Icy Plate Tectonics?

Today we are going to talk about one of Jupiter’s moons, Europa, an interesting world often spoken of in discussions about the possibility of life beyond Earth. Europa has, geologically speaking, a young surface. By geologically speaking, I mean the surface is ‘only’ forty to ninety million years old. This means that somehow, the older material is being ‘recycled’ into new material. On Earth, this is done by the crust going back into the mantle, melting partially or entirely, and coming back up to the surface as a changed material. So how does Europa–an icy moon that appears so different from our home planet–accomplish this recycling?
Two views of Europa. The left is a true-color image; the right is false color to bring out the more subtle differences. Image taken by the Galileo spacecraft. Image credit: NASA/JPL/DLR.
Two views of Europa. The left is a true-color image; the right is false color to bring out the more subtle differences. Image taken by the Galileo spacecraft. Image credit: NASA/JPL/DLR.
There is evidence for expansion on Europa’s surface, dilational bands similar to mid-ocean ridges on Earth. On Europa, warm ice is pushed upwards and spreads the ridges,  much like lava does on Earth. But this means there must be some form of contraction as well; otherwise Europa would be getting larger!  There are a few features on Europa that suggest contraction, but not enough to account for the expansion without a net gain in size. So something else has to be happening.
Today’s paper is “Evidence for subduction in the ice shell of Euorpa“, by Simon A. Kattenhorn and Louise M. Prockter, published in Nature Geoscience in 2014. Using data from the Galileo spacecraft, this looked at the possibility of tabular zones that could be areas of subduction, where the area is being removed from the surface area of the moon as it goes beneath the initial icy crust. The authors found eight items that support these tabular zones as being areas of subduction:
  1. Geological features are not the same on both sides of tectonic features: across a tabular region, ridges or dilation bands stop suddenly, not continued on the other side (which you would expect if the rest of the feature is going beneath the ice at this location). Across other features (not tabular areas), ridges and bands are offset, indicating a different type of motion in the plates, such as lateral motion.
  2. Missing surface area: the authors used geological markers to carefully recreate the surface as it was before various geological processes–but they ended up with an area of about 20,000 square kilometers (7722 square miles) that was missing. When such recreations are done on Earth, the reason for missing surface area is subduction. On Europa, a better word may be ‘subsumption’: the icy crust is recycled into a deeper layer of warmer ice (see Diagram 1), which is not quite the same as Earth’s crust descending into the molten mantle.
  3. Plate motion: in the previously mentioned reconstruction, there were five steps. After each step, some variable(s) would change, and the plates would all shift in their motion in a congruent way. The same things happens on Earth.
  4. The terrain did not go into the topography: the expanded terrain did not contract onto itself–this would have resulted in thicker ice with mountains that are probably 330 to 400 meters (1080 to 1312 feet) high, of which there is no evidence from the datasets.
  5. Different forms: the tabular subsumption zones do not look like anything else on Europa or other icy satellites, implying they are something new, not before seen or understood.
  6. Cyrolavas: the terrain near subsumpion is higher than its surroundings. This appears to be from an upwelling of material below the surface; essentially lava  made of ice (cyrolava). This only occurs on the crust directly above the subsumption, a phenomena notably similar to volcanoes occurring on the overriding plates of subducting materials on Earth.
  7. Differences in color: subsumption areas appear differently in false color images, suggesting some sort of difference in composition or chemicals. This is not entirely exclusive to subsumption, as dilational bands are a similar color. It might be caused by exposure to different ices usually deeper than the crust (which also happens with dilational bands), and thus could be influenced by the aforementioned cyrolavas.
  8. Strain patterns: in one area, the overriding plate had small lateral faults from strain, resembling a pattern seen in terrestrial [plate tectonics]
Diagram 1: A diagram of how subsumption might work on Europa. Taken from Katternhorn and Prockter, 2014.
Diagram 1: A diagram of how subsumption might work on Europa. Taken from Katternhorn and Prockter, 2014.
So what can we infer? It looks like material is subsumpted into a layer of warmer ice below Europa’s surface. This is promoted by the outer ice being denser than the warmer ice below.
Subsumption would in theory thicken the ice shell in ways not visible in the datasets, depending on the rates of contraction and subsumption. Several factors cause motion of plates of Earth, but which ones are applicable to Europa are not yet known. Also unknown is the rate of subsumption, but if its rate is similar to that of subduction of Earth, then enough subsumption zones could account for the young surface of Europa.
It is also not clear how cyrolavas are created, but they do indeed seem to be present. This is only in subsumption zones, though, so perhaps the subsumption process creates melting, or an upwards pressure in the fluids within the warmer ice layer.
More subsumption zones than were observed would be needed in order to explain the resurfacing of all of Europa. However, this study only looked at a portion of the world. More research would need to be done, but Europa looks like it might be the only place in the solar system with all the features of plate tectonics, aside from Earth!
Bibliography
Sources used to write this article directly:
Jet Propulsion Labratory. “Catalog Page for PIA00502: Natural and False Color Views of Europa.” N.p., n.d. Web.
The JPL photojournal page for the images of Europa used in the beginning of this post.
Kattenhorn, Simon A., and Louise M. Prockter. “Evidence for subduction in the ice shell of Europa.” Nature Geoscience 7.10 (2014): 762-767.
Today’s main paper. Also used for the diagram of subduction on Europa.
Marshak, Stephen. Earth: Portrait of a Planet. 3rd ed. W.W. Norton & Compant. Print.
Geology textbook, as a reference for subduction and plate tectonics on Earth.
Further Links
National Aeronautics and Space Administration. “About Europa.” N.p., n.d. Web.
A page about Europa and its connection to the search for life, which I also linked to in the first paragraph of this post.
National Aeronautics and Space Administration. “Europa.” N.p., n.d. Web.
The home page of a NASA site about Europa.
National Aeronautics and Space Administration. “Europa: Facts & Figures.” N.p., n.d. Web.
A NASA page of facts and figures concerning Europa.
National Aeronautics and Space Administration. “Solar System Exploration: : Galileo Legacy.” N.p., n.d. Web.
A NASA page about the Galileo mission and its legacy.

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