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The “Air” Up There

When NASA Goddard scientists Martin Cordiner and Conor Nixon took a look at the chemical make-up of the atmosphere of Titan using a millimeter wave telescope, what they found was surprising. Cordiner led an international team in a study of Saturn’s moon using the Atacama Large Millimeter/submillimeter Array (ALMA), a network of high-precision antennas in Chile. They looked at the concentrations of two organic molecules (hydrogen isocyanide (HNC) and cyanoacetylene (HC3N)) in Titan’s atmosphere. How? Nixon says, “The molecules emit at slightly different frequencies, so you can think of them as tiny ‘radio stations.’ We tune into a molecule the way you tune into a radio station, each has a characteristic frequency (actually, more than one.) When ALMA listens to one frequency and uses all its ‘eyes’ (dishes) to look, it can make a map of that molecule.”

The surprise they found was not so much that the gases containing these molecules appear to be concentrated over Titan’s poles, but that at high altitudes, the gas is shifted away from the poles. Says Cordiner, “The HNC molecule’s location in the atmosphere had never been mapped before, so when we saw that it was displaced from the poles, this came as a big surprise, and hinted that a peculiar set of forces (never before observed) must be at work for this molecule. We subsequently discovered that the high-altitude HC3N was displaced in a similar fashion to the HNC, showing that this effect was not just an isolated occurrence for the HNC molecule, but that it is likely a more general effect, impacting other organic molecules produced in Titan’s upper atmosphere.”

Titan's Atmosphere

High in the atmosphere of Titan, large patches of two trace gases glow near the north pole, on the dusk side of the moon, and near the south pole, on the dawn side. Brighter colors indicate stronger signals from the two gases, HNC (left) and HC3N (right); red hues indicate less pronounced signals.
Image Credit:

We interviewed both Cordiner and Nixon to learn more about their research.

NASA Blueshift: What do you do here at NASA Goddard? What is the primary focus for your research?

Conor Nixon: I study (mostly) planetary atmospheres, particularly the atmospheres of bodies in the outer solar system, especially Jupiter, Saturn and Titan. I don’t study airless bodies (asteroids or small moons) or inner solar system planets (Mars, Venus, Mercury) – at least not yet! I also do a little bit of work on predicting atmospheres of exoplanets.

The chemistry of atmospheres in the inner vs outer solar system is quite different. In the inner solar system the gases are mostly oxidized (CO2, O2, H2O) whereas in the outer solar system they are mostly ‘reduced’ (opposite of oxidized – so heavy elements mostly bonded to H instead of O – e.g CH4, H2, NH3 etc). Titan is somewhat in between since it is mostly N2, like the Earth, but still not much oxygen compounds.

In terms of techniques, I have mostly used infrared spectroscopy, especially spacecraft data from Cassini and Galileo. Sub-mm (ALMA) is new for me. I am the Deputy PI of one of the twelve instruments on Cassini (CIRS).

Martin Cordiner: I’m a researcher in astrochemistry and astrobiology at NASA, focusing my attention on the evolution of chemical complexity in the universe. I work on looking at how organic molecules are produced in interstellar clouds and the environments around young stars, as well as in cometary and planetary atmospheres. Basically, wherever molecules are found in space, I like to measure them using the most powerful telescopes on Earth, and try to work out how they got there.

NASA Blueshift: What makes Titan such an interesting and/or unique body to study? What can studying Titan teach us about our own planet? Can studies of bodies like Titan pave the way for future exoplanet/exomoon studies? (ie, Is there a bridge between studying our own solar system and exoplanet systems?)

Conor Nixon: Titan’s atmosphere is mostly N2 (95-98%, depending on altitude) like the Earth (78%). The surface pressure is also similar to the Earth (about 1.5x as much). However there are big differences: the surface is much, much colder (-179 C, -290 F) and the ‘bedrock’ is not rock, but water ice. Titan is about 50/50 rock/ice by mass, but the rock is all in the middle and the mantle/crust is water ice.

Titan has no free oxygen (Earth has 20%) and negligible amounts of water vapor due to the extreme coldness. However, methane gas in the atmosphere (2-5%) takes the place of water as a condensible, and forms clouds and rain, and on the surface, rivers, lakes and seas. Therefore Titan is the only other place in the solar system where there is rain falling onto a solid surface (today – Mars a long time ago!) and making fluvial-erosional landforms, such as channels and river deltas.

Therefore if you study rivers and lakes, there is Titan and Earth, that’s it!

Needless to say, Titan is the only moon in the solar system with a significant atmosphere, and the reason for that is currently unknown (although there are theories, of course.)

