Frozen Moons

This week, Keagan Lee, an Undergraduate Research Assistant working at PVL for the summer reports on some independent reading he has been doing on a fascinating solar system object: Europa. The image above is a well-known mosaic acquired by the Galileo Orbiter, which you can find on the Planetary Photojournal here.

By Keagan Lee

We like to think of Earth being in the “Goldilocks Zone” -- an area in a star system that is not too cold and not too hot so that liquid water can exist on its surface -- as if this is the ideal location in the solar system. We call Earth the “Blue Planet” because it has so much water. Ostensibly, yes. In our neighbourhood, we are the largest host of water; any water that made its way to Mercury (outside of the permanently shadowed polar traps) or Venus would be boiled off instantly, and it is too cold for water to exist in liquid form on Mars, at least currently. But water is much more likely to exist further out in the solar system where the effects of solar radiation are lessened because of the distance from the Sun and, as a consequence, is found in the form of ice. Europa, one of the moons of Jupiter, which is less than 1% the mass of Earth, is estimated to have more than twice the volume of water than Earth!

Europa belongs to a family of bodies known as icy moons, so-called because one of the primary building block of the moon is ice itself. Some of these moons include Callisto and Ganymede of Jupiter, Titan of Saturn, and Triton of Neptune. Each moon has more water (in ice form) than Earth, since water is less likely sublimate into vapor and escape to space. At these distances -- of more than 750 million kilometers from the Sun -- ice behaves like rock and makes up a hard crust on the outside up 100 kilometers thick. For comparison, if all the water on Earth was frozen, it would form a layer of ice 4 kilometers thick, a fraction of a percent of the planet’s diameter. Now what happens when the interior of the moon is heated by differentiation, the decay of radioactive isotopes, and tidal friction? The interior of the Earth is was, and still is, heated by these effects to over 6000 degrees Kelvin. All of these processes could be heating the interior of these moons. If we imagine that kind of heating on a smaller scale, it becomes possible that beneath these rock hard icy exteriors, the icy moons have liquid oceans.

Two possible interior scenarios for Europa are shown in this diagram from the photojournal.

This is not a new concept, and in fact there is good evidence for such internal structures within some of the icy moons such as Enceladus and Europa. Large plumes of water vapour, vented by geysers, have been detected coming off of these moons and sending particles out into space. Enceladus is the creator for Saturn’s E ring made of water vapour; a ring only faintly visible to sensitive instruments when observing Saturn. In addition, there is evidence that the crust of Enceladus is separated from it’s core, probably from a subsurface ocean of water. Contrary to the name, these icy moons are clearly not “frozen” per-se or inactive, they are active and dynamic bodies. So what’s going on? Scientist are saying volcanism.

Now obviously this isn’t volcanism as we know it, involving silicate rocks which make up Earth’s crust and mantle. This “cryovolcanism” involves a water-ice and other volatiles which make up the crust and subsurface ocean. What we are describing is just a different -- albeit generalized -- version of the crust-mantle-core structure we are used to on Earth only with different materials. Provided an opening, the liquid underneath will make its way to the surface and solidify, creating a surface feature. One major difference is that ice floats on water, whereas lava is more buoyant that solid rock. Thus if there is an opening in the rock, silicate lava will be forced up by buoyancy. Cryovolcanism would require a buildup in pressure in this subsurface-ocean / water-mantle and an avenue to release pressure. An alternative is if the cryolava was composed of other materials added into the mix, like ammonia. This would also make the cryolava more viscous, but not as viscous as silicate lava. This would mean cryovolcanoes would likely be shield volcanoes, which are flatter and wider.

All the clues point to a structure generally similar to that of Earth. But on Earth, volcanism is one part of a much larger system which includes plate tectonics. If there were localize warming zones under the crust, not only could we see volcanos and geysers, but also potentially icy plate tectonics. There is evidence for this as well on our friends Enceladus and Europa. There are clear fissures, ridges, and trenches scoring the surfaces of these moons similar to those created by plate tectonics. Additionally, where we would expect to see impact craters marking the surface, there are few meaning there has to be some process which is eroding or changing the surface topography. Icy plate tectonics is a possible process for resurfacing, along with cryovolcanism.

Presented with this evidence, the logical conclusion is to accept this idea of geo-dynamic systems on these moons where water-ice behaves similar to silicate rocks, but we can’t be totally sure. Our knowledge of the inner workings of the Earth are based on hundreds of years of data collected using sophisticated techniques. Methods such as studying the propagation of earthquake tremors, sea-floor mapping, ground penetrating radar, extensive geological field surveys, gravity measurements, and extremely accurate GPS positioning along with other space-based observations. The only opportunities we have had to collect data regarding the structure and dynamics of the icy moons is the occasional pass of a spacecraft. Ultimately, we need more information to be sure.

What we can say with certainty is the solar system is a wondrous and active place that is constantly pushing the boundaries of our knowledge and the realm of possibilities. And although the imagination tends to wander, often what we discover instead can be many times more interesting. Hopefully we will be able to send more spacecraft to these frigid worlds and see what mysteries are frozen in their icy surfaces.