Titan is the largest moon of Saturn, the second-largest moon in the Solar System, and is very peculiar. Its dense atmosphere makes it hard to study, but it's the only known body aside from Earth with liquid features on its surface. Though as it’s so cold there, the lakes and rivers are made of methane rather than water.
A particularly interesting feature of some of these lakes is their edges. Some of these bodies of methane have steep rims tens of meters in height, which are difficult to explain. As reported in Nature Geoscience, researchers saw similarities between these basins and explosion craters formed by interactions between magma and water.
Using data from Cassini, they think that on Titan, the explosions would likely be produced by pockets of liquid nitrogen trapped under the surface. It's believed that the Saturnine moon experiences colder and warmer periods (although still cold by our standards). The team thinks that during warm periods, ground-trapped liquid nitrogen expanded quickly, blowing holes in Titan’s surface.
The evidence is in the rims, which are raised compared to the surrounding regions. The previous theory saw them as analogous to Earth’s karst lakes, which are formed when subterranean caves collapse due to the rocks being dissolved in water.
"The rim goes up, and the karst process works in the opposite way," lead author Giuseppe Mitri, from G. d'Annunzio University in Italy, said in a statement. "We were not finding any explanation that fit with a karstic lake basin. In reality, the morphology was more consistent with an explosion crater, where the rim is formed by the ejected material from the crater interior. It's totally a different process."
The observational data that allowed this analysis was collected by the Saturn-studying Cassini mission, an international collaboration between NASA, and the European and Italian Space Agencies, that ended two years ago this week with a planned death dive into the planet's atmosphere. Not that that's stopped it from supplying us with plenty more data. Cassini’s radar data is so good, that researchers can really zoom in on features like these lakes, challenging previous assumptions with more explosive ones.
"This is a completely different explanation for the steep rims around those small lakes, which has been a tremendous puzzle," said Cassini Project Scientist Linda Spilker of NASA's Jet Propulsion Laboratory. "As scientists continue to mine the treasure trove of Cassini data, we'll keep putting more and more pieces of the puzzle together. Over the next decades, we will come to understand the Saturn system better and better."