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Saturn's largest moon Titan is by far the strangest place in the solar system: An unimaginably frigid world with a thick, opaque atmosphere where the clouds rain liquid natural gas, the "rocks" and mountains are composed of water-ice as hard as granite, and rivers of hydrocarbons run to organic chemical seas.  It is a world with eerie similarities to the processes that shape Earth, and yet is so far outside our frame of reference in temperature and bizarre chemistry that even visiting it with robotic probes presents unique technological challenges.  But most importantly, while Titan may someday become a human world, the most fascinating thing of all about the Orange Moon of Mystery is what may already live there.  In Vol. 3, we explore some general aspects of Titan's atmosphere.

The progress of our adventure so far (current in bold):

1.  The Sun
2.  Mercury
3.  Venus
4.  Earth (Vol. 1)
5.  Earth (Vol. 2)
6.  Earth (Vol. 3)
7.  Earth (Vol. 4)
8.  Earth (Vol. 5)
9.  Earth (Vol. 6)
10.  Luna
11.  Mars (Vol. 1)
12.  Mars (Vol. 2)
13.  Mars (Vol. 3)
14.  Phobos & Deimos
15.  Asteroids (Vol. 1)
16.  Asteroids (Vol. 2)
17.  Asteroids (Vol. 3)
18.  Ceres
19.  Jupiter (Vol. 1)
20.  Jupiter (Vol. 2)
21.  Io
22.  Europa (Vol. 1)
23.  Europa (Vol. 2)
24.  Ganymede
25.  Callisto
26.  Saturn (Vol. 1)
27.  Saturn (Vol. 2)
28.  Saturn (Vol. 3)
29.  Rings of Saturn
30.  Mimas
31.  Enceladus
32.  Tethys
33.  Dione
34.  Rhea
35.  Titan (Vol. 1)
36.  Titan (Vol. 2)
37.  Titan (Vol. 3)
38.  Titan (Vol. 4)
39.  Iapetus
40.  Minor Moons of Saturn
41.  Uranus
42.  Miranda
43.  Ariel
44.  Umbriel
45.  Titania
46.  Oberon
47.  Neptune
48.  Triton
49.  The Kuiper Belt & Scattered Disk
50.  Comets
51.  The Interstellar Neighborhood
52.  Updates
53.  Overview: Human Destiny Among the Worlds of Sol
54.  Test Your Knowledge
Titan's atmosphere in true color, as we would see it if we were descending into it:

PIA06236

5.  Atmosphere

(i) General Description

Titan has a thick and partly Earth-like atmosphere overwhelmingly composed of nitrogen (N2) with a minority of methane (CH4), with rarer and more complex hydrocarbon compounds responsible for the opaque orange haze that blocks the surface from view in visible light.  It's somewhat thicker, taller (due to Titan's lower gravity), and more massive than Earth's, but would still be manageable for humans in terms of pressure - although far too cold for a human to survive even for a moment without intense heating and bulky insulation.  Despite its bizarre and hostile conditions, the Titanian atmosphere is the most like our own of any other world in the solar system.

A common misconception about Titan is that its surface is obscured by clouds, but in fact clouds are not the same thing as haze: A haze is a uniform, shapeless region of an atmosphere that makes objects look progressively dimmer and less distinct the deeper they are from an observer's perspective.  Hazes occur at certain temperatures and pressures for a given substance, when a normally clear gas condenses just enough to start blocking light but not enough to form coherent structures (clouds) that block it mostly or completely.  The most common hazes on Earth are made of water, and tend to occur in valleys and coastal regions, while those of Titan are global, perpetual, and consist of a variety of different hydrocarbons.  This photo I took from a small mountain overlooking the San Gabriel valley of Southern California shows our familiar experience of haze:

P1180811

Notice that there are no horizontal borders to the obscuring region - it forms a more or less uniform plane that obscures more as you look deeper into it, with foreground objects mostly visible and the most distant background objects within the haze layer totally blocked.  This is why we can't see Titan's surface in visible light.  Actual clouds - regions where vapors and particulates stick together in loosely-bounded shapes and structures - aren't as pervasive in Titan's atmosphere, although they do occur regularly and are composed of some of the same substances as the hazes.  Compare the above with this image from the Huygens probe on descent:

Huygens Coming in For Landing 3  

Hydrocarbon hazes can condense into clouds on Titan when temperature and pressure change, in the same way that water hazes condense into clouds on Earth (or in the opposite direction, become transparent atmospheric vapor content), and that's partly why the haze in the Titan image above looks more "oily" than the water-haze in the Earth image - the conditions are probably closer to cloud-formation, and in places are actually fog.  Also like on Earth, the clouds that do form often condense even further and rain, which is how Titan has methane/ethane seas and lakes, so in some ways Titan is a bizarre analog of processes we experience but with totally different materials.  

It's usually difficult to see Titanian cloud structures in visible light because they tend to be closer to the ground, deep beneath higher-altitude haze, but sometimes they can be seen - in some instances spectacularly.  But clouds on Titan never consist of water: It's so cold that there is essentially zero water in the atmosphere, and (as far as we know) zero water in the lakes and seas except as a solid, granular seabed material like sand and rock.  

(ii) Bulk Properties

Titan's atmosphere varies between about 98% to 95% nitrogen, with the highest values occurring in the upper atmosphere.  The vast majority of the remainder is methane, which peaks at the surface due to its greater density than nitrogen, with trace amounts of other compounds.  This is considerably more than the 78% of Earth's atmosphere that's nitrogen, and has effectively zero oxygen because it was all bound up in water ice in the formation of the Saturn system.  This means that other oxygen compounds seen in atmospheres in the inner solar system, particularly CO2, are not present in any significant proportions.  But there is a little hydrogen (H2) by itself at 0.1% to 0.2%, which isn't really possible on a moon or terrestrial planet closer to the Sun - hydrogen is too light, and would just blow away into space under hotter conditions.  Comparing Titan's surface atmospheric composition to Earth's:

Earth vs. Titan Surface Atmospheres  

One important consequence of the lack of oxygen in the atmosphere is the fact that ozone (O3) - which protects Earth's surface from solar ultraviolet rays - is not available on Titan, but rather methane and other hydrocarbon compounds do the same thing by absorbing the high-energy UV photons and are chemically altered by them.  The hazes that shroud Titan are a result of this process breaking apart methane and diatomic nitrogen (a process known alternately as photolysis or photodissociation) into simpler molecules or even individual atoms that recombine in a variety of ways, some of which form obscuring hazes at the temperature and pressure of that part of the atmosphere.  Illustration:

PIA17240

Other, heavier byproducts of this process reach the surface as dark hydrocarbon granules that can be analogized to snows, and end up blown by the wind into lowland plains - particularly dry seabeds like Shangri-La.  However, these complex chemicals are at any time only a very small proportion of the atmosphere, and are of interest to scientists mainly because they're thought to resemble the prebiotic "soup" of early Earth.  But the hydrological cycle that drives Titanian climate is overwhelmingly based on the simplest and most abundant hydrocarbon in the atmosphere, methane.  This is what forms most of the clouds on Titan, and most of the composition of the lakes and seas.  It's basically the same stuff burned in your gas stove, just profoundly colder.

Titan's atmosphere is denser, 19% more massive, and hundreds of kilometers taller than Earth's, and also taller even than that of Venus.  This means it has the second thickest and second most massive solid-body atmosphere in the solar system after Venus, and the #1 tallest air column.  It has more gas overall than the air we breathe, and more nitrogen and methane specifically, but no appreciable quantity of oxygen.  

Importantly, the gas thins out much more slowly with altitude due to Titan only having 14% of Earth's surface gravity, meaning that it's less compressed.  As a result, spacecraft flying nearly 1000 km over the surface (more than double the height of the International Space Station over Earth) have had to correct their flight path due to atmospheric drag.  This is very helpful for landing spacecraft on Titan, since parachutes work well in the dense atmosphere and have plenty of time to slow down before reaching the surface, but blasting off from the surface and getting back into space could be challenging despite lower gravity requiring lower velocity.  Still, it is worth noting that balloons could float around Titan at altitudes that would be in hard vacuum around Earth or Venus - and from those heights (if not lower), Saturn and its other moons would probably be visible.

