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Europa’s Patchy Plate Tectonics

I keep an eye on recent findings about Europa because fine-tuning procedures for the science that missions like Europa Clipper and JUICE (Jupiter Icy Moons Explorer) will perform at the Jovian moon is an ongoing process that doesn’t stop at launch. The more we learn now – the more anomalies we uncover or processes we begin to glimpse – the better able we’ll be to adjust spacecraft observing strategies to go after the answers to these phenomena. A new study teaches us a bit more about Europa’s plate tectonics, the only solid evidence of tectonics we know of other than Earth’s. And it will take new high-resolution imagery to confirm the theories put forth within it.

Appearing in the Journal of Geophysical Research: Planets, the paper looks at the processes that evidently govern the evolution of the fractured Europan surface the Galileo mission revealed to us back in the 1990s. What’s intriguing here is the identification of Europan tectonic plates in the context of deep time. If a tectonic plate shows a broad continuity, it is bounded by surface discontinuities. The authors show that a sequence can be established based on how discontinuities cut through surface features, and we can begin to make statements about how the surface changes.

Image: A complex pattern of ridges and bands named Arachne Linea is seen in this false-color image of Europa’s surface taken by the Galileo spacecraft on 26 September 1998. New research shows that this landscape was formed by the jostling of nearby tectonic plates. Credit: NASA/JPL-Caltech/SETI Institute.

The work, led by Geoffrey Collins (Wheaton College, MA), looks at three areas on Europa – spread out in terms of latitude so as to include the high northern and southern latitudes as well as the equatorial areas – to reconstruct what they would have looked like before surface plates moved. It’s fascinating to contrast the findings with what we see on Earth, because on Europa tectonic movement is scattered, with some areas showing no plates at all. That reveals a regional process in which plate travel distance can be less than 100 kilometers. Plate tectonics, in other words, occurs only in limited areas on Europa, covers only a small degree of motion across the surface, and only appears to operate intermittently. It does not appear to be happening now, at least in the areas the paper surveys.

I come back to Europa Clipper, the mission that should benefit from this analysis, for although we’ve studied plate tectonics on Europa before, the work has been hampered by lack of imaging data at high enough resolution to attain a widespread view. The authors argue that the improved imaging from missions like Clipper and JUICE will detect many more areas with plate-like motions to add to their three examples. This should highlight the fact that while Earth’s tectonic plates form a global system, Europa’s are sharply confined in a process that is likely local or regional.

Note the stop-and-start aspect of plate movement, as discussed in the paper (italics mine):

In all of the study areas, young ridges and ridge complexes overprint the plate boundaries. The young ridges do not accommodate offsets like those seen in the plate boundaries. Thus, whatever process was driving the plate motions came to an end, and is not actively driving plate motions today in any of the areas studied. The relationship between the Castalia Macula study area and the older Belus area to its north… demonstrates that the plate tectonic-like behavior on Europa did not occur all at the same time. Combined with the previous conclusion, we develop the view that plate tectonic-like behavior on Europa occurs in regional patches and turns on and off at different times in different places.

Image: This is Figure 19 from the paper, illustrating tectonic change over time. Caption: An area exhibiting plate-like motions north of Belus Linea is outlined by a red dashed line. The purple lines in the south are plate boundaries CM2 and CM4 from the Castalia Macula reconstruction… The blue line shows a ridge that is crosscut by CM2 and CM4, and extends all the way south to be crosscut by Acacallis Linea (off the southern edge of this figure). The Blue Ridge crosscuts the green ridge, which crosscuts all of the candidate plate boundaries in the area north of Belus. This shows that all of the plate-like activity in the area north of Belus is older than the activity in the Castalia Macula study area. Orthographic projection centered at 15°N, 135°E. Credit: Collins et al.

The study of these three areas of Europa implies that something stops plate motion from persisting and spreading, possibly because of the nature of the surface material or else the mechanism that forces plate motions in the first place. Here’s an interesting point that has further ramifications for future observations by our spacecraft: Is convergence – where surface material is lost and pre-existing terrain must be reconstructed – always apparent from the limited data we have available? We’ll need plenty of high resolution imagery from future spacecraft to make the call on that.

There is much for Europa Clipper to examine in terms of how tectonics functions here, not the least of which is the question of what drives the plate motions thus far observed on Europa. If plate tectonics is indeed as sharply limited as this study implies, just what is it that forces a plate movement and then apparently shuts it down?

And in terms of that ever-fascinating ocean below the ice, is surface material drawn in large quantities into the ice shell, and are there times when the lower crust or ocean is exposed while the plate motion occurs? Reconstructing plate motions is an open investigation with serious implications for habitability. Europa Clipper and the JUICE mission should give us data that sharply refine our modeling of surface and ocean.

The paper on Europa’s tectonics is Collins et al, “Episodic Plate Tectonics on Europa: Evidence for Widespread Patches of Mobile‐Lid Behavior in the Antijovian Hemisphere,” Journal of Geophysical Research: Planets Vol. 127, Issue 11 (06 November 2022). Full text.

