Nitrogen in Titan’s lakes may cause them to fizz
Cassini captured this mosaic of images showing the northern lakes and seas of Saturn’s moon Titan on Feb. 17, 2017. The mission’s final close Titan flyby is planned for April 22. Image & Caption Credit: NASA/JPL-Caltech/Space Science Institute
The hydrocarbon lakes of Saturn’s largest moon, Titan, may contain nitrogen that, at times, causes them to bubble and fizz, according to a NASA study that simulated conditions on Titan’s surface.
Conducted at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, the simulation showed that high levels of nitrogen could dissolve in the very cold methane rains that supply the large moon’s lakes, seas, and rivers.
It also confirmed that the nitrogen can separate out of these lakes and bubble up to their surfaces in response to minor changes in the lakes’ temperatures, air pressure, and composition. As the nitrogen separates out, it fizzes much like a can of carbonated soda does when first opened.
The study was done based on data sent back by the Cassini Saturn orbiter that showed variations in the compositions of Titan’s lakes from one region to another, with some having higher levels of ethane than methane.
“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,” explained JPL’s Michael Malaska, lead author of a study on the findings published in the February issue of the journal Icarus.
The trouble with bubbles
The release of nitrogen from the lakes, known as exsolution, produces a large number of bubbles. The process is more likely to occur during Titan’s seasonal changes when the methane lakes become warmer.
Bubbles and fizz are a concern for NASA because they could cause trouble for a future robotic probe sent to swim or float in the lakes. Heat generated by the probe could trigger the same process, possibly leading the probe to become surrounded by bubbles that compromise its ability to steer or remain stable.
Radar images from Cassini showed a strange island-like feature in one of Titan’s hydrocarbon seas that appeared to change over time (series of images at left). One possible explanation for this “magic island” is bubbles. Image & Caption Credit: NASA/JPL-Caltech/ASI/Cornell
Nitrogen bubbles might be responsible for a phenomenon Cassini has observed during several flybys of Titan. Within the lakes and seas, Cassini’s radar observed what seemed to be small islands appear and disappear. On at least one occasion, an island that had apparently disappeared later reappeared. Several possible theories have been proposed to explain this, including large fields of bubbles.
“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 study co-author Jason Hofgartner, who is also a co-investigator for Cassini’s radar team.
Titan lake simulation
In a simulated hydrocarbon lake, the researchers successfully separated nitrogen out of an ethane-rich solution. The ethane, which, unlike water, is denser as a solid than as a liquid, froze at the “lake’s” bottom, thereby displacing the dissolved nitrogen gas that then fizzed up to the surface. However, in Titan’s lakes, the nitrogen doesn’t move only in one direction; it moves into both the methane and ethane before it can escape.
“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’.”
He compared the process to the movement of carbon dioxide into and out of oceans on Earth.
Titan’s last flyby
Cassini’s last Titan flyby will occur on April 22, when it will use its radar beam to observe the moon’s northern seas. The observation has been designed by the probe’s radar team to focus on the mysterious “islands”, should they appear, using brightness levels to differentiate between suspended solids, waves, and bubbles.
The Titan flyby will alter Cassini’s course, initiating the first of 22 dives between the innermost rings and the giant planet, ultimately ending the 20-year mission with a September 15 plunge into Saturn’s atmosphere.
Video Courtesy of NASA / JPL-Caltech / M. Malaska
Laurel Kornfeld
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.
