(CNN) When NASA's Cassini mission surveyed Saturn's largest moon, Titan, in 2013, scientists encountered a mystery they couldn't solve: the "magic islands."
Radar images showed unexplained bright regions in Ligeia Mare, the second largest liquid body on Titan's surface. The sea is 50% larger than our own Lake Superior and is comprised of liquid methane, ethane and nitrogen.
That environment makes the "magic islands" a less-than-ideal vacation spot. But the question remained: What are they?
Now, researchers have an answer. The bright regions that looked like islands are actually thousands of bubbles. It turns out Ligeia Mare can sometimes be an actively fizzing sea, according to a new study in Nature Astronomy. But this activity is ephemeral, which is why it has been difficult to classify and pin down.
"The surface looks very calm, at least in the light of RADAR observations, and from time to time we have streams of big bubbles a few centimeters in diameter reaching the surface," Daniel Cordier, the lead study author, told CNN via email. Cordier is a research scientist at the Centre National de la Recherche Scientifique in Paris.
There is no equivalent to this kind of activity on Earth.
The bubbles are formed when the nitrogen, ethane and methane mixture becomes unstable at the temperatures and pressures researchers believe to exist at Ligeia Mare's depths. The high concentration of nitrogen at the surface and the ethane at the bottom of the sea mix through hydrodynamic circulation. But these chemicals separate due to pressure, sending nitrogen bubbles back to the surface.
The researchers discovered this through a numerical model using Cassini's data. A previous study suggested the result through a chemical experiment in a lab.
Titan, larger than both our own moon and the planet Mercury, is unique in our solar system. It is the only moon with clouds and a dense atmosphere of nitrogen and methane, which gives it a fuzzy orange appearance. It is 60% greater than our own atmosphere, meaning it exerts the kind of pressure you feel at the bottom of a swimming pool, according to NASA.
It also has Earth-like liquid bodies on its surface, but the rivers, lakes and seas are comprised of liquid ethane and methane, which form clouds and cause liquid gas to rain down from the sky. The surface temperature is so cold at negative 290 degrees Fahrenheit that the rivers and lakes were carved out by methane -- the way rocks and lava helped to form features and channels on Earth.
Titan is also believed to have an internal liquid water ocean, like Jupiter's moon Europa and one of Saturn's other moons, Enceladus. Last week, NASA announced that Europa and Enceladus' oceans have some or most of the ingredients necessary for life as we know it.
But how does Titan compare? First of all, it's bigger. It's also entirely unique in its possession of a dense atmosphere, which has obscured the observations that researchers have tried to make of Titan.
"Contrary to the case of Enceladus where active geysers are well observed, there are only a few cryovolcano candidates at the surface of Titan, and there is really no convincing evidence of their activity," Cordier said. "Titan's internal water ocean may be similar to that of Europa or Enceladus, but the existence of a form of life in these oceans belongs, for the moment, to the domain of science fiction."
Given its complex chemistry, it's safe to say that Titan isn't hospitable to humans. But it is attractive to researchers.
The Cassini mission comes to an end later this year, but proposed mission concepts already exist for a type of "Titan airplane" called the AVIATR (Aerial Vehicle for In-situ and Airborne Titan Reconnaissance) and a submarine that would explore Titan's seas.
In the meantime, Cordier wants to investigate how waves are generated on the surface of Titan's seas, and its cryovolcanic activity. But he believes the biggest question waiting to be answered about Titan is where its complex atmosphere originates from, given the fact that researchers don't know the source of its methane.
"In the future, perhaps we will discover some 'exo-Titan' among exoplanets, making our Titan the prototype of a new class of celestial objects," Cordier said.