Titan’s lakes might be fizzing with nitrogen bubbles

A shot by Cassini of the lakes Kraken Mare and Ligeia Mare near Titan’s north pole. Credit: NASA

Featured image: A shot by Cassini of the lakes Kraken Mare and Ligeia Mare near Titan’s north pole. Credit: NASA.


One more study reporting cool things about my favourite moon this week. Researchers from Mexico and France have found that the conditions exist in which the lakes of nitrogen, ethane and methane around Titan’s poles could be fizzy with nitrogen bubbles. In technical terms, that’s nitrogen exsolution: when one component of a solution of multiple substances separates out. In this case, the nitrogen forms bubbles and floats to the surface of the lakes, becoming spottable by the Cassini probe. The results were published in the journal Nature Astronomy on April 18.

The Cassini probe has been studying Saturn and its moons since 2004. In 2013, its RADAR instrument – which makes observations using radio-waves – found small, bright features on some of Titan’s lakes that winked out over time. These features have been whimsically called ‘magic islands’ and there has been speculation that they could be bubbles. The Mexican-French study provides one scientific form for this speculation.

The researchers used a numerical model to determine how and why the nitrogen could be degassing out of the lakes. Specifically, they extracted estimates of the temperature and pressure on the surface and interiors of the Ligeia Mare lake from past studies and then plugged them into simulations used to predict the properties of Earth’s oil and gas fields. They found that the bubbles could form if the solution of methane, ethane and nitrogen was forced to split up at certain temperatures and pressures. So, the researchers had to figure out the simplest way in which this could happen and then the likelihood of finding it happening in a Titanic lake.

When the lake’s innards are not forced to split up, they’re thought to exist in a liquid-liquid-vapour equilibrium (LLVE). In an LLVE, two liquids and a vapour can coexist without shifting phases (i.e. from liquid to vapour, vapour to liquid, etc.). The researchers write in their paper, “In the laboratory, LLVEs have been observed under cryogenic conditions for systems comparable to Titan’s liquid phases: nitrogen + methane + (ethane, propane or n-butane).” While cryogenic conditions may be hard to create on Earth’s surface, they’re the natural state of affairs on Titan because the latter is so far from the Sun. The surfaces of its lakes are thought to be at 80-90 K (-190º to -180º C), with the lower reaches being a few degrees colder.

For an LLVE-like condition to be disrupted, the researchers figured the lake itself couldn’t be homogenous. The reasons: “A sea with a homogeneous composition that matches that required for the occurrence of an LLVE at a specific depth is an improbable scenario. In addition, such a case would imply nitrogen degassing through the whole extent of the system.” So in a simple workaround, they suggested that the lake’s upper layers could be rich in methane and the lower layers, in ethane. This way, there’s more nitrogen available near the surface because the gas dissolves better in methane – and also because it could be dissolving into the top more from the moon’s nitrogen-rich atmosphere.

Over time, the lake’s top layers could be forced to move downward by weather conditions prevailing above the lake, and push the material at the bottom to the top. But during the downward journey, the rising pressure breaks the LLVE and forces the nitrogen to split off as bubbles. Given the size and depth of Ligeia Mare, the researchers have estimated that nitrogen exsolution can occur at depths of 100-200 m. The bubbles that rise to the top can be a few centimetres wide – not too small for Cassini’s RADAR instrument to spot them, as well as in keeping with what previous studies have recorded.

Of course, this isn’t the only way nitrogen bubbles could be forming on Ligeia Mare. According to another study published in March, when an ultra-cold slush of ethane settling at the bottom of the lake freezes, its crystals release the nitrogen trapped between their atoms. Michael Malaska, of NASA’s Jet Propulsion Lab, California, had said at the time:

In effect, it’s as though the lakes of Titan breathe nitrogen. 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 Mexican-French researchers are careful to note that their analysis can’t say anything about the quantities of nitrogen involved or how exactly it might be moving around Ligeia Mare – but only that it pinpoints the conditions in which the bubbles might be able to form. NASA has been tentative about sending a submarine to plumb the depths of another Titanic lake, Kraken Mare, in the 2040s. If it does undertake the mission, it could speak the final word on the ‘magic islands’. Ironically, however, NASA scientists will have to design the sub keeping in mind the formation of LLVEs and nitrogen exsolution.

But won’t the issue be settled by then? Maybe, maybe not. Come April 22, Cassini will fly by Titan’s surface at a distance of 980 km, at 21,000 km/hr. It will be the probe’s last close encounter with the moon, as mission scientists have planned to take a look at some of the smaller lakes. After this, the probe will fly a path that will take it successively through Saturn’s inner rings. Finally, on September 15, NASA will perform the probe’s ‘Grand Finale’ manoeuvre, sending it plunging into Saturn’s gassy atmosphere and unto its death, bringing the curtains down on a glorious 13-year mission that has changed the way we think about the ringed planet and its neighbourhood.

Published in The Wire on April 20, 2017.



