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Europa: Two Takes on Plate Tectonics

Could plate tectonics occur on Europa? It’s an intriguing notion because the surface crust of the Saturnian moon offers oxidants and other chemicals useful for life. The process called subduction, in which a tectonic plate slides underneath another plate and sinks, would offer a way for these materials to come into contact with the subsurface ocean. We would have, in other words, a first-class mechanism for feeding the ocean with nutrients.

Image: Previous studies have hinted that something like subduction may have been happening on Jupiter’s moon, Europa. A new study provides geophysical evidence that it could indeed be happening on the moon’s icy shell. Credit: NASA/JPL-Caltech/SETI Institute.

There has been earlier evidence of possible plate tectonic activity on Europa, based on studies of surface geology. In places, the icy crust seems to be expanding, and here we can draw an analogy with mid-ocean ridges on Earth, which spread as a result of tectonic activity. In our oceans, material from the crust can sink into the mantle because it is cooler than the mantle material, and therefore denser. Negative buoyancy results, empowering subduction.

A temperature gradient could be in play on Europa. One line of thinking is that the ice shell on the moon is bi-layered, with the thin and cold outer layer atop a somewhat warmer layer of convective ice. Push colder ice from the surface into the warmer ice and it would sink until it warmed to the temperature of the ice around it. The image below illustrates how this might work, and is based on work that was done in 2014.

Image: A subduction model for Europa based on temperature, drawing on a 2014 paper by Simon Kattenhorn (University of Idaho) and Louise Prockter (JHU/APL), This conceptual illustration of the subduction process shows how a cold, brittle, outer portion of Europa’s 20-30 kilometer-thick ice shell moved into the warmer shell interior and was ultimately subsumed. A low-relief subsumption band was created at the surface in the overriding plate, alongside which cryolavas may have erupted. Credit: Noah Kroese, I.NK.

But a new study out of Brown University hints at another, perhaps more robust option. The work of Brandon Johnson, who developed it with the help of graduate students in a class on ocean worlds he was teaching, the paper looks at differences in salt content on Europa. The team’s computer modeling tracked the temperature and compacting of ice as it sinks into warmer ice below, analyzing density changes as warm ice becomes easier to compact. The temperature, salt content and level of ice compaction determine the buoyancy of the ice.

Johnson’s work shows that variations in salt content in Europa’s ice shell of no more than a few percent can cause subduction to occur. Moreover, we have several mechanisms for producing these variations in salt, with upwellings of water from the ocean — suggested by surface geology — offering similarities to magma rising from Earth’s mantle. This process, along with possible cryovolcanism, could deliver salty ocean water directly onto the surface.

Thus we have a mechanism for making salt variations global on Europa, and a process that can feed the ocean below with nutrients. If subduction is occuring on Europa, it would not necessarily be dependent on temperature differences. Or perhaps both processes are in play. In any case, Europa would be only the second place in the Solar System where plate tectonics occurs, and it would carry astrobiological implications. “If indeed there’s life in that ocean, subduction offers a way to supply the nutrients it would need,” Johnson says, and adds:

“It’s fascinating to think that we might have plate tectonics somewhere other than Earth. Thinking from the standpoint of comparative planetology, if we can now study plate tectonics in this very different place, it might be able to help us understand how plate tectonics got started on the Earth.”

The paper is Johnson et al., “Porosity and salt content determine if subduction can occur in Europa’s ice shell,” accepted at the Journal of Geophysical Research: Planets 4 December 2017 (abstract). The earlier Kattenhorn and Prockter paper is “Evidence for subduction in the ice shell of Europa,” Nature Geoscience 7 (2014), 762-767 (abstract).

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{ 16 comments… add one }
  • John walker December 5, 2017, 17:37

    Katterhorn and Prockter’s ngeo article is still behind a pay wall but later received a free 20page supplement which describes their deductive process. https://images.nature.com/original/nature-assets/ngeo/journal/v7/n10/extref/ngeo2245-s1.pdf
    Wiley has the full JGR paper from Johnson et al on offer: http://onlinelibrary.wiley.com/doi/10.1002/2017JE005370/pdf

  • Alex Tolley December 5, 2017, 17:56

    An interesting idea on its own, but I doubt it has much impact on the issue of life. Yes, it moves oxygen species from the surface to the ocean, but these are slow processes and life does not need oxygen. The terrestrial analogy might be that photolysis adds O2 to the early atmosphere, but of course, this wasn’t needed for abiogeneis, and photosynthesis is the main source of O2 that is then used by the evolution of aerobic respiration.

    If the dissolved O2 is high enough, it might just make terraforming the Europan ocean that much easier, allowing a low density of marine organisms to survive without needing photosynthesis to support them.

    • Mauldred December 6, 2017, 10:42

      Indeed, I would also rather think that the necessary nutrients for life on Europa are coming nowadays more from the seafloor hydrothermal vents than from the icy crust (carbonated asteroids and sulfur traces transported from Io’s eruptions along Jupiter’s magnetic field lines probably give a little help but not so much).