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{ 16 comments… add one }
  • Brett December 2, 2022, 16:13

    I wonder if it’s underlying vulcanism beneath the ice layer, in the ocean. Maybe whenever Europa has volcanoes (or a set of them) erupt in a particular area, it thins out and fragments the ice above it, allowing it to start moving and drifting. When that volcano wears out, it then hardens up again and the process repeats elsewhere.

  • Geoffrey Hillend December 2, 2022, 18:22

    Using archetypal theory and physical principles we can come to some interesting conclusions. Earth and Water would be described by two different archetypes that fall under the Great mother archetype. Consequently, we should distinguish or differentiate between these. Plate tectonics on our Earth, made of rock are not exactly the same as fractures and plates on water. In other words, frozen water might not have a lot of resurfacing of plates since there are no subduction zones. Maybe tidal forces and tidal heating from the core and water beneath will cause cyrovolcanistic re surfacing and new cracks?

  • Alex Tolley December 2, 2022, 18:26

    So the reasons for these calculated ice crust tectonic plate movements are speculative. The obvious one, is that hot plumes from the rocky core cause upwelling and a ridge (as we saw in an earlier post). But the authors state that the mechanism of the required subduction is hard to determine. While the evidence of subduction is in the record as they calculate, is it possible that the ice crust is increasing in area too?
    The argument that Jupiter’s gravity is a possible cause is interesting, only in that it was also invoked for the internal heating of Europa and therefore the plumes.
    The authors state early on that this is the only moon where tectonics occurs. Does this mean that Enceladus (and presumably other icy moons with subsurface oceans) are static, despite plume evidence on Enceladus?

    It would seem to me, perhaps naively, that just as ocean ridge spreading on Earth can be shown over time by magnetic data, some equivalent to showing age should be possible on Europa. For example, peroxide levels might be expected to be lower in fresher material as exposure to Jupiter’s radiation has had less time to work on the surface water ice. Could other features, such as dust, organic material, or other material that would be time-dependent be of importance? Subduction zones should have no difference in these materials across the zone and should be the same as the nearby surface.
    If warm ice is being pushed to the surface at the ridges, shouldn’t these be detectable by IR or other sensitive heat measurements?
    Lastly, do the spreading ridges tell us something about the structure of the rocky core below the ocean? Are there structural hot zones that drive the plumes, or are plumes randomly generated by transiently weak spots that change as the core is “massaged” by Jupiter’s gravity?

  • Geoffrey Hillend December 3, 2022, 16:03

    Due to tidal forces from Jupiter, there is a great possibility that there is some kind of plate tectonics and volcanism on the surface crust. The problem is the oceans which are assumed to have a current and planet wide circulation are a medium between the frozen surface ice of that ocean. Consequently, the idea that is might have subduction zones in it’s idea is dubious. The ice plates are supposed to be floating on soft ice and there might be some subduction of those plates in certain places, but there can’t be a rift with the floor spreading like on our ocean floor and therefore no fixed subduction zones over time like on Earth’s crust. It does not mean there is not any subduction. It is clear by the few craters as has been written in the literature that Europa’s surface must undergo a renewal and loss of craters which is cannot only explained by the micrometeorite resurfacing which is not very deep. Maybe whole layers of the ice are melted by the oceans internal heat and volcanism and then refrozen from below? Thermal infra red cameras and spectrometers will certainly be helpful. I would like to see what the JWST shows.

  • Geoffrey Hillend December 3, 2022, 16:30

    If the ice shell has different speeds in different places there might be collisions and subductions of the crust. https://astronomy.com/magazine/ask-astro/2015/06/europan-tides

  • Steve Bowers December 4, 2022, 9:57

    If Europa’s mobile-lid tectonics is intermittent, perhaps it is driven by changes in the orbit of Europa, or changes in the tidal interactions between the Galilean moons. A small change in eccentricity might release a lot of energy into the system.

  • wd December 4, 2022, 14:23

    Not having looked at the paper yet, but considering the conundrum, water ice charts for states in 3 dimensions can include temperature, pressure and … drum roll … density. If temperature increase is considerable at subsurface levels on Europa, then ice near the surface and near the
    liquid boundary can go through several state transitions worth review.
    Densities from our nominal value close to 1.0 can vary a couple tenths.
    If it unit mass ice expands or drops at the right points, that might induce subduction and cycling.

    • Andrei December 5, 2022, 7:42

      You’re thinking about the discovered other phases of ice there, well the conditions might be right for Ice II on Europa (also Ganymede and Callisto) but none of the others, the pressure and other conditions are not extreme enough for the other phases. Uranus and Neptune are considered to have a whole list of those weird forms of ice however.

