Tracking Changes in Titan’s Atmosphere

by Paul Gilster on November 29, 2012

Even as New Horizons continues to push toward Pluto, now just past the halfway point between the orbits of Uranus and Neptune, we’re continuing to get excellent data from the much closer Cassini spacecraft around Saturn. Cassini’s composite infrared spectrometer (CIRS) is probing the circulation and chemistry of Titan’s atmosphere, tracking how gases like benzene and hydrogen cyanide are distributed and affected by changes in temperature and circulation. We’re getting a closeup view of how chemistry and atmospheric circulation modify climate on the distant moon, information that shows a good deal of change over a short period of time.

The latest from Cassini, written up in a paper just published in Nature, involves a shift in seasonal sunlight that is apparently tied to the reversal in circulation around the moon’s south pole. Earlier in the mission the air here was rising. Now there is strong evidence for sinking air. Nick Teanby (University of Bristol, UK) notes how quickly the transition has occurred given the fact that Titan’s year is the equivalent of nearly 30 Earth years, and changes here generally occur slowly. Fortunately, Cassini is in the right position to see the changes in progress.

Image: This true color image captured by NASA’S Cassini spacecraft before a distant flyby of Saturn’s moon Titan on June 27, 2012, shows a south polar vortex, or a mass of swirling gas around the pole in the atmosphere of the moon. The south pole of Titan (3,200 miles, or 5,150 kilometers, across) is near the center of the view. NASA/JPL-Caltech/Space Science Institute.

In Cassini’s first years at Saturn, Titan’s north pole was associated with a high-altitude hot spot at a time of northern winter. A visible ‘hood’ of dense high altitude haze remained lit by sunlight even though much of the northern latitudes remained dark. As the southern hemisphere moved into autumn and began to tilt away from the Sun, the north pole hot spot began to disappear, while similar features — evidently induced by the compression of sinking air above the pole — started showing up at the south pole. The reversal from uplift to sinking air evidently occurred in a period of six months around the equinox of August 2009, when the Sun was directly over Titan’s equator. Within two years of the reversal, some of the gases CIRS tracks had increased 100-fold.

“Next, we would expect to see the vortex over the south pole build up,” said Mike Flasar, the CIRS principal investigator at NASA’s Goddard Space Flight Center in Greenbelt, Md. “As that happens, one question is whether the south winter pole will be the identical twin of the north winter pole, or will it have a distinct personality? The most important thing is to be able to keep watching as these changes happen.”

The story involves more than changes in Titan’s vertical winds. The CIRS instrument also found that complex chemicals are being produced at altitudes up to 600 kilometers above the surface, showing us that atmospheric circulation extends a good deal higher than expected. In fact, complex chemistry and vertical movement in the atmosphere occur above the haze layer first detected by the Voyager spacecraft. What had been thought to be a ‘detached’ layer of haze may actually be a transformational area where small haze particles combine before descending deeper into the atmosphere, where they produce the orange color characteristic of the moon.

The paper is Teanby et al., “Active upper-atmosphere chemistry and dynamics from polar circulation reversal on Titan,” Nature Vol. 491 No. 7426 — I’ll have a link to the abstract when Nature‘s servers, currently down, get back online. More in this JPL news release.

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James M Essig December 1, 2012 at 8:21

We need to send a floatation lander and also a submersible to ply the hydrocarbon seas of Titan. A airplane or balloon would be good as well.

Other folks who read this, feel free to offer your own perspectives. I notice so far that this thread and the previous one have very few or no comments yet. All you astrobiologist types and planetary science folks, feel free to offer your comments in respose to my above comments.

As an occasional commentor at this website, I would like to hear what you have to say on the matter. I am thinking about getting into some sort of lobbying efforts to advocate such planetary science missions although I have no idea how to go about such. I would get involved in lobbying for interstellar propulsion system design which is my favorite topic, but TZ-CD, the 100 Year Starship Program, Icarus Interstellar and other great organizations all ready have that covered.

Rob Henry December 2, 2012 at 2:14

James M Essig, I think there are a few aspects of Titan’s situation that are underrated. The one that concerns me the most is as follows.

Titans atmosphere is very complex and its chemistry correspondingly hard to predict. One exception to this rule is the release of hydrogen by methane due to the absorption of two very narrow bands of uv light. This is such a simple and efficient way to release H2 that it is complete at very high altitudes (600-800km from memory). Anyhow, this, when combined with the thermal rate of loss, should dictate a minimum level of hydrogen in the outer atmosphere. This would be expected to be augmented by more H2 released deeper in the atmosphere by more complex process. Instead, current data suggests a flow the other way. This flow represents 0.4mg/sqkm, and, under the unrealistic assumption that no more hydrogen is released by process involving uv absorption in the smoggy lower atmosphere, this is the amount of hydrogen absorbed by the ground. This is about 20W of chemical activity per square kilometre, compared to 100,000 W/sqkm of photosynthetic dark reaction energy that is available for life in the Earth’s oceans.

