Titan: Nitrogen Bubbles and ‘Magic Islands’

With Cassini now in the final stages of its mission, we can look forward to just one more close flyby of Titan, the 127th targeted encounter, on April 22. ‘Targeted’ means that Cassini has to use its thrusters to position itself optimally for the flyby. The first of the images below, by contrast, comes from a ‘non-targeted’ flyby, one of several anticipated for 2017.

The close pass will give researchers a chance to probe the moon’s northern seas one last time, which may prove useful in the investigation of the transient features some have dubbed ‘magic islands.’ Even as these studies proceed, Cassini will also be using the Titan flyby to alter its course enroute to the series of plunges through the gap between Saturn and its innermost rings now being called the Cassini Grand Finale. The spacecraft will plunge into Saturn’s atmosphere on September 15.

Image: As it sped away from a relatively distant encounter with Titan on Feb. 17, 2017, NASA’s Cassini spacecraft captured this mosaic view of the moon’s northern lakes and seas. Cassini’s viewing angle over Kraken Mare and Ligeia Mare was better during this flyby than previous encounters, providing increased contrast for viewing these seas. Because the spacecraft is peering through less of Titan’s haze toward Kraken and Ligeia, more details on their shorelines are visible, compared to earlier maps. Credit: NASA/JPL-Caltech/Space Science Institute.

But back to those ‘magic islands.’ Several previous Cassini flybys of Titan have revealed the radar signature of small areas on the seas that have appeared and then disappeared. In one instance, the feature re-emerged in subsequent imagery. Have a look at Ligeia Mare in the image below. With a total area of about 130,000 kilometers, this huge Titan lake is 50 percent larger than Lake Superior. The Cassini flyby on April 22 will re-observe this area. Similar transient features have been found on Kraken Mare, the largest of Titan’s seas. In both cases, we’re learning that Titan’s seas are more active places than originally thought.

The April 22 observations may help us to distinguish between floating or suspended solids, waves or bubbles as the culprit for the ‘magic islands,’ assuming that Cassini finds the phenomenon again. And if bubbles seem an unlikely cause, consider a new paper in the journal Icarus, which gives us an interesting look into how nitrogen behaves as it interacts with methane and ethane in conditions like those on Titan’s surface.

Image: These images from the radar instrument aboard NASA’s Cassini spacecraft show the evolution of a transient feature in the large hydrocarbon sea named Ligeia Mare on Saturn’s moon Titan. Analysis by Cassini scientists indicates that the bright features, informally known as the “magic island,” are a phenomenon that changes over time. They conclude that the brightening is due to either waves, solids at or beneath the surface or bubbles, with waves thought to be the most likely explanation. They think tides, sea level and seafloor changes are unlikely to be responsible for the brightening. The images in the column at left show the same region of Ligeia Mare as seen by Cassini’s radar during flybys in (from top to bottom) 2007, 2013, 2014 and 2015. The bottom image was acquired by Cassini on Jan. 11, 2015, and adds another snapshot in time as Cassini continues to monitor the ephemeral feature. The feature is apparent in the images from 2013 and 2014, but it is not present in other images of the region. Credit: NASA/JPL-Caltech/ASI/Cornell.

Through simulations of Titan’s surface conditions, JPL researchers have learned that a great deal of nitrogen can be dissolved in the cold liquid methane that rains out of Titan’s skies and flows through its rivers and lakes to the seas. This JPL news release likens the nitrogen release to the fizz that happens when you open a bottle of carbonated soda. The nitrogen bubbles would vary with the composition of the moon’s lakes and seas, depending on the concentration of ethane vs. methane, as JPL’s Michael Malaska, who led the study, explains:

“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.”

This release of nitrogen could be a widespread phenomenon on Titan, and one that could also mark changes in season as methane seas warm and cool during the year. So the possibility exists that the ‘magic islands’ may be explained by fields of bubbles emerging from below. Nitrogen, like carbon dioxide being absorbed in Earth’s oceans, moves in both directions.

“In effect, it’s as though the lakes of Titan breathe nitrogen,” Malaska said. “As they cool, they can absorb more of the gas, ‘inhaling.’ And as they warm, the liquid’s capacity is reduced, so they ‘exhale.'”

The team’s simulations also show that when ethane ice forms — it appears on the bottom of a simulated Titan lake, rather than, like water on Earth, floating on the top — nitrogen can be released. So we have plentiful ways of coaxing bubbles out of liquid on Titan.

Whether or not these nitrogen bubbles are the cause of the ‘magic islands’ on Titan, we need to learn as much as we can about them. Proposals to send robotic vessels to float through Titan’s seas may have to be modified to account for them, for heat from the probe could cause bubbles to form around its various surfaces, possibly inducing problems in stability. The JPL work shows that even the slightest changes in temperature, air pressure or composition can cause the absorbed nitrogen to separate rapidly, a process we’ll have to anticipate.

The paper is Malaska et al., “Laboratory measurements of nitrogen dissolution in Titan lake fluids,” published online at Icarus 2 February 2017 (abstract).