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.
- Core: Density indicates an iron and sulfur rich core - probably molten. Absence of a global magnetic field suggests that the core does not have strong convection currents.
- Mantle: Silicate. Models indicate a rapid mantle convection. Data from the Galileo probe indicate the presence of a thin layer of liquid magma (10-20% of mantle mass) beneath the lithosphere. An asthenosphere on steroids.
- Lithosphere: The lithosphere must be very thin - maybe 40 km?
Hi'iaka Patera from NASA
- Io has no known impact craters, therefore its entire surface is very young, the result of constant volcanic resurfacing.
- Just as volcanic activity drives the differentiation of oceanic and continental crust, Ionian volcanism is speculated to drive a differentiation of the upper and lower lithosphere.
- Io's volcanoes seem to tap two distinct types of magma:
- Silicate magma similar to Earth's (E.G. Tvashtar Catena.)
- Sulfur-compond magma, including sulfur dioxide (SO2). (E.G. giant plume eruption)
- Over 60% of Io's surface is volcanic. Oddly, Ionian volcanoes do not form elevated mountains. Instead, they appear as paterae (sing. patera), large rimmed depressions. But Io has mountains up to 16 km. high that seem to be formed by local lithospheric motion. The mountains are not volcanic, but are frequently associated with volcanoes or form the rims of paterae. Hi'iaka Patera, (right) is flanked by a pair of highland blocks that look as if they should fit together.
Europa surface with lineae and domes from Wikipedia
6. Europa:A rocky world with icing.
- Despite its icy outer layers, Europa's density is close to that of terrestrial bodies (see above).
- It has a metallic core and rocky mantle, like Io only smaller, masked by a thin "outer mantle" of ice (~130 km).
- Like Io, Europa experiences tidal heating, although considerably less because its orbit is less eccentric. Nevertheless, the fact that it is locked in a 2:1 resonance with Ganymede insures that tidal heat will be a continuous feature.
Topography: Europa is the smoothest solid object in the Solar System. The surface is of ice marked by:
- Rare impact craters, indicating that Europa's surface is a maximum of 30 Ma.
Europa lineation types from Amazon Web Service
- Lineae (lines and bands). These are classified as:
- Troughs: Simple elongate cracks (above - a)
- Double ridges: Parallel ridges separated by a trough (above - b)
- Bands: Sets of parallel ridges (above - c)
Cycloid lineae from The Escapist
- Cycloidal: Sets of arcing line segments (right).
- Small domes and depressions.
Conamara Chaos from Wikipedia
- "Chaos" - regions of chaotically fractured ice fragments. (E.G. Conamara Chaos Image scale is 50x30 km.)
- Surface topography seems consistent with the idea of a liquid ocean. Schmidt et al, 2011 propose that chaos regions marked the locations of large pockets of water rising through the solid crust.
- Magnetic enigma: Unlike Io, Europa has a weak global magnetic field. We have no reason to expect its metallic core to convect - Io's doesn't. But it could arise from electrical currents in saline water.
- "Line tectonics": Superficially, Europa's band lineae look like Earth's sea-floor spreading zones. Does some combination of warm ice, slush, or water well upward along fractures created by Jupiter's tidal forces, creating new crust? Probably not much. Even the broadest bands are narrow. Nevertheless, Kattenhorn and Prockter, 2014 claim to have identified subduction zones that correspond to them. (Link to schematic image.)
- Tidal flexing: Using cross-cutting relationships, it is easy to identify relatively young lineae. We would expect them to be aligned with the direction of tidal stretching. In fact, the most recent ones are, but older lines are increasingly misaligned. Odd. We'd expect Europa to exhibit synchronous rotation. Apparently Europa's crust is mechanically decoupled from its mantle and rotates at a slightly different rate from the mantle and core. Strong evidence for a liquid water layer.
- Daily cracks: Using a geophysical model, Hoppa and Tufts, 1999. have shown that each arc of a cycloid linea represents the propagation of a crack over a single Europa day. We can actually count how long it took them to form!
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:
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.
- Differentiation: Not clear, earlier estimates held that Titan was not well differentiated into ice and rock - maybe closer to Callisto than to Ganymede. There seems to be:
- a partially differentiated silicate-metallic core
- an icy mantle and crust.
- Unlike the small icy moons, Titan's topographic features seem to result from compression, not expansion as in smaller icy moons. It is massive enough that much of its icy mantle is thought to take the form of high-pressure ice, a crystal morph of ice that is actually more dense than liquid water (Mitri et al. 2010). As a consequence Titan has shrunk rather than expanded as it has cooled, like an icy version of Mercury and in stark contrast to worlds like Dione. This is thought to be the source of its enigmatic non-volcanic mountains (right).
