Comparative tectonics on ten wolds II - The outer Solar System

Io with erupting Tvashtar Catena from Wikipedia

5. Io:

Now imagine a world with so much internal heat that, unlike Venus, where volcanic overturn punctuates long periods of quiescence, its outer layers are constantly overturning. That world is Io, with its powerful tidal heating. The energy imparted by tidal heating is two orders of magnitude greater than that of radiogenic heat. That's lots of heat to dissipate. It's surface heat flow is 30 times that of Earth. As a result, Io experiences constant and widespread volcanic activity characterized by long-lived eruptions. (Two eruptions are visible in the Galileo image at right).

Differentiation: Although inhabiting the outer Solar System, Io is a terrestrial planet in terms of size and composition, but enriched in more volatile elements, particularly sulfur. As hot as it is, it is hard to see how Io could avoid being differentiated.

Hi'iaka Patera from NASA
Lithosphere tectonics: Current conjecture is that weight of constantly deposited volcanic material drives the movement of small blocks of lithosphere on a local scale but that there is no global lithospheric movement. Hard to tell, of course, given that so much is buried under fresh volcanic deposits.


Europa surface with lineae and domes from Wikipedia

6. Europa:

A rocky world with icing.

Topography: Europa is the smoothest solid object in the Solar System. The surface is of ice marked by:

From Wikipedia
The great debate addresses how much of Europa's icy mantle might be "molten" - a subsurface ocean of liquid water. One model holds that the mantle is ductilely deforming warm ice. The other that liquid salt water lies anywhere from 30 km to mere hundreds of meters beneath the brittle surface. Recent works favors the liquid ocean model:

Does the water of the deep ocean ever gush out of these cracks? Roth et al. 2014 report observations from the Hubble Space Telescope of what appeared to be a cloud of water vapor over Europa's southern hemisphere in 2013. The first direct evidence for Enceladus-style eruptions? Future observations are needed to verify.


Ganymede from Wikipedia

7. Ganymede:

Density and moment of inertia indicate roughly equal amounts of ice and heavier stuff (silicates and metals). Thus, Ganymede has:
  • Core: made of iron sulfide and iron
  • An Inner mantle made of silicates
  • An Outer mantle made of ices

    Magnetic surprise: The Galileo spacecraft determined that Ganymede's magnetic field is roughly three times that of Mercury. That's much stronger than expected. It seems to be generated by electrical currents in a Europa-style sub-surface ocean of salt water. In Ganymede's case, this ocean is separated from the surface by a thick icy lithosphere.

    Lithosphere tectonics: We have mentioned the dichotomy of its surface between ancient crater-saturated terrain, and slightly younger grooved terrain reminiscent of Europa's surface. Tectonically, Ganymede seems to be a large fossilized version of Europa. Ganymede apparently experienced greater tidal heating in the distant past than it does now, due to changes in its orbital eccentricity, such that for an interval, it experienced enough tidal heating to power the formation of grooved terrain and occasional cryovolcanoes, but this did not last long enough for Ganymede's entire surface to be remodeled, as Europa's has. Ganymede's wild time is long past and both the ancient dark terrain and the grooved terrain are quite ancient.


    Dione from Wikipedia

    8. Dione - a representative small icy world:

    Compositionally, the smaller icy worlds are like Callisto or Ganymede - up to one half ice in addition to rock and metals. Their surfaces typically are ancient and saturated with craters, but often display parallel arrays of fractures - "normal faults" that form graben valleys resembling stretch marks. Thought to result from their actual expansion as they have cooled and more of their water has frozen. Dione (right) is our example but most of Saturn or Uranus' moons fit this pattern (not Enceladus or Miranda!).

    Recently, Hammond et al., 2016 have noted similar tectonics on Pluto, suggesting that it, too, has or once had a subsurface "magma" of liquid water.


    Parallel ridges in Xanadu region of Titan from NASA Cassini

    9. Titan:

    The study of Titan tectonics is in its infancy, however evidence is beginning to suggest that Titan's interior is as unique as its surface.


    Enceladus with tiger-stripes from Wikipedia
    10. Enceladus: Lest we should forget, tiny Enceladus shows a dichotomy between ancient cratered terrain and young grooved terrain like Ganymede's, all painted white by snow falling from its giant water/ice geysers. But the big differences:


    Weird Miranda from Wikipedia
    Bonus worlds for the future! We have much more to learn about comparative tectonics!

    Alas, no expeditions are planned for your lifetimes. Maybe your grandchildren will see new information from these worlds, but we must be satisfied with what Voyager 2 and New Horizons have given us.

    Key concepts and vocabulary:
    Additional reading: