As the only moon in our solar system with a dense enough atmosphere and large liquid reservoirs on its surface, Saturn's moon Titan is of great interest to scientists. However, despite possessing a nitrogen-dominated atmosphere much like Earth, it possesses very little oxygen, with the rest of its atmosphere mostly made up of methane and trace amounts of other gases such as ethane. Furthermore, these chemical compounds are sometimes found on the moon's surface in liquid forms.
Titan's pools of methane and ethane in liquid form have caused scientists to speculate that it might have hydrocarbon lakes and seas, and now, new data from NASA/ESA Cassini-Huygens mission reveals that more than 620,000 square miles of Titan's surface are covered in liquid. In total, there are three large seas near the moon's north pole, all of them surrounded by smaller lakes in the northern hemisphere.
The team made their findings using the Cassini radar instrument, the same tool that was used back in 2014 to confirm that the Ligea Mare, the second largest sea on Titan, is rich in methane.
"Before Cassini, we expected to find that Ligeia Mare would be mostly made up of ethane, which is produced in abundance in the atmosphere when sunlight breaks methane molecules apart," said Alice Le Gall, a member of the Cassini radar team from the French laboratory LATMOS and lead author of the new study. "Instead, this sea is predominantly made of pure methane."
The methane composition of this sea could be the result of numerous processes.
"Either Ligeia Mare is replenished by fresh methane rainfall, or something is removing ethane from it," Le Gall said. "It is possible that the ethane ends up in the undersea crust, or that it somehow flows into the adjacent sea, Kraken Mare, but that will require further investigation."
Using radar observations of the heat given off by Ligeia Mare, as well as data from a 2013 experiment that bounced radio signals off of the sea, the team determined the compositions of the liquid sea and the sea bed by separating each of their contributions to the sea's observed temperature.
"We found that the seabed of Ligeia Mare is likely covered by a sludge layer of organic-rich compounds," Le Gall said.
In addition, they used Cassini temperature measurements to determine that the shoreline surrounding the sea is likely porous and rich in liquid hydrocarbons.
"It's a marvelous feat of exploration that we're doing extraterrestrial oceanography on an alien moon," said Steve Wall, deputy lead of the Cassini radar team at NASA's Jet Propulsion Laboratory. "Titan just won't stop surprising us."
The findings were published in the Feb. 25 issue of the Journal of Geophysical Research.