- Hemingway et al. 2013, have revealed that Titan's mountains are associated with negative gravitational anomalies, indicating that they "float" on deep roots that extend into the subsurface ocean.
Enceladus with tiger-stripes from Wikipedia
- The youngest grooves in Enceladus's grooved terrain, called "tiger stripes" have water gushing from them. Do these grooves expand over time, like Earth's mid-ocean rifts? Do they simply shut down when newer volcanic grooves form? That answer will await a new generation of robot spacecraft.
- Current research (E.G. Nimmo et al., 2007) assumes that Enceladus' tiger-stripes formed through tidal flexing, as did Europa's lineae. Confirmation comes from:
- The observation that older fractures are offset from the direction of tidal stress like in Europa. That would indicate that Enceladus also also has a global subsurface ocean that decouples its crust from its interior.
- But note: Iess et al., 2014 revealed that Enceladus' subsurface ocean is concentrated in the southern hemisphere, based on moment of inertia data. How far that ocean currently extends toward (or past) the equator is debated. Cadek et al., 2016 have the last word for now, reporting that:
- Enceladus' ocean is global, decoupling the solid lithosphere from the rocky core
- Enceladus' lithosphere is on average only 8 - 22 km thick, but that it thins to about 5 km in the south polar region!
- Of course, the beauty of Enceladus, is that its ocean spews out its cracks. Indeed Hedman et al., 2013 indicated that the amount of water erupted varies with Enceladus' orbital position as Saturn's varying tidal pull opens and closes the volcanic conduits when Enceladus is in different positions in its orbit.
Thus, the tidal origin of Enceladus' cryovolcanism seems pretty clear. Not surprising considering that its orbital eccentricity and period (e=0.0047, p=1.370 days) are similar to those of Io (e=0.0041, p=1.769 days) and that this is maintained over time because Enceladus and Dione have a 2:1 orbital resonance.
Weird Miranda from Wikipedia
- In the Uranus system, Miranda (right) possesses a surface dichotomy and evidence of cryovolcanism at least as strong as what we had for Enceladus, prior to the arrival of Cassini. Maybe we need only a Cassini-style mission to the Uranus system to catch it in the act.
- In the Neptune system, weird Triton cries out for further study.
- As New Horizons data reveal Pluto's secrets, researchers will eventually get past their fixation on its nitrogen glaciers, and start looking at the behavior of the icy bedrock over which they flow. Hammond et al., 2016 represents the beginning.
Key concepts and vocabulary:
- Magnitude of heat flow
- Impacts absent - surface very young
- Extreme differentiation
- Patera (pl. paterae)
- Mountains associated with local movement of lithosphere
- Differentiation with thin ice/water mantle and crust
- Impacts rare - surface young
- Linea (pl. lineae)
- Underground ocean or soft ice?
- Line tectonics
- Magnetic field
- Differentiation with thick ice mantle and crust
- Magnetic field suggests deep liquid water layer
- Linea (pl. lineae)
- Surface dichotomy
- Ancient cryovolcanoes
- Small icy worlds
- Arrays of parallel fractures
- Expansion of ice upon cooling
- Partially differentiated with thick ice mantle and crust
- High pressure ice
- Non-volcanic mountains reminiscent of Mercury's scarps.
- Tiger stripes
- Active worlds for future study
- Ondrej Cadek, Gabriel Tobie, Tim Van Hoolst, Marion Masse, Gael Choblet, Axel Lefevre, Giuseppe Mitri, Rose-Marie Baland, Marie Behounkova, Olivier Bourgeois, and Anthony Trinh. 2016. Enceladus's internal ocean and ice shell constrained from Cassini gravity, shape, and libration data. Geophysical Research Letters June 2016, preprint.
- M. M. Hedman, C. M. Gosmeyer, P. D. Nicholson, C. Sotin, R. H. Brown, R. N. Clark, K. H. Baines, B. J. Buratti and M. R. Showalter. 2013. An observed correlation between plume activity and tidal stresses on Enceladus. Nature 500, 182-184.
- L. Iess, D. J. Stevenson, M. Parisi, D. Hemingway, R. A. Jacobson, J. I. Lunine, F. Nimmo, J. W. Armstrong, S. W. Asmar, M. Ducci, P. Tortora. 2014. The Gravity Field and Interior Structure of Enceladus. Science 344(6179) 78-80.
- Giuseppe Mitri, Michael T. Bland, Adam P. Showman, Jani Radebaugh, Bryan Stiles, Rosaly M. C. Lopes, Jonathan I. Lunine, Robert T. Pappalardo. 2010. Mountains on Titan: Modeling and observations. Journal of Geophysical Research 115(E10).