Titan's lakes may have been formed by explosions of nitrogen
A new study based on radar data from NASA's Cassini deep-space probe suggests that some of the smaller methane lakes on Saturn's largest moon, Titan, may have been formed by explosions of warming nitrogen. If true, this would mean that there are at least two mechanisms for forming Titanian lakes.
While there are thousands of bodies in our solar system, Titan is one of a handful that is of special interest to scientists because it is the only body other than the Earth where stable liquids sit on its surface.
Though Titan has a temperature of about -179° C (-290 °F), it also has an atmosphere with a pressure 1.45 times that of the Earth. Obviously, this is much too cold a place for liquid water, but a mixture of methane and ethane that fills Titan's lakes, streams, and seas, and the moon even has its own methane cycle that is similar to the Earth's water cycle, with evaporation, cloud formation, and rainfall.
It's also thought that this similarity also extends to how the lakes formed, with the liquid methane dissolving the bedrock ice and frozen organic compounds in a manner like that of the karstic lakes of Earth, which are formed by the effect of water on soluble rocks like limestone, gypsum, and dolomite that forms subterranean caverns that collapse and flood.
According to an international team of scientists led by Giuseppe Mitri of G. d'Annunzio University in Italy, that may be true for the larger lakes, but the radar profiles collected by Cassini in 2017 of the smaller lakes that have very steep rims that soar well above sea level suggest that instead of being formed by methane dissolving down, they were formed by methane exploding up.
"The rim goes up, and the karst process works in the opposite way," says Mitri. "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."
NASA says that the mechanism behind this is based on the idea that Titan has gone through a series of ice ages over the past 500 million or one billion years. Methane acts as a greenhouse gas that warms the moon, but sunlight breaks down methane and it has to be replenished by geological forces. When this happens, nitrogen builds up in the atmosphere and rains down in the cooling atmosphere to collect as subsurface pools. As these warm, they vaporize with explosive force, forming the lakes.
"These lakes with steep edges, ramparts, and raised rims would be a signpost of periods in Titan's history when there was liquid nitrogen on the surface and in the crust," says Cassini scientist Jonathan Lunine of Cornell University in Ithaca, New York.
The research was published in Nature Geosciences.