Icy Mysteries of Europa

I originally had another topic planned for today—it was written and scheduled, and of course I will still upload it at some point—but then my area had a rare winter storm, blanketing everything in ice and snow, and I thought to myself that it would be appropriate to do a blog post on Europa, one of the Solar System’s iciest little worlds. Here is that post!

Captured by Voyager 2 during its flyby of the Jovian system.

Europa is a darling of the planetary science community. The second-closest satellite of Jupiter, with a mass of around 0.008 Earths, it is immediately notable for an almost craterless surface riven by fissures. External observations and modeling have indicated the presence of a planet-spanning subsurface ocean, heated by tidal forces; while we do not know for a concrete fact that this ocean exists, the evidence is quite strong, and given the likelihood of liquid water Europa is viewed as a primary contender for extraterrestrial life.

For this post I intend to write about the moon’s outer crust, a scientific marvel in itself. Just going outside today, and handling the ice encrusting every surface in the neighborhood, has gotten me thinking about the properties of frozen water—especially its translucence, and lightness, and plasticity. Chemically the ice of Europa is no different from that on my driveway1, it just exists on a far grander scale. Out there, on a moon hundreds of millions of kilometers from Earth, a dynamic icy crust moves in complex ways, dictated only by the laws of physics and the surrounding environmental conditions.

The view from Galileo, during its expedition in the 1990s. Color contrast has been enhanced in this image.

The most obvious features on Europa, visible even to the Voyager probes as they flew past at great distance, are the dark striations known as lineae2. These are somewhat mysterious, but they are believed to be caused by eruptions of warm ice through the colder surface material. Divergent plate boundaries would be the closest Earth analogue. They do not rise very far above the surrounding terrain—Europa is, overall, the smoothest solid world in the Solar System—and in the larger ones, up to twenty kilometers wide, lighter bands run in the middle of the darker material.

The action of Jupiter’s gravity predicts a pattern for these striations, which they do not follow. Older lineae diverge more from the prediction. This is believed to be evidence for movement of the icy crust with respect to the core, with a full revolution taking 12,000 years. I’m a little surprised it takes that long; given the presence of a global ocean, the crust would be completely detached from the rocky interior, so you’d think it could spin as fast as it darn well pleased.

Chaos terrain.

Europa’s moon-spanning glacier harbors many other interesting phenomena. Among these is chaos terrain—regions were the ice is jumbled together seemingly without rhyme or reason. In 2011 researchers at the University of Texas at Austin3, led by Dr. Britney Schmidt, created a new four-step model of chaos formation. This model matched quite well with existing data, but it also predicted the presence of vast lakes not far beneath the ice. For a future space mission, these would be far easier to reach than the subglacial ocean, which is buried under many kilometers of icy crust.

The actual thickness of the ice is a matter of contention within the scientific community. A detailed treatment of the subject would likely take up an entire blog post of its own, but here I will attempt to convey the consensus, such as there is one. Wikipedia gives a figure of 10-30 kilometers, out of 100 kilometers’ depth for combined ice and liquid water. Other theories propose a thinner crust of only a few kilometers, maybe as little as 200 meters in places (!). Another source, the website of the Universities Space Research Association, has narrowed it down to 19-25 kilometers. Suggestions for crust thickness are based on a variety of factors, including the plasticity of impact craters and the hypothesized tidal heating Europa undergoes from Jupiter’s gravity4.

A NASA illustration showing two possibilities. Note the convection currents.

The surface of the ice sheet is believed—as of recent research, in 2018—to host pointed ice formations known as “penitentes,” which are found on Earth in high and cold areas of the Andes Mountains. These are formed when sunlight causes deposits of ice to sublimate from solid to gas; the formation of small depressions leads to a positive feedback loop, and eventually the ice takes the form of jagged spires. On Europa the penitentes are believed to be most prominent at the equator but they may dominate much of the surface. Since they can be more than 15 meters tall, with little space between them, they may pose a serious problem for spacecraft attempting to land there.

Penitentes in Chile. ESO, CC BY 4.0 https://creativecommons.org/licenses/by/4.0, via Wikimedia Commons

From only a handful of probe expeditions to the Jovian system—with only one, Galileo, making repeated close-up observations—we have nevertheless gained a great deal of information on Europa, enough at least to raise a thousand interesting questions. How thick is the ice sheet? Are there lakes just beneath the surface? How extensive are the deadly ice spikes? With only a thick layer of ice and tidal heating from Jupiter, Europa has evolved into a world of fascinating complexity, as if the glaciers of Antarctica expanded to cover a moon three thousand kilometers across. Ice is versatile stuff.

Thanks for joining me for another week, everyone! Next Sunday we may have a guest post by a good friend of mine, so stay tuned.

Further Reading:

  1. Well, this isn’t exactly true. H2O is H2O, but when frozen it takes on different crystalline structures, or none at all. Under Earth conditions it forms what is known as ice Ih, with a hexagonal structure. Ice as found in the frigid Outer Solar System often has no structure to speak of; this is known as amorphous ice, and particularly in the Galilean moons it is present in various quantities, with Callisto mostly comprising ice Ih and Europa mostly comprising amorphous ice (due to intense radiation breaking down crystals).
  2. The Latin translates, unsurprisingly, to “lines.”
  3. Also including researchers from John Hopkins University and the Lunar and Planetary Institute in Houston. I had some difficulty writing a sentence that mentioned these and didn’t trip over itself, so they go in a footnote instead.
  4. We can’t really measure this directly, but there’s a lot that can be learned from computer models.

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