(iii) Greenhouse and Anti-greenhouse Effects

The thermal environment on a world with an atmosphere is not easy to predict, which is why scientific views about what kind of exoplanets could be habitable are constantly changing.  If Earth had a different atmosphere, it could either be a ball of ice or an inferno, and to a lesser extent it has been both in its history: Its location in the solar system only determines what kind of materials it started out with and how much energy reaches it, not what happens to that energy.  So facts play out on Titan that both cool and warm its surface.

Greenhouse gases (of which methane is an especially potent one) selectively allow sunlight to enter the atmosphere but won't let the resulting heat leave, and that actually makes Titan warmer than it otherwise would be if it had less methane.  But at the same time, its haze layers produce an anti-greenhouse effect, reflecting 90% of the sunlight that reaches them back into space while allowing surface heat to escape.  The former raises Titan's surface temperature by 21 kelvins (21 C° or 38 F°) while the latter reduces it by 9 kelvins (9 C° or 16 F°), so the warming wins out by 12 kelvins (12 C° or 22 F°).

That fact should serve as a worrisome parable about the power of the greenhouse effect to overcome countervailing influences, and casts doubt on the efficacy of geoengineering solutions to climate change on Earth.  Let me reiterate: Titan's clouds and hazes block 90% of sunlight from reaching the surface, but the surface is still 12 C° hotter than its surroundings in the Saturn system due to methane.  So it would be a wonderful gas to let loose on Mars or other worlds with a cold problem, but pretty insane that we're pumping it and CO2 into the atmosphere of this planet.

The irony of Titan is that it gets wetter the colder it gets, because it's on the upper end of the temperature scale for methane and ethane being liquid.  So there's a "vast wasteland" of hotter possible temperature conditions where Titan would be a desert with no liquid at all because the methane and ethane would all be gas while the water ice surface would still be too cold to contribute water vapor to the atmosphere.  But if it were globally somewhat colder than it is, we'd see the currently dry equatorial seas fill up with dark hydrocarbon liquid and maybe even overflow into global oceans.  This is one of the exotic ironies of alien environments: That you actually have to go far from familiar circumstances before things once again start to behave in familiar ways.  Titan - the only other solid body in the solar system with a thick atmosphere, rains, and surface seas - would be a lot harder to live on than Mars, which has none of them.

(iv) Layers, Temperatures, and Pressures

Even with what little we know about it, Titan's atmosphere is actually more complex than Earth's, and as mentioned earlier, a lot taller:

Titan Atmosphere

What this shows is that UV is absorbed by the upper atmosphere, visible light is reflected by different layers (bluish colors by the upper atmosphere, reddish by the lower), but the whole column is transparent to infrared - which is why the surface images taken from space that were shown in Vol. 2 were all in IR wavelengths.  UV and visible images are only useful where Titan is concerned for looking at the atmosphere, if your perspective is above it.  A demonstration of the principle - Titan in visible (left), Titan in UV (middle), then Titan in IR (right):

Visible, UV, and IR

The fact that you get that blue halo in visible images is because of how the upper atmosphere reflects or scatters it (I'm not quite clear on which), so it's possible that if you were in a high-altitude balloon on Titan that the sky would be dark blue.  UV tends to show more detail in the atmosphere because there's a lot more absorption going on, so you get those deeper, more intricate halos, and in full-face images can see cloud structures and weather patterns more clearly.  A closer UV image of the upper-atmosphere shows details that aren't discernable in visible images:

PIA06160 (UV)

