Back in 2013, a bright, mysterious feature on Saturn's moon Titan suddenly appeared in images taken by the Cassini mission's radar instrument. The strange anomaly, not seen in previous images, inexplicably emerged in Ligeia Mare, the second-largest of Titan's seas, located near its north pole.
During a follow-up flyby just days later, the feature — informally dubbed a "magic island" — had vanished. And then
two similar anomalies were spotted the following year.
Scientists had a few possible explanations for the phenomenon, but a new study points to a theory that may prove to be the most likely reason: Titan's seas are bubbling with nitrogen, creating island-like features.
The frozen moon's seas aren't made of water. Titan is so cold, averaging −290 °F (−179 °C), that it could only have seas of liquid hydrocarbons like methane and ethane. The combined Cassini and Huygens missions determined that cold liquid methane rains from the skies on Titan and collects in rivers, lakes, and seas.
But how might so-called "islands" suddenly appear in such an environment? A recent experiment by scientists at the Jet Propulsion Laboratory simulated the conditions on Titan, and it showed that just slight changes in temperature, air pressure, or composition can cause the nitrogen to rapidly separate out of a solution, just like the bubbling fizz when a bottle of champagne or soda is opened.
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"Our experiments showed that when methane-rich liquids mix with ethane-rich ones — for example from a heavy rain, or when runoff from a methane river mixes into an ethane-rich lake — the nitrogen is less able to stay in solution," Michael Malaska of JPL, who led the study, said in a
"The basic idea is that lots of bubbles (or other phenomena) would give the same type of signal as solid land," Malaska added in an email to Seeker. "The radar would bounce off the bubbles and give a similar backscatter signature."
This composite image shows an infrared view of Saturn's moon Titan from NASA's Cassini spacecraft, acquired during the mission's ''T-114'' flyby on Nov. 13, 2015. Credit: NASA
His team's study showed that significant amounts of nitrogen can be released by the mixing of hydrocarbons, and also when there is a change in temperature. The bubbling release of nitrogen, called exsolution, could create large enough features to be picked up by Cassini's instruments.
"We show that significant volumes of nitrogen gas will be released from Titan lake fluids on heating, and that significant volumes of nitrogen gas will be absorbed by Titan lake fluids on cooling," the team wrote in
their paper, published in the journal Icarus. "The densities of the lake fluids will be affected by nitrogen dissolution. We also show that mixing of two cryogenic fluids of different composition can lead to the release of large amounts of nitrogen gas. This has implications for lake fluids, bubble formation, and geological phenomena."
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"Thanks to this work on nitrogen's solubility, we're now confident that bubbles could indeed form in the seas, and in fact may be more abundant than we'd expected," said Jason Hofgartner of JPL, who serves as a co-investigator on Cassini's radar team and was a co-author of the study.
NASA has noted that a fizzy liquid like this could also cause problems for any potential future lander missions to Titan, especially a spacecraft that would have the ability to float on or swim through Titan's seas. Excess heat emanating from a probe might cause bubbles to form around its structures, such as propellers used for propulsion, making it difficult to steer or maintain the probe's stability.
Cassini's current mission is slated to end in September, and as the probe has moved closer to Saturn, the imaging team has recently released
some stunning views of the planet's moons and rings. The closer orbit also allows for more surveys of Titan, and new images of the moon's lake region have just been released.
Some lakes on Titan are as big as the Great Lakes in the US or the Caspian Sea, which lies between Europe and Asia. Cassini has even captured sunlight glinting off of one of the lakes, with hints of waves on other lakes.
Cassini will make its final close flyby of Titan — the 127th such maneuver — on April 22. During the flyby, the spacecraft will sweep its radar beam over Titan's northern seas one last time. The radar team designed the upcoming observation so that if magic island features are present this time, their brightness may be useful for distinguishing between bubbles, waves, and floating or suspended solids.
The flyby also will turn Cassini's course to begin its final series of 22 dives through the gap between Saturn and its innermost rings, known as Cassini's Grand Finale.
The 20-year mission will end with the probe plunging into Saturn's atmosphere on September 15.
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