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Cryovolcanism and Geologic Analogies

Part One: Analogous Volcanic Activity

You might be wondering what I mean by “geologic analogies.” Well, you have good reason because it’s not immediately obvious. What I’ll be talking about here is water. Yes, water can imitate rock…once it’s frozen. Where can you find this kind of behavior? Well, you can probably find it in the arctic and sub-arctic regions in some places, but the only place I can personally verify it occurs is around the Great Lakes of the US.

Have you ever heard of an ice volcano? They occur, and erupt, along the shorelines of Michigan (and likely Wisconsin, Minnesota, and Ontario). I’ve photographed the features on Lakes Michigan, Huron, and Superior. Whether or not they occur on the other lakes is unknown to myself (and is questionable due to their smaller size). The structures typically occur within several hundred meters of shore, but always over the lake. I’ve observed them to range in height from about one foot to over ten feet.

The formation of ice volcanoes requires several conditions to be met. First, the air temperature must be below freezing (otherwise, there will be no ice…obviously). Second, there must be high surf (which is why smaller lakes don’t have these features). The large waves, trapped by the ice, punch through the surface and erupt. In this way, ice volcanoes are strikingly analogous to littoral cones at basaltic volcanoes. The energy of the waves must find a point to disperse, and do so by punching up through the crust of ice. In this way, also, the water can be thought of as molten, and the ice as rock. This can seem abstract as we’re all used to seeing water as liquid, but it is a molten solid at room temperature the same way basalt is a solidified liquid at room temperature.

The third condition required for the formation of ice volcanoes can come in one of three forms. That condition is this: there must be a feature that causes the waves to punch upward at a certain point.

Those of you who have taken physics or live near the open water know that wave height increases when it moves over shallower water. Consequently, waves swell when they approach the shore or move over a sandbar.

And this is what happens. The waves pass over a sandbar or approach the shore and grow to a point at which they punch through the ice. This means that arcs of ice volcanoes typically follow the shore and trace out sandbars nearby. This is probably also why ice volcanoes can’t be found far away from land when the ice sheets spread. The eruptions are usually pretty short lived because they require windy conditions and a small ice field. That is, once the ice sheet extends to a great distance beyond the volcanoes, wave energy will be dissipated into the ice before it reaches the eruption site. Moreover, the lake ice will thicken and may clog eruptive vents.

The following will read more as a field description from a one-hour hike through a pair of ice volcano arcs along the shore of Lake Superior near Marquette:

Coming down from the road, it is difficult to determine where the underlying beach ends and the lake begins. Where beach is exposed, the sand is locked in a matrix of ice. This results in some unusual sandstone-style erosional features. The first volcanic arc appears to be only a few meters from the shoreline…perhaps twenty or thirty at the most. This arc is composed of extinct volcanoes averaging 3-5 feet in height and spaced 5-15 feet apart. One or two meters beyond this arc lies a small cliff, one to two feet in height. Perhaps 60 yards out from this lies the second arc, possibly tracing a sandbar or submerged ridge of rock. These volcanoes are significantly larger, ranging from 5-20 feet in height. The volcano bases tend to be around three times the height in diameter. All activity is long dead and ice on the lake-ward side of this arc is a foot or two lower in elevation from the rest. There are no visible volcanoes beyond this arc. Ice is covered in a layer of firn-like material; that is, a very icy snow. In many places (at times, surprisingly far from the volcanoes) the ice is actually a conglomerate. It is composed of transparent ice clasts locked in a matrix of very “rough” looking ice. Clasts are subrounded and range from 2-10 centimeters along their long axis. Many have an unexpected half-moon shape. Inside the clasts are small grains of sand in very small abundance. Near the lakeshore is a beautiful fault line, with several centimeters of lake-ward horizontal displacement (no vertical, as opposed to the ice cliffs elsewhere). Not only does this fault show extensional features, but it also displays wonderful small (2-4 cm) strike-slip features in its structure.

Part Two: Cryovolcanism

So why, you might be wondering, did I spend so much time talking about this “analogous activity?” Cryovolcanism refers to a volcanic eruption…of ice and water (cryo meaning frozen, as in cryogenic). If I had started this article with that concept, you might have thought it was bizarre. Now it doesn’t seem so strange. This word is only used to refer to these eruptions on other planetary bodies. Where does this occur and how do we know? Well, that gets more interesting.

