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Evidence for a Shift of Europa’s Icy Crust

A hypothesis about an astronomical object snaps into sharper detail when it can be tested. Thus the new findings on Europa and the movements of the ice shell that covers its ocean, which are the subject of a paper in Geophysical Research Letters. The work of Paul Schenk (Lunar and Planetary Institute, Houston) and colleagues, the paper argues that the shell has rotated by about 70 degrees during the last several million years. Clearly, such movement can only happen with a shell floating freely over a liquid ocean beneath, and Europa Clipper should be able to tell us more.

Remember, we are talking about a geologically young surface on this Jovian moon, as indicated by, among other things, the relative smoothness of the terrain and the paucity of impact craters. All that is consistent with ice in motion in one way or another. Schenk’s team homes in on large global-scale circular patterns that can be made out by reference to Galileo and Voyager data, previously identified features that could only have been formed during a reorientation of the shell. The process moves the outer shell with respect to the moon’s spin axis, and is known as true polar wander (TPW). The implications are striking, according to Schenk:

“Our key finding is that the fractures associated with true polar wander on Europa cross-cut all terrains. This means that the true polar wander event is very young and that the ice shell and all features formed on it have moved more than 70° of latitude from where they first formed. If true, then the entire recorded history of tectonics on Europa should be reevaluated.”

Image: Perspective views of fractures on the surface of Europa formed during true polar wander. The large cracks crossing the scene from left to upper right are ~3 kilometers (1.9 miles) wide and 200 meters deep. The double ridges crossing the scene are similar in width. Credit: P. Schenk/USRA-LPI.

The reorientation of an ice shell has been proposed for a variety of icy worlds, with the best cases, according to the paper, being made for Pluto and possibly Ganymede, the latter a world we’ll be able to analyze up close with the arrival of the JUICE mission in the 2030s. There is speculation about true polar wander as well on Miranda and Enceladus, among others.

Maps produced from Galileo and Voyager data are at the heart of this paper, as Schenk worked with Isamu Matsuyama (University of Arizona) and Francis Nimmo (UC-Santa Cruz) to correlate large fractures on the Europan surface with concentric circular depressions that had already been identified. The team analyzed the global map largely at 200-meter resolution, with highest-resolution in some areas reaching 40 meters per pixel.

The fracture systems are related to the circular true polar wander tectonic patterns previously found, according to the paper, and we can get a sense of their age because the fractures cut across all known terrains. The team concludes that the global orientation of the ice shell had to have been one of the last major events to occur on Europa. Moreover, the thickness of the ice shell, a key factor in any attempt to reach the ocean below, may have increased with time.

The work makes predictions that we can test when the Europa Clipper mission reaches Europa. “In addition to generating global-scale tectonic features, true polar wander also produces global-scale gravity and shape perturbations, which affects gravity and shape constraints on the interior structure,” says co-author Matsuyama.

The spacecraft is to complete our map of Europa, including high resolution images of critical surface features, allowing scientists to determine more precisely the age of Europan fractures and depressions. From the preprint:

A return to Europa will be necessary to map out the full distribution of all known and candidate TPW-related features on Europa (including troughs, fissures, plateaus, folds, etc.) to determine their extent, stratigraphic ages, structural strain and other characteristics. These will constrain TPW processes and timing, as well as properties of the ice shell during the epoch in which they formed and will be key objectives of NASA’s Europa Clipper mission. Any putative previous TPW episodes may also be resolved in global high-resolution mapping in the form of cryptic troughs or fractures.

The paper is Schenk et al., “A Very Young Age for True Polar Wander on Europa from Related Fracturing,” in process at Geophysical Research Letters (abstract).

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{ 8 comments… add one }
  • Alex Tolley August 28, 2020, 9:42

    (…) the paper argues that the shell has rotated by about 70 degrees during the last several million years. Clearly, such movement can only happen with a shell floating freely over a liquid ocean beneath (…)

    It has been suggested that Mars has had polar wander of about 20 degrees:
    True polar wander on Mars, and I really a Planetary Society meeting perhaps 2 decades ago where the speaker suggested that polar wander had been perhaps 60 degrees due to the Tharsis bulge.

    In the case of Mars, does it imply that a “liquid” mantle was once present to allow the wander, much as Earth has today? In the Europa case, the liquid ocean should allow a considerable decoupling of the ice crust from the moon’s body. Does this mean that wander is must faster than on bodies with more viscous material connecting the crust?

  • Gary Wilson August 28, 2020, 16:18

    “This means that the true polar wander event is very young and that the ice shell and all features formed on it have moved more than 70° of latitude from where they first formed.” I will try to get access to the paper but can anyone explain why this is the case. Why is it a recent event? What caused this to happen? Has the subsurface ocean changed dramatically in the recent past to cause such a huge TPW? Maybe I’m missing the whole point here but I don’t understand the rationale behind saying the TPW event is very young.

    • Paul Gilster August 28, 2020, 19:59

      The full text of the paper is available at the link in my article. The TPW is known to be young because it cuts through all kinds of terrain, indicating it occurred later than the formation of these different features. But the paper explains it well, I think. Again, both abstract and full text are available.

      • Laintal August 30, 2020, 6:17

        Hi

        “Why is it a recent event?” Gary you seem to have all same questions that I had here too, One can only assume that the ice shell has been moving since Europa formed and the recent shift is the one we can see evidence of.

        I’ll try and read the paper tommrow

      • Mike Serfas August 30, 2020, 13:26

        I did not see the mentioned link but, waving back at Alexandra, it seems the authors cite https://www.sciencedirect.com/science/article/abs/pii/0019103589900535 and by extension https://ui.adsabs.harvard.edu/abs/1989Icar…81..220O/abstract – in short the solar and tidal heating can change the thickness of the ice causing the poles to swing outward under some conditions. I will admit that on consideration, I’m not actually understanding just how that works…

    • Alex Tolley August 30, 2020, 18:50

      While I don’t fully understand their argument, it seems to me that they are saying that the features postdate know earlier features, and therefore must be more recent. However, it is not clear to me why much earlier TPW event[s] evidence could not have been erased by resurfacing. If this resursurfacing is not the case then this leaves several options:

      1. Europa is a very young moon
      2. TPW was absent prior to 70 mya.

      For option 2, it might imply the subsurface ocean was not liquid to allow the crust to move, or that there was no imbalance in the crust that would initiate TPW during its prior history. For option 1 to be true, the consequences for life by abiogenesis would indicate that it would be absent.
      I find both options unlikely. It seems to me that TPW has occurred in teh past but that the evidence of the surface features has been destroyed, just as most of the meteor impacts on Earth have been destroyed by erosion, unlike the Moon.

      • Mike Serfas September 2, 2020, 12:45

        According to the authors, “The timescale for the shell to undergo thermal/thickness adjustment following TPW is ~d^2/k, where d is the shell thickness and k is the thermal diffusivity. For a 20 km thick shell, this timescale is 12 Myr, less than the ~50 Myr mean age of Europa’s surface (Bierhaus et al., 2009).”

        That paper is available: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2009JE003451

  • Gary Wilson August 30, 2020, 18:56

    Thanks Paul and Laintal. I’m going to try to read it as well :).

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