    • John walker December 6, 2017, 16:36

      Were Europas subduction rates similar to Earth’s 20-80mm/year then complete resurfacing would be possible on time scales commensurate with the crater record(50Mio a). Radiolytic oxygen species combined with iogenic sulphur, Na and Cl could influence oceanic composition thus generally influencing biospheric conditions even if they were to play no direct role in a nutrient cycle. However, Katterhorn and Prockter currently believe that the putative subduction seems not to exit into the ocean but rather in a malleable (warmer) lower ice shell.

  • Rob Flores December 6, 2017, 0:53

    Do we know if the ice furrows on Europa have changed in any
    large scale(visible) way? since voyagers visit (some ridge line shifting etc) ?

    This presupposes that the surface ice sheet is detached from the inner core (whether icy or not). is that a requirement for this type
    tectonic activity? Reason to ask is that while I can imagine a core
    and ice sheet differential rotation rate, I can’t imagine It lasting
    4.5 billion years. at some point due to friction the core and surface
    Ice sheet would come to have minimal rotational difference.

    If that is so, then the furrows maybe straight up gravitational
    uplifiting of the ice crust. which would not be a very weak
    tectonic plate effect

    • John walker December 6, 2017, 16:45

      The expected movement is well below the limits of our instruments. Probably only millimeters per year. On your second point, tectonic processes don’t depend on differential rotation but rather thermal convection. Btw. the ice shell is separated from the silicate core. Europa is thought to have a global ocean.

    • John walker December 6, 2017, 17:12

      The expected movement is well below the limits of our instruments. On your second point, the tectonic processes here don’t depend so much on any potential asynchronous rotation but rather buoancy differentials and thermal convection. Btw. the ice shell is indeed though to be separated from the silicate core. Europa would thus have a global ocean.

  • Michael Fidler December 6, 2017, 9:17

    Something else may be increasing the salt as well as producing the convection on Europa to fuel the plate tectonics. Impacts from comets, Jupiter is continual pelted by comets and asteroids, the 1994 Comet Shoemaker–Levy 9, Anthony Wesley spotted a large Jupiter impact event in 2009 and numerous other bright flashes. Historical observations going way back have also been drawn and reported. The point is that both Ganymede and Callisto have grazing and fresh craters on their surface so with Europa closer to Jupiter the chances of impacts from objects embedded in its gravity well (Hill sphere) should be more common. Jupiter has a whole of family comets that are short-period with orbital periods less than 20 years around the sun. Over the long time periods many of these could eventual impact Jupiter as well as the large satellites of Jupiter. This could also be part of the reason that Io is so active.
    M Dwarfs systems such as Trappist 1 would also have similar activity since it is just like Jupiter in diameter but much more massive and has a massive Kuiper belt of comets. Short orbital periods of its seven planets would also increase the likelihood of impacts and would probably be on the orbital paths forward facing hemispheres of these tidally locked worlds. Just as on Earth the chance of seeing meteors are higher in the early morning sky.

    • Michael Fidler December 6, 2017, 22:47

      Organic molecules make up half of Comet 67P.

      Researchers present list of comet 67P/Churyumov-Gerasimenko ingredients.

      https://phys.org/news/2017-12-comet-67pchuryumov-gerasimenko-ingredients.html

      “The dust that comet 67P/Churyumov-Gerasimenko emits into space consists to about one half of organic molecules. The dust belongs to the most pristine and carbon-rich material known in our solar system and has hardly changed since its birth.
      The current findings also touch on our ideas of how life on Earth came about. In a previous publication, the COSIMA team was able to show that the carbon found in Rosetta’s comet is mainly in the form of large, organic macromolecules. Together with the current study, it becomes clear that these compounds make up a large part of the cometary material. Thus, if comets indeed supplied the early Earth with organic matter, as many researchers assume, it would probably have been mainly in the form of such macromolecules.”

  • Michael December 6, 2017, 13:14

    I think it is possible to ‘decompose’ our way through the ice into the ocean below by using a long fibre optic UV laser. It would be incredibly hard but the view using the fibre optic as a light conduit would be amazing.

  • andy December 6, 2017, 13:21

    Any further updates on the question of whether the possible plumes erupting from Europa are real or not?

    • Paul Gilster December 6, 2017, 13:45

      As far as I know, the issue hasn’t been settled. Let me dig around a bit and see if there’s anything more recent.

    • John walker December 6, 2017, 15:24

      Latest I have is that ALMA observations failed to confirm a temperature anomaly that Galileo had detected near Pwyll Crater, the area identified as the origin for the 2 Hubble plume reports of 2014 and 2016.

  • Nicky December 8, 2017, 17:55

    I wonder if the tidal heating is enough to drive convection that is enough to cause subduction and also spreading.

  • Michael December 12, 2017, 11:03

    My issue with subduction and ‘tectonics’ is the colour, surely the whole surface should be red. Could it be that the salts are been concentrated in the surface of the subduction zones much like how salt is displaced out of seawater ice. These subduction zones would be of great interest as they are likely to host low melting point regions for ‘melting’ point probes and for that matter life. These salts offer nutrients for life as well as protection from the harsh radiation environment.

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