Welcome to Ganymede

For a time, Ganymede was thought to be the perfect example of cryovolcanism. It was thought that the smooth surfaces photographed by the Voyager spacecraft were the result of resurfacing via volcanic processes (pic, planetary structure). This theory was shot down, however, when higher resolution images taken by the Galileo satellite showed that the surface is actually highly fractured by large rifts (example). The icy surface shows evidence of both ductile and brittle deformation, but no fluid flow structures (pic). It is possible that volcanic processes have occurred there long ago, but all evidence currently points to tectonic processes. We’re not looking for places that could have experienced cryovolcanic activity, but for bodies that are actively showing this behavior.

Hope on Europa

Here we move to another satellite of Jupiter. The crust of Europa is composed entirely of ice, but it is thought to possess a sizeable liquid-ocean below (planetary structure). This moon shows a striking lack of topography, which is evidence of extensive resurfacing. The highest peaks of Europa reach only several hundred meters and the few impact craters present are even smaller. The body is crisscrossed by a sizeable network of ridges that seem to be resurfacing the planet.

Not everything is the way it appears, however. Cryovolcanism, it seems, is not nearly so important as we once thought. For example, many features that were once thought to be ice-lava flows may actually be the result of warming from below. To picture this, imagine scratching up a block of ice; you next warm it to 32 o F. What will happen is a gradual disappearance of the surface features; eventually the surface will refreeze and be relatively scratch-free. On Europa, some locations seem to have been heated and re-melted from below. This is partly evidenced by large isolated blocks of heavily carved ice atop these regions. It is thought that these blocks represent the old ice and were floated along the surface atop a thin layer of dense water-like material.

This doesn’t kill the idea of cryovolcanism on Europa, however. There remain strange flow-like features and the ridges are particularly suspicious. The ridges show numerous parallel ridge-like features within them, indicating an extensional volcano-tectonic origin, much like what occurs along our oceanic rift zones.

What is the driving force for these rift eruptions, though? Well, like Io, the answer is probably “tidal forces.” Jupiter’s gravity should cause tides to rise several dozen meters beneath Europa’s icy crust. This may cause the rifts to open and close each orbit, allowing the interior water-ice slurry to rise upward toward the crust (since liquid water is less dense than ice).

Moving toward Enceladas, Titan, Tethys, and Dione

Tethys and Dione are both poorly imaged and so we ought to be cautious when talking about volcanic activity. Both show graben-like features (which are surface collapse features); Dione has thin wispy terrain nearby which may be the result of frost from an energetic cryovolcanic eruption.

Far more promising, however, are Enceladas and Titan. Enceladas shows extensive resurfacing, but more importantly, it has the highest albedo of any satellite in the solar system. Albedo is a measure of how much light is coming from a surface, usually reflective. This high albedo is thought to be caused by an H2O frost (since the albedo closely matches that of freshly fallen snow). Large faults and fractures cut across the surface while it lacks the expected abundance of impact craters. We don’t, however, have a definite reason for it to be volcanically active. Tidal heating due to the gravitational conflict between it, Saturn, and the moon Mimas is simply inadequate. It’s small size and composition effectively rules out radiogenic heating (such as occurs on Earth). It may have been influenced by other moons in the past, which are now gone. Interestingly, Enceladus lies within the densest portion of Saturn’s outermost ring. It’s currently consuming the ring, but also seems to be its source. The ring probably formed in response to a massive impact in the past (which may have supplied that mysterious heat), although it may have also formed through vigorous volcanic venting.

Saturn’s moon Titan, however, makes Enceladas look mundane in comparison. To begin, this moon is itself larger than Mercury or Pluto. It’s large size and density allows for radiogenic and gravitational melting of the interior to compliment the tidal friction caused by it’s eccentric orbit around Saturn. Titan has a thick atmosphere that keeps the surface relatively warm (-197 o C), which is composed of mostly of nitrogen. Interestingly its atmosphere is four times denser than ours and exerts 60% more pressure at its surface than Earth’s. Sizeable quantities of methane also inhabit the atmosphere while HCN, CO, and Ar are all present as well. This strange composition has fueled speculation as to this atmosphere’s source. It is thought that it may have formed as ammonia erupted cryovolcanically from Titan’s surface. This ammonia was then transformed gradually to nitrogen through ultraviolet photolysis and the emission of hydrogen.

The real kicker for proposed cryovolcanic activity is the remaining presence of methane. This is because methane is destroyed via uv photolysis as well. The fact that it still exists in great quantities indicates that it may still be erupted from the surface at this time. At Titan’s surface temperature and pressure, methane behaves much the same way water does on Earth. This has caused some experts to speculate that its surface may contain methane lakes and experience methane rain and snowstorms. The dense atmosphere and possible lakes probably inhibit the exsolution of gases meaning that the ice-based lavas are probably effusive and may form pillow-style lavas.