Martin Cordiner: I’d like to add that Titan’s nitrogen and methane atmosphere is believed to be similar to what the Earth’s atmosphere could have been like billions of years ago, before plants and algae emerged and filled the atmosphere with oxygen. Studies of the chemistry occurring now on Titan may thus provide insights into the processes taking place on the young Earth that eventually gave rise to the complex chemical machines that we humans refer to as ‘life’.

NASA Blueshift: Why was the discovery of these molecules being shifted away from Titan’s poles a surprising result? What new information does this give you about about Titan’s atmosphere?

Conor Nixon: So, the basic ‘glow’ (or emission) itself is understood: these are molecules in Titan’s middle atmosphere that have been seen before. The curious part is *where* they are glowing in these new maps, since we haven’t had anything comparable to ALMA before, which has made these first-of-a-kind 2D maps. Any increased emission can be due to one of two things: (1) there are more molecules in this place or (2) they are hotter in these places. At this point, we don’t know which of those two things is causing the enhancement.

It’s definitely intriguing that we’re seeing this glow shift away from the poles at 300-500 km altitude, while at lower altitudes it really seems to be over the poles.

Mixing by fast-blowing winds should mix the gases around in a circle at a given latitude quite quickly, much more quickly than chemistry can change the gas abundances. HNC is quite short-leved (2-5 Earth days), so it’s less certain that it couldn’t have a daily difference (from day side to night side, or dawn to dusk) than it is for HC3N, which is really long lived (1 Earth year.) However then the bigger mystery is why dawn concentration in the south and dusk in the north?

Martin Cordiner: This discovery tells us that Titan’s mid-to-upper atmosphere can be much more variable than is generally considered. Titan’s ‘weather’ just got a lot more complicated.

NASA Blueshift: Are there hypotheses to explain this phenomena yet?

Conor Nixon: Rather than the gases being increased in certain spots, it could just be that the gases are hotter. So then we’d need to explain a temperature ‘hot spot’ – which actually has many of the same difficulties as the explanation of increased gases – simply that the winds would redistribute the energy quickly. I am still wondering if this could be some kind of ‘aurora’ or at least aurora-like heating, but it seems awfully deep in the atmosphere for ions to penetrate. We are still trying to check that hypothesis more quantitatively.

Another explanation is that it could be a kink in circulation cell, that causes it to veer away from the pole at high altitude.

Martin Cordiner: From looking at ALMA archival data, I am under the impression that what we observed could have been a rare, transient phenomenon on Titan. It may be the result of a peculiar circulation pattern or nonuniform heating or cooling of the atmosphere that occurred at a particular moment in time around the time of our observations. We are already planning observations to examine this possibility in more detail.

NASA Blueshift: What new questions does this raise about how Titan’s atmosphere works?

Conor Nixon: This is definitely causing a rethink of Titan’s middle atmosphere, since previously we haven’t ever suspected that we could have this kind of effect.

Martin Cordiner: How can Titan’s chemistry/physics give rise to these apparently paradoxical emission patterns? Do the brightness hotspots correspond to increased chemical production at these locations, or is there some kind of unexpected and bizarre heating/cooling pattern in effect? Can high-altitude circulation phenomena explain our results? Was this a one-off observation, or will we see it when we look again at HNC and other molecules?

NASA Blueshift: Given these new questions, what is in the future for research on Titan’s atmosphere, in both the short and longer term?

Conor Nixon: The main tools we have right now are Cassini, plus ground-based telescopes (ALMA, and optical telescopes) and space-based telescopes (Hubble, and eventually JWST).


Near-infrared image of Titan, captured by the Visual and Infrared Mapping Spectrometer on Cassini.
Image credit: NASA/JPL-Caltech/University of Arizona/University of Idaho

Cassini is the main driver right now, since it is so close and can look at everything in detail. But when Cassini ends in 2017 ALMA and JWST will be crucial.

There are still numerous open questions, including the origin and longevity of the atmosphere (it may be transitory), and many details about how the atmosphere works, what the lakes/seas are made of, and the structure of the interior.

Martin Cordiner: Using ALMA, we plan to return to Titan in the coming years to map its molecular species in more detail. The overall objective is a complete understanding of Titan’s atmospheric physics and chemistry, that will provide new insights into the processes occurring on planets nearby, in our own Solar System, as well as those orbiting other stars in the distant universe.

NASA Blueshift: Thank you!

For more reading, there is a press release and a Tech Times article.

As far as James Webb Space Telescope observations of Titan, there is a recently released PDF flyer by Space Telescope Science Institute on this very topic (lead authored by Nixon). Titan has a 29.5 Earth-year annual cycle and it will be northern summer when JWST launches and begins observations in 2018. JWST will be able to do long-term monitoring of the changing spatial distributions of gases, clouds and hazes on this moon and help scientists to better understand the chemistry and dynamics of its seasonal cycle.

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