Cryovolcanism on Titan

Titan, the largest moon orbiting the planet Saturn, has long been thought to be a likely place for volcanic activity. Volcanic activity requires internal heat. Titan's large size and substantial density suggest that plenty of gravitational and radioactively generated energy is available for melting its interior. Titan has a substantial layer of water-ammonia liquid lying beneath its surface. Therefore, unlike volcanism on Earth, which is the eruption of molten (melted) rock, volcanism on Titan would be cryovolcanism, which is essentially the eruption of icy water, sometimes mixed with other materials-likely ammonia and methane in Titan's case

Titan's thick atmosphere is about ninety-five percent nitrogen, with a few percent of methane. The methane in Titan's atmosphere is broken down by sunlight so that it recombines with other constituents of the atmosphere, forming organic compounds such as ethane.For this process to continue, the methane must somehow be replenished. One thought is that large liquid bodies on the surface (perhaps liquid methane or ethane) could re-supply the atmospheric methane; at Titan's temperatures (-176°C at the surface), methane behaves much like water on Earth. The Cassini spacecraft, which orbited Saturn and studied the planet and its moons, revealed large bodies of liquid on Titan's surface, perhaps enough to replenish the atmospheric methane, but another possibility is that cryovolcanism supplies methane and other gases to the atmosphere.

Cassini results suggest that cryovolcanism has indeed been a significant geological process on Titan. The craft carries a radar instrument that can peer through the clouds and haze to the never-before seen surface. It showed that several large liquid flows were spread across Titan's frigid landscape. Some, particularly those that appear to come out of craters (surface depressions), are likely to be cryovolcanic, though some researchers argue that some of these flows could possibly be rivers. Titan's surface has fluvial (river) activity, as shown by plenty of branched channels, indicating that rivers of liquid methane run there. Cryovolcanism can also cause flows, so the challenge is to identify which process caused a particular flow deposit. Some of the flows seen in the radar images are more likely to be cryovolcanic than fluvial, particularly those that appear to come out of craters. The craters are elongated rather than circular, indicating origin by volcanic eruptions rather than by impact (collision with objects from space). The association of flows with non-impact craters is hard to explain by any process other than volcanism. Titan may still be actively cryovolcanic: Cassini observed period brightening of infrared light at two locations that could not be explained by changes in cloud cover. It is possible that active cryovolcanism, perhaps in the form of the release of gas, causes the brightness changes. When the radar instrument observed these locations, they showed flow features that could be due to cryovolcanism.

Whether or not Titan is currently actively volcanic, it is likely that it was in the past. The Huygens spacecraft, which landed on Titan on January 14, 2005, obtained other evidence that cryovolcanism may have occurred on Titan. Although the amazing surface images did not show any features that could be unambiguously interpreted as cryovolcanic, Huygens did make a surprising finding. It detected a variant of the element argon in Titan's atmosphere. This variant is formed from the element potassium, and its presence in the quantities measured means that the atmosphere must be in communication with a reservoir of potassium. Titan is large enough to have differentiated, that is, it evolved into compositionally distinct layers, with the denser materials sinking to the center. Therefore, it is likely that most of the potassium-bearing material is in the rocks that form Titan's core (center). Cryovolcanism would be one means by which this material might be brought to the surface.