experiment with 0.5 cm brittle layer and 8 cm ductile layer where (B) is the uninterpreted version of (C). The bottom panel (D) and (E) show the cross-section of an extensional experiment with 0.5 cm brittle layer and 2 cm ductile layer where (D) is the uninterpreted version of (E).

When the experiments are scaled to the conditions on Europa, it is inferred that the folds formed when Europa’s ice shell was ~3 km thick. Most hypotheses for Europa’s current ice shell thickness range from 10-50 km, implying that Europa’s ice shell has significantly thickened over the past 60-100 million years. Therefore, I hypothesize that the Ridged Plains resulted from global cooling, associated with an initial volumetric expansion of the ice crust, triggering compression at the onset of the latest resurfacing event. This hypothesis suggests that Europa’s ice shell has not been in steady state over its ~60 Myr visible surface history.

Further Reading:

Leonard, E.J., Yin, A. and Pappalardo, R.T., 2020. Ridged plains on Europa reveal a compressive past. Icarus, p.113709. doi: 10.1016/j.icarus.2020.113709

Hubbert, M. K., 1937. Theory of scale models as applied to the study of geologic structures, Geological Society of America Bulletin, 48(10), pp. 1459–1520. doi: 10.1130/GSAB-48-1459.

Leonard, E. J., R. T. Pappalardo, and A. Yin, 2018. Analysis of very-high-resolution Galileo images and implications for resurfacing mechanisms on Europa, Icarus, 312. doi: 10.1016/j.icarus.2018.04.016.

Nimmo, F., 2004. Stresses generated in cooling viscoelastic ice shells: Application to Europa, Journal of Geophysical Research: Planets (1991–2012), 109. doi: 10.1029/2004JE002347.

Pappalardo, R. T., M. J. S. Belton, H. H. Breneman, M. H. Carr, C. R. Chapman, G. C. Collins, T. Denk, S. Fagents, P. E. Geissler, B. Giese, R. Greeley, R. Greenberg, J. W. Head, P. Helfenstein, G. Hoppa, S. D. Kadel, K. P. Klaasen, J. E. Klemaszewski, K. Magee, A. S. McEwen, J. M. Moore, W. B. Moore, G. Neukum, C. B. Phillips, L. M. Prockter, G. Schubert, D. A. Senske, R. J. Sullivan, B. R. Tufts, E. P. Turtle, R. Wagner, and K. K. Williams, 1999. Does Europa have a subsurface ocean? Evaluation of the geological evidence, Journal of Geophysical Research: Planets (1991–2012), 104(E10), pp. 24015–24055. doi: 10.1029/1998JE000628.

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