Pedagoguery

Whenever we send a robotic probe out into the solar system, we find surprises. The Mariner 9 probe found that Mars had the largest volcano in the solar system: Olympus Mons. Voyager discovered volcanic activity on Io, a moon thought too small to sustain volcanism. Voyager also discovered nitrogen geysers on Triton, a moon of Neptune. Cassini is no different. Many of the surprises it discovered deal with the moon Enceladus.

The first images of Enceladus by Voyager 2 already hinted at its strangeness. Bright white, like new snow, with extensive uncratered terrain, it was clear that some resurfacing had occurred. However, at a mere 500 kilometers across, it was far too small to generate much heat on its own. And, unlike Io, there were no strong orbital resonances to generate tidal heating. Add to that the fact that it was at the thickest part of the broad, diffuse E ring meant that there was something strange going on in its vicinity.

Enter Cassini. Unlike the Voyager probes, which were flybys, Cassini is an orbiter. Like the Galileo probe at Jupiter, it entered into orbit around Saturn and stays for several years, compiling more and more observations. Cassini's mission was designed to get a good look at Enceladus. Fortunately, Enceladus' position as one of the innermost moons of Saturn proved fortuitous. Of the larger moons, only Mimas orbits closer to Saturn. (Mimas also marks the innermost edge of the E ring.)

Several sets of observations of Enceladus have answered some questions, but deepened the mystery in other ways. Like Voyager did to Io, Cassini caught Enceladus in the act: several jets were observed emerging from the southern limb of Enceladus when it was back lit by the Sun. An equatorial flyby showed that the “smooth” terrain was actually deeply fractured and grooved, evidence of several episodes of severe tectonic activity in its past. A later flyby of the southern latitudes of the moon revealed a roughly circular patch that is completely crater-free, and is marked by a handful of deep, roughly parallel cracks dubbed “tiger stripes”. The tiger stripes are nearly evenly spaced, and run for about 130 kilometers, ending in hook-shaped bends. The region is sharply delineated at about 55 degrees of latitude by a meandering circumpolar boundary of concentric mountains and valleys. The whole arrangement seemed to recall processes similar to what is happening on the Earth's mid-Atlantic ridge; a place where the crust is spreading apart. The volcanic activity appeared to be coming from the tiger stripes. When Cassini passed through one of the jets, instruments recorded water, nitrogen, carbon dioxide, and methane. Cassini's spectral imager revealed that the terrain at the bottom of the tiger stripes was considerably hotter than it should be – 180 kelvins, as opposed to the 70 kelvins that would be expected by heating through sunlight. In addition, the energy output at those areas was an astounding 60 watts per square meter. Compare this with Yellowstone's geothermal area, where the energy output is a mere 2.5 watts per square meter. Something unusual was going on.

What could be the source of the energy driving Enceladus? It is far too small to have a store of radioactives large enough to do the job, but there is an orbital resonance with the moon Dione. Unfortunately, this is not sufficient to account for the observed level of heating. However, there is an interrelationship between the non circularity of Enceladus' orbit, the degree of interior cracking of the crustal ice, and strength of the heating. Here is how the scenario is thought to go. Enceladus starts with a nearly circular orbit, which is gradually made more eccentric due to the orbital resonance with Dione. Tidal stresses due to the non circular orbit cause cracks to form in the ice crust. As the blocks of ice shift past each other each orbit due to tidal stresses, frictional heating builds up. The heat builds up faster than it can dissipate, potentially melting ice along the cracks and allowing s subsurface sea to accumulate. The tidal stresses and heating dissipate orbital energy, re-circularizing the orbit. Eventually, the cooling causes the cracks to seal up, returning the moon to its original state.

More investigation of the moon is in store, but it remains intriguing. It is yet another place, like Europa and possibly Mars, where liquid water is thought to exist, and thus, where life is possible.

Next time, what is dark energy?