A new study has found that Ligeia Mare, one of the many large seas on Saturn's moon Titan, is surprisingly made mostly of pure liquid methane. The sea is surrounded by wetlands and the seabed might be composed of a sludge of carbon- and nitrogen-rich material.
Titan is one of the most remarkable objects in the Solar System. It is larger than our Moon and even the planet Mercury, and it has a complex dense atmosphere. Most importantly, it is the only other place apart from Earth where we have found large liquid bodies on the surface.
The discovery was possible thanks to data from the Cassini spacecraft, which has been orbiting Saturn since 2004. The NASA/ESA probe’s radar recorded the thermal emission from Titan’s seas between 2007 and 2015, and astronomers were able to work out their seasonal variations, depth, and composition.
“We expected to find that Ligeia Mare would be mostly ethane, which is produced in abundance in the atmosphere when sunlight breaks methane molecules apart,” explains Alice Le Gall, lead author of the new study, in a statement. “Instead, this sea is predominantly made of pure methane.”
Although the finding is surprising, the researchers put forward a couple of explanations for the lack of ethane. They think that either methane rains replenish the sea, or the ethane is being removed and flowing either to the nearby sea Kraken Mare or in an undersea crust.
In the paper, published in the Journal of Physical Research: Planets, the researchers show that as Ligeia Mare goes from winter to spring, it experiences a warming of less than 2°C (3.6°F). It might not seem like a big difference since Titan has an average temperature of -179.5 °C (-291.1 °F), but this variation is responsible for the cycle of material on the moon.
Cassini also looked at the temperature difference between the sea and the surrounding land without finding any difference. This could indicate the presence of wetlands along the shore. The spacecraft was also able to make the first-ever detection of the bottom of an extraterrestrial sea. As it turns out, Ligeia Mare reaches a depth of 160 meters (525 feet) along the radar track.
"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, in a statement. "Titan just won't stop surprising us."
Ligeia Mare, shown here in a false-color image from the international Cassini mission, is the second largest known body of liquid on Saturn's moon Titan. NASA/JPL-Caltech/ASI/Cornell