This may sound low, but an Europan ecosystem that is geothermally driven would have its level of activity capped at a maximum of 0.001 W/sqkm, yet a few think that it is possible that very slow living higher forms might develop there. If that anomalous hydrogen flow is confirmed AND due to life then we know that Titan’s MINIMUM activity is 20,000x this. To me we should be thinking of a way of capping the maximum as that is really what we should be contemplating as we evaluate immediate future exploration priorities. My attempt to do so follows.

If life there is drive by uv induced production of high energy chemicals and H2, then the energy available from other chemical reactions in the lower atmosphere should only be about an order of magnitude more by my guess, perhaps capping it around 200W/sqkm. And if it is photosynthetically driven, then we must ask is there any reason to believe that it is as inefficient as ours, or as limited by mineral deficiency. About a third of visible light, but I believe no uv, perpetrates to Titan’s surface, so if photosynthesis is as efficient there as sugar cane under ideal laboratory conditions, that maximum should be around 30,000 W/sqkm. That figure says GO THERE NOW.

Rob Henry December 2, 2012 at 4:55

James, I should address another suggested problem that keeps recurring – that life at Titan’s temperatures must be very slow, however this currently implied minimum activity actually makes this a positive. Let me explain.

If we want to find life then the greater biomass the easier the task. If activity levels are equivalent, then the MINIMUM biomass surface density on Titan should be 20/100,000 = 0.02% that of our oceans, but if it is a thousand times slower it should have 20% our biomass density. As I mentioned above, I believe that this minimum is almost certainly about order of magnitude too low. Furthermore, if life there is confined to the seas (as life on Earth was through most of its history), our oceans would likely look the poorer!

Rob Henry December 2, 2012 at 5:00

Oops. I did not make it clear that by “this minimum” I meant minimum biological activity, not minimum biomass.

James M Essig December 3, 2012 at 12:25

Hi Rob;

Thanks for the information and enthusiasm. We must go to Titan very soon. The complexity of hydrocarbon chemistry is beconing for us to send probes in droves and eventually, manned missions.

ljk December 4, 2012 at 11:00

James M Essig said on December 1, 2012 at 8:21:

“We need to send a floatation lander and also a submersible to ply the hydrocarbon seas of Titan. A airplane or balloon would be good as well.”

Actually a helicopter might work best in Titan’s thicker atmosphere:

http://www.lpi.usra.edu/meetings/outerplanets2001/pdf/4003.pdf

I do not care if we get to Titan by handcart, we must explore that world! Even if there is no native life present, it is among the most exciting and dynamic places in the Sol system. We could visit many places in our celestial neighborhood at once, if we had our priorities straight.

Not sending missions to Titan is almost a scientific and cultural crime.

ljk December 4, 2012 at 11:04

3 December 2012

Text & Images:

http://news.stanford.edu/pr/2012/pr-titan-saturn-moon-120312.html

TITAN, SATURN’S LARGEST MOON, ICIER THAN THOUGHT, SAY STANFORD SCIENTISTS

Scientists have long suspected that a vast ocean of liquid water lies under the crusty exterior of Titan, Saturn’s largest moon. New analysis suggests that the internally generated heat that keeps that ocean from freezing relies on the moon’s interactions with Saturn and its other moons.

A new analysis of topographic and gravity data from Titan, the largest of Saturn’s moons, indicates that Titan’s icy outer crust is twice as thick as has generally been thought.

Scientists have long suspected that a vast ocean of liquid water lies under the crust. The new study suggests that the internally generated heat that keeps that ocean from freezing solid depends far more on Titan’s interactions with Saturn and its other moons than had been suspected.

Howard Zebker, a professor of geophysics and of electrical engineering at Stanford University, will present the findings at the annual meeting of the American Geophysical Union (AGU) in San Francisco on Tuesday (Dec. 4) at 1:40 p.m. PST in Room 2005 of Moscone Center West.

Zebker is part of the team interpreting radar data of Titan acquired by NASA’s Cassini spacecraft, which has been orbiting Saturn since 2004. He has been studying the topography of Titan, and has combined improved radar measurements of the moon’s surface with newly released gravity measurements to make the new analysis.

