There are only two objects in the solar system with sustained pools of liquid on their surface: Earth and Titan. On Earth, we have a well-understood water cycle that keeps liquid water flowing on the surface of our planet. On Titan, the process is believed to be conceptually similar but based on liquid methane rather than water. There are signs that Titan has an alkanological cycle similar to the hydrological cycle on this planet. But how Titan actually got its lakes has been something of a mystery until now. Researchers are suggesting that the lakes may actually have formed as the result of explosions, not erosion.
Many of Titan’s lakes are what is known as Sharp Edged Depressions (SEDs). They have circular or irregular shapes that are generally not thought to be impact craters. On Earth, many of these types of lakes are karstic, meaning they form when liquid (water on Earth) undermines the geology beneath an area and it collapses, forming a depression that then fills. However, there aren’t many substances in Titan’s crust that are believed to be susceptible to dissolution in the first place. The organic material that rains out of Titan’s atmosphere and collects at the poles is insufficient to create an erodible organic sedimentary layer. The researchers write:
The presence of raised rims in SED basins undermines the karst lake model for SEDs with raised rims. According to the karst model, the lake basins on Titan should be produced as dolines formed by collapse, dissolution or subsidence of the terrain; such processes do not produce rims. While SEDs with raised rims are not formed by a karstic process, their presence in a karstic-like environment is not excluded.
The team has an alternate proposal for how the lakes may have formed — nitrogen bombs. On Earth, certain characteristic structures are produced by phreatic or phreatomagmatic eruptions. When seawater comes into contact with magma, the result can be a substantial steam explosion. Maars and other forms of tuff produced by this kind of explosion have characteristics that fit the observed characteristics of Titan’s lakes. We see that many Titan lakes have ramparts of material built up around them, and this could correspond to debris ejected from the newly formed crater as a result of a nitrogen explosion.
The theory isn’t perfect, because there’s not as much debris around the various lakes as might be expected on Earth — but this could be explained by differences in Titan’s natural composition and its planetary evolution. One theory is that at some point in the distant past, Titan’s atmosphere was dominated by nitrogen, not methane, and the moon was much colder. This could have been the case if the level of methane in Titan’s atmosphere was lower than it is today. As the amount of methane increased and the planet warmed, small temperature variations could have produced an extreme pressure rise in the nitrogen-dominated aquifers. There may even be evidence of this kind of event occurring on Neptune’s moon Triton during the Voyager 2 flyby.
Scientists are still studying the characteristics of Titan’s geology to determine whether this is a plausible hypothesis, but we could know more in fairly short order. The NASA Dragonfly mission to Titan is set to gather an unparalleled amount of information about this distant moon, shedding new light on its history and continued evolution.