A submarine on Titan in 2040

An artist's conception of the proposed Titan Submarine, which NASA could land on Titan around 2040 to explore the depths of Kraken Mare, the moon's largest hydrocarbon lake.
An artist’s conception of the proposed Titan Submarine (conceived before the latest design was released), which NASA could land on Titan around 2040 to explore the depths of Kraken Mare, the moon’s largest hydrocarbon lake. Image: NASA

Nothing bespeaks humankind’s potential more than the following statement: Around 2040, NASA plans to splash down a submarine to explore a liquid hydrocarbon lake on Titan.

Fore more than a decade now, Titan has captivated astronomers not simply by being Saturn’s largest moon by far but also with its vast seas of liquid methane and ethane. NASA has its eyes on the largest such lake, called Kraken Mare, located near the moon’s north pole. The Cassini mission helped map the lake in great detail since it reached the Saturnian system in 2004, accompanied by the Huygens probe that landed on the moon’s surface in 2005. Thanks to them, we know Kraken Mare has an intricate shoreline and deposits of water-soluble minerals around it. According to the scientists who authored the article describing the submarine, these features “hint at a rich chemistry and climate history”.

They continue: “The proposed ~1-tonne vehicle, with a radioisotope Stirling generator power source, would be delivered to splashdown circa 2040, to make a ~90-day, ~2,000 km voyage of exploration around the perimeter, and across the central depths of Kraken.” While its design is by no means final (it’s described as a “first cut”), that NASA is considering exploring Titan in great detail belies its interest in the moon as well as continued commitment to studying the Saturnian system in general. Note that the agency cancelled the development of the proposed Titan Mare Explorer – a nautical surface probe – soon after 2013 to channel the funds into developing Stirling radioisotope generators, which we now find could be used to power the submarine.

Notwithstanding future budgetary cuts, delivering such a vehicle to the surface of a faraway moon might just signify the next leap in astronautical engineering. As the scientists remark,

Even with its planetary application aside, this exercise has forced us to look at submarine vehicle design drivers in a whole new way.

The current design has been developed by scientists from the JHU Applied Physics Laboratory, the NASA Glenn Research Center, and the Penn State Applied Research Lab. It will be presented at the 46th Lunar and Planetary Science Conference in Texas, during March 16-20.

1970s Space Shuttle ditching tests at Langley show lifting bodies can make safe landing on liquid.
1970s Space Shuttle ditching tests at Langley show lifting bodies can make safe landing on liquid. Image: ‘Titan Submarine: Vehicle Design and Operations Concept for the Exploration of the Hydrocarbon Seas of Saturn’s Giant Moon’ by Lorenz et al

Around 2040, they expect to be able to deliver it to Titan on board a ‘spaceplane carrier’, essentially a repurposed US Air Force DARPA X-37. According to them, Titan’s thick atmosphere could allow the carrier to descend to the surface at hypersonic speeds, following which attempt a soft-landing on the Kraken Mare. Finally, “the backshell covering the submarine would be jettisoned and the lifting body would sink, leaving the submarine floating to begin operations. (Alternatively, the submersible could be extracted in low level flight by parachute).”

Once inside, it will explore tidal currents in Kraken Mare, use a camera mounted on the mast to explore the shoreline landscape, make meteorological observations, analyze sediments from the seabed, and study trace organic compounds to learn how they evolved.

The slender low-drag hull has propulsors at rear, and a large dorsal antenna at the front of which is a surface camera is mounted in a streamlined cowl. A sidescan sonar, seafloor camera, and seafloor sampling system are visible on ventral surfaces.
The slender low-drag hull has propulsors at rear, and a large dorsal antenna at the front of which is a surface camera is mounted in a streamlined cowl. A sidescan sonar, seafloor camera, and seafloor sampling system are visible on ventral surfaces. Image: ‘Titan Submarine: Vehicle Design and Operations Concept for the Exploration of the Hydrocarbon Seas of Saturn’s Giant Moon’ by Lorenz et al

The submarine itself looks conventional apart from a large dorsal antenna and two cylindrical buoyancy tanks that jut out of the upper surface. According to its designers, the antenna was shaped so to be able to send data across billions of kilometers to Earth. And such large buoyancy tanks are necessary because the lake the submarine will explore is composed of methane and ethane, whose densities range from 450 kg/m3 to 670 kg/m3, as well as to counter the unique drag effects arising due to the dorsal antenna.

Another complication is thermodynamics. Titan has a frigid surface, cold enough to keep methane, whose boiling point is -161.5 degrees Celsius, in its liquid form. As a result, extra heat rejected from the submarine’s radioisotope power source could cause the surrounding methane and ethane to bubble. As the scientists explain, this results in “heat transfer uncertainties” as well as the potential to interfere with sonar observations. At the same time, the vessel must also be heavily insulated to allow the power source to warm its insides.

NASA first announced its intention to explore Kraken Mare with a submarine in June 2014, elaborating that the mission would help scientists learn more about the history and evolution of organic compounds in the Solar System, in turn a “critical step” along the path to understanding the formation of life. Earth and Titan are the only two objects in the System to host liquid lakes on their surfaces, albeit of different compositions.