Titan has long intrigued scientists because of its similarities to the Earth. Like Earth, Titan appears to have a layered structure, crudely similar to the concentric layers of an onion, albeit far less edible.

“Titan probably has a core that is a mixture of ice and rock,” said Zebker. The core is overlain by the ocean and icy crust.

The rock in the core is thought to contain radioactive elements left over from the formation of the solar system. As in Earth’s core, when those elements decay, they generate heat. On Titan, that heat is crucial to keeping its ocean from freezing solid.

As Titan orbits Saturn, Titan is slowly spinning on its axis, one spin for each trip around Saturn. Still, that spin is enough for the gravity instrument onboard Cassini to measure the resistance of Titan to any changes in its spin — also called the moment of inertia.

“The moment of inertia depends essentially on the thickness of the layers of material within Titan,” Zebker said. Thus, he and his graduate students were able to use that data to calculate the moon’s internal structure.

“The picture of Titan that we get has an icy, rocky core with a radius of a little over 2,000 kilometers, an ocean somewhere in the range of 225 to 300 kilometers thick and an ice layer that is 200 kilometers thick,” he said.

Previous models of Titan’s structure estimated the icy crust to be approximately 100 kilometers thick. So if there is more ice, then there should be less heat from the core than had been estimated. One way to account for less heat being generated internally is for there to be less rock and more ice in the core than previous models had predicted.

That all seems simple enough, but there is a complication. Titan is not a true sphere. Its shape is distorted by the gravitational pull of Saturn, making the moon sort of oblong along its equator and a little flattened at the poles.

From measurements of the observed gravitational field of Titan, one can compute what the shape of Titan ought to be. But the new data show that Titan’s shape is much more distorted than would be predicted by a simple gravitational model.

That discrepancy means the internal structure of Titan isn’t quite so simple.

[The rest of this article is hyperlinked at top.]

James M Essig December 5, 2012 at 10:59

Hi ljk;

You could not have put it better.

Finding any form of aninal life would be totally awesome on th surface, in the lakes, or below the ground.

I watched a TV special on exobiology about a few years ago where mention of prospects for life or lack thereof on Titan was one of the primary themes of the broadcast. One of the artistic depictions of a hypothetical animal on Titan looked like a furry snow ball with its mouth wide open making a monotonic crying sound that sounded like a cross between a human infant cry, a cat meow, and a women holding a note at an opera. The creature actually looked and sounded very cute.

Animal life of all kinds may be out there including intellegent personal forms.

The helicopter idea is one that I never did think of before but which is excellent.

I’d also like to see submersible mission to Europa. Io, and other icy moons.

Provided 20 billion dollars was allocated for interplanetary robotic science missions, we could have some real fun over the next two decades all the while waiting in anticipation of our first missions to other star systems.

ljk December 6, 2012 at 10:48

We should focus first on exploring Titan for the sake of knowing more about the moon itself. Any native life forms there, which are no guarantee, should be thought of as a scientific bonus, not the focus or even goal.

That has been the problem with Mars exploration since the days of Mariner 3 and 4. Look at how bent out of shape people got about that one overblown word by one guy at JPL on upcoming information from the Curiousity rover because the public thought it was going to be a life discovery announcement.

There may be life on Mars somewhere or at least its fossils, but it should be clear by now there are no canals, thoats, or plants. Even the microbes seem to be in hiding. Mars should be better studied for itself and what we do know. I am amazed it took so long for NASA and others to send instruments to the Red Planet to truly analyze the surface first, rather than focusing on expensive portable biology labs.

I am as big a fan of alien life as anyone and I support all aspects of its search and understanding. However, I have seen what happens to space projects when the focus is on finding life on worlds where we do not know the fuller picture, such as with Viking. It was considered a failure when the life reports came back as ambiguous despite all the other amazing things those two robot landers did and found. That is why the next Mars surface mission did not happen for almost two decades.

For the sake of future missions to Titan and elsewhere, I do not want to see that happen again because everyone is focusing on the life issue. Let us see what that amazing world is really like first, then we can determine if the search for native organisms is worth it. And NASA or whoever does the mission should make that VERY clear from the start, so there are no more miscommunications and disappointments than necessary.

James M Essig December 7, 2012 at 11:37

ljk;

You make some good points. Even the Earth’s moon should be explored if only for the possibilities of mining operations to collect rare elements and isotopes, a prominently potentially useful isotope being Helium-3.

The knowledge we can gain about Titan can help us obtain an understanding about how its surface and atmosphere may be able to evolve life in the event that no life is currently present. In the latter case, give stellar evolution about 1 billion to 3 billion years or more and Titan may become a temperate environment, perhaps even for deep future human habitation.