  • Geoffrey Hillend December 4, 2022, 17:43

    The problem with the density is that ice is less dense than water. I don’t think there can be soft ice without some water in it. The whole subduction idea does not work unless there is more salt than in the frozen ice above the warm ice below it. Salt increases density. Ice floats one water since it is less dense than water. https://www.sciencedaily.com/releases/2017/12/171204112918.htmI
    Also, I have a hard time with the thought experiment of convecting warm ice. There is not a lot of wiggle room or temperature differences in warm, soft ice for convection currents. Where is the ice being recreated? It would have to be cryanovolcanoes or fissures with gases coming out. Cracks, melting and meteorites could account for the loss of craters over time. Maybe the Europa clipper will give us some more hints.

    • Alex Tolley December 5, 2022, 0:29

      Is there any possibility that the higher density of peroxide or its lower melting point would be sufficient to create the needed density change or melting to allow either subduction or more likely ice loss as meltwater?

      • Geoffrey Hillend December 6, 2022, 18:37

        Saltwater also has higher density, but why should the surface ice have more salt in it than the ice lower down. The other problem when ice is being subducted, we have to have some new ice coming up. Maybe some convection currents from below. Another problem is that the subduction of land masses and plates on Earth are huge, so it makes sense that we can have mountains being raised by plate collisions and subduction. The plates are spread apart at the rifts where there is convection of the hot mantle. I don’t know if ice can mimic that process. I can’t rule it out though and as you wrote that infra red and ground penetrating radar will also help, but we will have to wait for the Europa clipper. The JWST might help.

        All I can say is that it seems rather far fetched since we won’t have the same pressures with an ice collisions floating on water as two large land masses or plates which are more denser and massive. It seems too convenient and explanation and analogy. I can imagine ice plates colliding and having different cleavage angles, but the subduction idea is hard to work out in a thought experiment. Certainly there can be melting right above the ocean.

  • James M Essig December 5, 2022, 13:31

    Awesome indeed! I hope NASA sends an ice melter probe to one of the tectonic plate boundary cracks.

    I would love National Geographic to do a story in the 2050s about any discovery of extra-terrestrial fish.

    I can imagine organically farmed Europian fish. I might try a one kilogram fellet if they could get the price down to $100/kg.

  • Mike Serfas December 5, 2022, 19:19

    It’s time to put up the bat signal regarding geologic activity – potentially leading to volcanic eruptions and floods of liquid water – on modern day Mars: https://phys.org/news/2022-12-giant-mantle-plume-reveals-mars.html With InSight somewhere in the area…

    Big problem: I didn’t find an ArXiv link for this paper.

  • Marc linquist December 11, 2022, 15:58

    Gravitational tidal forces from Jupiter would probably be enough to produce the needed surface tension to cause the divergent boundary separation. This would then fill with water from below to freeze in situ, forming a future buttress that the tectonic crustal ice sheet now lie against. Later, as the gravitational tidal force (GTF) wane and reduce its pull on the crust, it would leave the overall crustal ice plate matrix with a net positive internal load of gravitational potential energy (GPE) as it supported itself between its newest divergent boundary infill of ice and the overall planetary wide crustal matrix. The GPE load will over time be distributed into the form of those observed raised ridges that crisscross the crustal surface. This is the long term method of storing GPE on both Europa and Earth. On Earth there are two levels that develop depending on the level of GPE available. Most often the energy is diverted into subduction when a weak compression ridge fails to become a convergent boundary. And at rarer times, the convergent boundaries are overwhelmed by greater GPE levels and the energy is diverted into mountain complexes, they form many of the mountain ranges we see that were formed simultaneously over the last 5 million years that are referred to as the Neotectonic Period.
    https://www.electroplatetectonics.com/

  • ljk December 15, 2022, 19:34

    Enceladus’ ocean even more habitable than thought

    Posted by

    Paul Scott Anderson

    December 15, 2022

    https://earthsky.org/space/enceladus-ocean-alkaline-habitability/

    The new study here:

    https://astrobiology.nasa.gov/news/details-of-enceladus-subsurface-ocean/

  • ljk December 20, 2022, 12:08

    Europa’s icy crust may let more material into hidden ocean than thought

    By Sharmila Kuthunur published about 2 hours ago

    “It’s like the Titanic times 10.”

    New research shows that even smaller impacts can warm and soften enough ice on Jupiter’s moon Europa to send material sinking into the underlying ocean. This finding explains a new transport mechanism that could deposit important ingredients into the moon’s ocean.

    The icy shell of Jupiter’s moon Europa is marked with craters, most of which are from small impacts that dent the moon’s surface but are not big enough to penetrate all the way to its underlying ocean. Now, researchers have shown that impacts that penetrate even halfway through the ice shell accumulate enough meltwater to sink through the rest of the ice and into the underlying ocean.

    “Once you get enough water, you’re just going to sink,” Evan Carnahan, a doctoral student at the University of Texas at Austin and lead author on the new research, said in a statement. “It’s like the Titanic times 10.”

    Full article here:

    https://www.space.com/europa-comet-strikes-life-melt-ice-shell

    The research is described in a paper published Nov. 28 in the journal Geophysical Research Letters.

    https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022GL100287

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