On the right: Titania, Ariel, and Miranda

In the moons of Uranus we can see some unusual features. Umbriel and Oberon are both heavily cratered, as would be expected. The 35% of Ariel’s surface that was imaged by Voyager showed the lowest crater density of any of the Uranian satellite, though images by Voyager 2 showed that it contained many more crater than previously thought (still fewer than one might expect). More importantly, it has what appear to be channel-like structures (though this should not be regarded as great evidence since the images are low resolution [remember what happened at Ganymede]). There seem also to be graben structures on the surface, with possible flows emanating along the floors. This may have been the result of crustal extension caused by expansion, a global loss of spin, or a decrease in the tidal bulges. Some craters lie partially buried by these flow structures are around a kilometer thick and show significant topography, indicating that the flow was highly viscous (comparable to rhyolite). The most likely flow composition, if these are flows, would be an icy ammonia mix with methanol or a methanol-like chemical to add viscosity.

Titania shows large tectonic features as well, which include planet-sized rifts. Some of these rifts form grabens tens of thousands of square kilometers in area and up to 5 kilometers deep! Titania lacks significant evidence for cryovolcanism, but it’s highly active tectonic setting suggests that evidence may exist.

The big mystery is the moon Miranda. Miranda is only 470 km in diameter (a little larger than Connecticut), yet is carved nearly beyond belief. Enormous faults and coronae mar the surface, though not so many craters. One particular fault, near Inverness Corona, shows 10 km of sharp relief. A leading explanation for the strange appearance of Miranda is that it collided with another moon or asteroid long ago. The combined planetary body was thus slammed together chaotically. With dense material scattered about the surface and possibly light material nearer the center, Miranda might have experienced density-driven changes with heavy material sinking and the light rising. Melting may very well have been involved and are somewhat evidenced by extremely smooth areas within Elisnore Corona.

Finally to Triton

At Neptune we find the final evidence of cryovolcanism in our solar system, on its moon Triton. Unlike other moons where we have evidence of cryovolcanism, at Triton we actually observed activity! As Voyager 2 passed under its south pole, two plumes could be seen jetting to around 8 km above the surface and traveling 150 km downwind. In addition to this, more than 100 additional dark streaks cross the polar region, indicating significant past activity. Closer to the equator, bizarre “cantaloupe-like” structures seem to be formed by volcanic flooding while some craters seem to have been resurfaced by a fluid material. With a surface temperature of –235 o C, nitrogen readily freezes. It seems, however, that the surface may see seasonal temp fluctuations high enough to occasionally turn nitrogen to a fluid. We have detected abundant nitrogren and methane at the surface with smaller quantities of carbon monoxide and carbon dioxide ices. Water is also expected to be present, but masked by the other materials (and thus unidentifiable from recent photographs). Triton has a thin atmosphere composed of these ices, which traverse the planet, pole-to-pole, every year making it isothermal.

It is thought that the nitrogen geysers may be formed through heating of the surface by the sun, although the sun appears merely as a bright star here. Triton’s southern hemisphere was just approaching its summer solstice during the flyby, which indicates that activity may also be seasonal. Possibly dark carbon-based ices beneath the nitrogen may absorb light and trap heat; since an increase of 10 C from the surface temperature will cause nitrogen to expand 100-fold, this may be causing the eruptions. One year for Triton equals about 165 of ours; moreover its orbit is extremely bizarre. Triton’s orbit is oblique (not circular) and opposite to the rotation of Neptune, making it the only moon in the solar system to do so. Its origin is probably the Kuiper belt and was likely captured somewhat violently through a collision with another moon or drag through Neptune’s atmosphere.

Conclusion

Well, there you go. Hopefully you’ve got a better understanding of how this bizarre form of volcanism acts. There are infinite possibilities for volcanism in our solar system and beyond, though at the moment this science is purely academic. There is no practical reason for pursuing this science at the moment, though hundreds of years from now, who knows? It should give you an idea of how variable this science is and in how many ways it can be applied. Finally, I hope you’ve seen how analogous volcanic processes can occur on earth, particularly along the great lakes. As already mentioned, there is no real useful application to this science at present, but it is interesting and may prove important for future generations. If you’re interested in learning more about the subject, you’ll have to look toward scientific journals.






Helpful pages on individual moons:
Europa
Ganymede
Enceladas
Titan
Tethys
Dione
Ariel
Titania
Miranda
Triton