Not sure how we could use the copious quantities of hydrocarbons on Titan, though, but my feeling is that perhaps there might be found a cheap way to capture, store, and transport the carbonaceous gases and liquids and use these potential natural resources as feed stock for industrial practices either in interplanetary colonies, on the surface of Mars, the polar regions of Mercury, on Earth’s moon, or on Earth. However, we would need to be careful with how much carbon we introduce into the Earth biosphere/atmosphere because of the global warming issues.

Perhaps slow moving solar sail type barges can be used to transport the feedstock and resulting industrial products. Since the time criticality for the economic aspects of each batch of chemicals will likely not be an issue with continuous and well estabilished infrastructure, solar sails may be our least cost option for slow transport of the goods.

Such hydrocarbon feedstock might be excellent for insitu or nearly so light sail and microwave sail manufacture. One thought that comes to mind is the manufacture of Nylon, Kevlar, Zylon, Kapton, as well as graphene, carbon nanotubes, artificial diamond fiber and the like.

Even in the absense of any immeadiately appearent industrial use for Titan’s hydrocarbons, I tend to feel that free markets and industry will find a way to harvest and use Titan’s vast resources.

Rob Henry December 9, 2012 at 6:09

James M Essig, when I look at Titan I can see no resources of hydrocarbons. Hydrogen and Carbon are extremely common a in our solar system, and even on other moons of Saturn, we could extract C and H with so much less lifting against gravity, that they would be the cheaper source ever if those hydrocarbons were manufactured.

However I can see nitrogen there that could be available for terraforming Mars. there are several keys to this. The first is that the atmosphere largely nitrogen (not methane). The second is that Mars also has an atmosphere. The third, is that, if we are very clever, we can deflect a vessel around an atmosphere in the same manner as if it is slingshoting around a giant planet for gravitational assist. This gives us free energy to scoop up a bit of that atmosphere from Titan and dump it on Mars.

James M Essig December 9, 2012 at 18:06

Hi Rob;

The nitrogen scoop. Now that a cool idea! By the way, have you heard of the latest scoop on a proposed form of high energy density molecular nitrogen fuel. Might be good for single stage back and forth from Earth to Mars manned craft and likewise with regard to the outer reaches of the planetary solar system.

I saw a web reference for such a material about a year or so ago and the conjectured specific impulse is very high.

With all of the nitrogen in the solar system, such a fuel may be ideal for interplanetary commerce.

Rob Henry December 10, 2012 at 18:28

Hi James, that N5+N5- fuel suddenly looks far more interesting to me in this context than in the single stage to orbit one where it originally appears. The implied specific impulse is actually rather low, being around half that of the standard LOX/LH mix. Its real advantage comes in storage.

By its very nature, this hydrogen scoop would have to be heavy in comparison to each additional payload of scooped N2. Here things work in reverse to normal spacecraft mass considerations, and the more deadweight it carries the better. Also it would have to fit in some dense aerodynamic capsule. Finally the game of celestial billiards it must play dictates that there will be occasions when it will do several Titan scoops before it revisits Mars.

Putting that all together, we can unpack a big mass of cheap solar cells between these visits, and use the power to compact that N2 into that solid storable form. Its ISP might be low, but it would still be powerful it used in Oberth effect situations, and is sufficient for cause corrections.

Though I believe your reasoning faulty, you might have hit the final answer that we may one day come to.

James M Essig December 11, 2012 at 14:50

Hi Rob;

Thanks for the enthusiasm. Nitrogen happens to be one of my favorite elements because of its capacity for stored chemical energy. As a young boy about 40 years ago, I enjoyed the notion of using nitrogenic processes to produce large and fun TNT fire crackers, something I never actually did do. However, indeed, with the huge amount on nitrogen in the solar system, the prospects for merely modest specific impulse generation for sporting around the solar system seems great.

Rob Henry December 12, 2012 at 6:28

James, if such wave rider type vessels do turn out to be one of the few technologies that work as planned (here the problem is to get lots of lift for very little drag), then it seems to me that we can choose between hydrogen/helium or carbon dioxide, or N2 or N2/O2 (or a combination thereof) from which to make a space fuel obtainable for a few cents per ton. The gas giants would have by far the highest encounter speeds, so perhaps H2 (that would be optimal to refuel nuclear engines) is out, and then N5+N5- looks better and better.

So, under those highly unusual circumstances, your fuel might end up being the most common fuel, perhaps augmented by NO2/hydrazine (a hypergolic combination with a specific impulse 50% higher) when enough hydrogen can be scrounged.

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