Titan: Nitrogen Bubbles and ‘Magic Islands’

by Paul Gilster on March 16, 2017

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.

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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.

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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).

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{ 8 comments… read them below or add one }

RobFlores March 16, 2017 at 12:16

I though I read a prior article stating that there was very little Ethane in
those lakes. That it is by and large liquid methane. Such a large effect from
a small amount of Ethane?

Also, I don’t know the porosity of those lake bottoms. If they are much
like limestone in behavior, could it be that many lakes are Hydro (or should I say Petro) dynamically connected? How could this be checked?

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DJ Kaplan March 16, 2017 at 17:20

The sheer number of different kinds of chemicals detected on Titan, plus the endless ways that these chemicals seem to be interacting, makes it hard to believe that there is no living process (or something similar) on Titan. We must investigate this world.

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Adam March 16, 2017 at 17:37

Fascinating. I wonder what kind of life could use nitrogen? Dissolved in hydrocarbons…

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RobFlores March 17, 2017 at 13:11

I have not seen extensive papers on speculating on why
Titan developed so differently than Callisto. Maybe someone can point
to them.

1) Both moons have nearly the same Orbital Period.
2) Both a very close in size with Titan 10%+ larger.
3) Both lie just outside stronger effects of their Jovians Radiation belts.
4) Avg Surface Temp Callisto 134K Titan = 93K.

Yet, Titan has a 95% Nitrogen surface atmosphere.
What accounts for the large amount of Methane present today
We don’t see traces of anything like this on Callisto.

Yes, the amount of solar radiation at Jupiter is larger than
Saturn, but both are small amounts compared what Earth Receives at 1AU
Jupiter ~ 1/25th Saturn ~ 1/90th, both are fairly weak. And a reminder
that earlier in it’s History the Sun had substantially weaker output.

Titan Just sticks out and defies the rules for icy moons in the outer
solar system.

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RobFlores March 20, 2017 at 11:12

Here is response to the question as to why Titan turned out so different
from Ganymede and Callisto, via Ask the Astronomer.

http://curious.astro.cornell.edu/physics/59-our-solar-system/planets-and-dwarf-planets/moon/168-why-does-titan-in-spite-of-being-smaller-than-mars-have-an-atmosphere-more-dense-than-earth-s-intermediate

QUOTE
“We don’t really know why only Titan has a thick atmosphere, while the structurally similar Ganymede and Callisto don’t. Temperatures may have been too high (well above ~40K) in the Jovian subnebula due to the greater gravitational potential energy release, mass, and proximity to the Sun, greatly reducing the NH3-hydrate inventory accreted by Callisto and Ganymede. The resulting N2 atmospheres may have been too thin to survive the atmospheric erosion effects that Titan has withstood.”

So maybe there should be a new Class of Moons, call it
Nitrogen Existence Zone. This would be a moon forming where solar radiation is 1/50 to 1/100 compared to Earth, Orbiting outside radiation belt of Jovian the size of 1/4 to 1/3 of Jupiter masses, with a mass equal to or greater our LUNA.

What is the frequency of Titan type Moons out there. in the galaxy.
it’s probably less rare than Earth Twins in the HZ, but still sparsely
spread.

P.S. It looks like it takes a fairly massive Jovian to wind up with large moon. (triton is a captured moon, our moon was captured absorbed and re-emmited)

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DJ Kaplan March 17, 2017 at 15:47

If the nitrogen is thrown off as a by-product of some para-metabolic process, then it’s involving one of the cyanide-related compounds on Titan. Something is consuming cyanide and excreting nitrogen maybe?

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Michael March 19, 2017 at 3:38

I am wondering if it is on a fault zone, if you look in the main picture top left there appears to be a long trench. Now if you look at the last small picture you will notice a faint under liquid trench, it lines up with the top left one to a large degree.

Reply

Michael March 19, 2017 at 8:58

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