The uppermost haze layer - and the one responsible for Titan's opaque orange-brown color in visible light - is composed of chemicals called tholins that form out of the carbon, hydrogen, and nitrogen atoms freed from CH4 and N2 by photolysis.  One major tholin substance is C6H6, better known as benzene.  It's formed by a number of simpler chemical byproducts of methane dissociation such as hydrogen cyanide (HCN), acetylene (C2H2), and ethane (C2H6), as well as some ionized hydrocarbons.  We have some of this same stuff on Earth, but we know it by the term smog.  Ironically, what terrestrial life finds toxic could theoretically play a role in a Titanian biosphere.  An illustration explaining tholin formation - the "Da" refers to a unit of molecular mass:

477px-Formation_of_tholins_in_Titan's_upper_atmosphere.svg

Below the tholins are a number of other hazes and particulate clouds where other substances, both simpler and more complex, reach haze points or condensation points as the temperature and pressure change with altitude.  There are so many different substances involved that might have their own distinct behavior, their own haze layers, their own clouds even that rain or snow in relatively small quantities, that it really serves to show how much simpler Earth's environment is with its basically all H2O cloud and haze systems (apart from smog, which is trivially rare on Earth by comparison).   But by bulk proportions, methane and ethane dominate - especially at lower altitudes where rain clouds are seen.  

Going back to the pressure-temperature-altitude diagram from earlier, we can see that pressure goes down more gradually with altitude than on Earth, such that if you were standing on the summit of a mountain the height of Everest (8.8 km high) you wouldn't even have gotten down to 1 bar - Earth sea level pressure.  This is partly due to the fact that pressure begins at Titan sea level 45% higher than that, or equivalent on Earth to being in a mine shaft 3.3 km underground or 4.6 meters (15 feet) underwater.  That's not too bad from a human perspective.

Unfortunately, at about 94 K (-179 °C / -290 °F) surface temperatures would be instantly lethal - equivalent to bathing in cryogenic liquid oxygen.  If you were to step out into the Titanian environment wearing the world's most efficient parka, you would still turn into a statue in a split-second.  Even the most robust, temperature-hardened spacesuits in existence today couldn't handle it because they're designed for vacuum temperatures rather than convecting air - they'd keep you alive until the cryogenic air sucked the power out of the batteries, which would at most be a matter of minutes.  But it does get better at higher altitudes: Notice from the earlier diagram how temperatures drastically increase from altitudes of about 50 km to 200 km, getting into a stable range of about 160 K (-113 °C / -171 °F) to 170 K (-103 °C / -154 °F).  These are Mars winter temperatures, and appropriately enough begin at Mars-equivalent pressures - something that could be convenient for balloon-based exploration or even cloud settlement.  More on that later.

One other thing to notice is that as you ascend from the surface up to about 50 km, the temperature actually goes down before going up again, and that explains the primary cloud layer being labeled as "methane-nitrogen": As the temperature jags into deeper cryogenic territory, it approaches liquid nitrogen temperatures (77 K) where atmospheric N2 would start to condense into clouds.  However, it doesn't get cold enough to rain liquid nitrogen, so it remains as vapor when conditions are cold enough to form that kind of cloud.  The methane and ethane, however, do condense further and drive the hydrological cycle.

So there are roughly speaking three temperature regions of the atmosphere: The relatively warm upper atmosphere, a transition region where the hazes occur, and then the cryogenic lower atmosphere with the cloud layers and liquid cycles that interact with the surface.  Below is a semi-real image of the lower atmosphere and surface, processed and partly extrapolated from the Huygens spacecraft during its descent: It seems very likely to be what a human would see looking out a window at the gloomy environment, although I don't know what local time of day it was during the Huygens descent, so I can't say if it ever gets brighter than this:

titan_25km (processed and rendered)

---

I had intended to completely cover the atmosphere in this volume, but it turns out Titan is even more complex than I thought it was, so I'll be ending this entry here and continuing in at least one more volume.  Topics that remain to be covered in detail: Winds, hydrology, cloud formations, climate, seasons, potential biosphere, human relevance, and Titan's physical future.

Originally posted to Troubadour on Sun Jun 16, 2013 at 08:24 AM PDT.

Also republished by SciTech, Astro Kos, and Community Spotlight.

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