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. 4, we explore the winds, hydrology, climate, and seasons of Titan, leaving potential for life and human future for the fifth and final volume later.
The progress of our adventure so far (current in bold):
1. The SunTitan in true-color, showing the Southern-polar vortex as it's often visible:
4. Earth (Vol. 1)
5. Earth (Vol. 2)
6. Earth (Vol. 3)
7. Earth (Vol. 4)
8. Earth (Vol. 5)
9. Earth (Vol. 6)
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)
19. Jupiter (Vol. 1)
20. Jupiter (Vol. 2)
22. Europa (Vol. 1)
23. Europa (Vol. 2)
26. Saturn (Vol. 1)
27. Saturn (Vol. 2)
28. Saturn (Vol. 3)
29. Rings of Saturn
35. Titan (Vol. 1)
36. Titan (Vol. 2)
37. Titan (Vol. 3)
38. Titan (Vol. 4)
39. Titan (Vol. 5)
41. Minor Moons of Saturn
50. The Kuiper Belt & Scattered Disk
52. The Interstellar Neighborhood
54. Overview: Human Destiny Among the Worlds of Sol
55. Test Your Knowledge
Due to Titan's slow, orbit-synchronous rotation, the moon only rotates once every 16 Earth days or so (383 hours), so coriolis effects play very little role in Titanian winds the way they do on Earth. However, this leads to some Venus-like wind patterns, which is another world that rotates slowly while having very rapid atmospheric motion. Basically, the overall atmosphere rotates in the same direction as Titan, but faster, which is called super-rotation. You can see some of the resulting cloud-banding in IR and UV spectra images, although it's not fast enough to produce the chevron-shaped cloud systems seen on Venus:
As you would expect, wind speeds vary across both latitudes and altitudes, but the global wind patterns appear to be part of a single continuous longitudinal system called a Hadley cell. Relatively warm air (and on Titan, "warm" is indeed extremely relative) rises to high altitude at the pole currently in Summer, circulates toward the opposite pole, and then descends where it returns as colder air and rains methane and ethane.
At higher altitudes, wind speeds can be hurricane-force (several hundred kilometers per hour) while nearer the ground, due to the thickness of the air, it's much slower and tends to be relatively gentle - but the rapid motions may at times occur at ground level, so operating for long periods of time on the surface may prove tricky. This would be especially problematic because of the cryogenic temperatures of the wind: High speed + dense air + ultra low temperature = suck the energy right out of a probe or a habitat, not to mention whatever structural or stability issues come into play under such winds.
You might think dense air and high wind speeds could be useful for wind-based power generation, but in Titanian temperatures it's thermodynamically a losing proposition. More on that in the concluding entry where we get into possibilities for future human exploration and habitation.
(vi) Hydrology & Cloud Formations
Many different substances go into the complex hydrological environment of Titan, but the vast majority of it appears to involve only two - methane and ethane, both simple hydrocarbons, with the latter being chemically derived from the large abundance of the former:
Separate ethane and methane cloud formations occur under different conditions, different latitudes, and different preferred altitudes, but both occur in greater profusion, size, and duration in colder temperatures. As a result, clouds of both kinds are more prominent in polar regions during winter, but can also occur elsewhere, though they tend to be more fragmentary and transient under other circumstances. The two types of cloud don't look any different though - you wouldn't know ethane from methane clouds just by looking them, and their respective rains probably look the same too.
There are semi-permanent cloud features at the poles though that appear to be seasonally related: The North polar region currently has a large "cap" of high-altitude hazes best seen in UV images that doesn't occur in the South:
And a large, very well-defined vortex has persisted at the South pole, as seen in the image at the top and others - this one is true color, so it's what you'd see from orbit with your own eyes:
It's more subtle from a distance and in monochrome:
You can actually see the Southern polar vortex in motion:
Clouds elsewhere tend to be less distinct and less stable, but are observable in IR:
After a rain, there are sometimes transient lakes and puddles that have been observed even at the relatively arid mid-latitude and equatorial regions, although it seems they evaporate pretty quickly. Just for the sake of wonder, let's try to put ourselves in the scene and imagine it: If the methane and ethane rains were chemically pure, they would just look like liquid natural gas - clear, basically - but on Titan they would be polluted with all sorts of organic gunk, especially once they reached the ground.
We can say the rivers and lakes are relatively viscous, slow-moving, resistant to waves and ripples, and probably dark in color. Observations have been made of polar seas, and they haven't yet seen any wave action, which could mean not only that the liquid itself is viscous, but might have some film of sticky material on top of it. One might speculate that a "raging torrent" on Titan probably doesn't make as much noise as a water river, but instead would flow somewhat "gooily" over the terrain. This is just speculation, but fun to imagination.
The terrain of Titan is pretty shallow, so the standing bodies of liquid are as well - only a few meters deep in most places: You might call them very broad puddles rather than lakes or seas. You could wade into them for dozens of kilometers, if not the entirety of the body in some of the smaller cases (of which the following is NOT one, but gives an idea of some shallow regions):
We don't know much about how those kinds of liquids behave under those conditions, in that gravity, at those temperatures, so that's an active area of investigation and theorizing. Odd fluid behavior could occur - it might be sticky, or crawl up the shores a bit, or something. The most fascinating things we'll learn about Titan in the future are, first and foremost, what it is we don't know. As it is, we still don't know what it is we don't know. We can test these things in labs that simulate Titanian conditions, but so far there hasn't been much of a reason to try, especially on a macroscopic level that would be relevant to human technology.
One thing we do know is that the geography of Titan is overwhelmingly shaped by the fluid flows and not nearly as much by tectonics, hence the topographical smoothness. This is why the liquid geography looks a lot like Northern Canada - basically a lot of glacial and runoff- or evaporation-based erosion rather than the stark features of an uplifted, rifted, or seismic-based topography. So the terrain in the polar regions tends to be swiss-cheesed with lakes rather than having a few huge seas, although there are a handful of larger bodies. A part of the North in false color:
In an earlier entry, we saw an air-column diagram of the Titanian atmosphere. Here's a more detailed version that shows the complexity of the hydrological process, including the processes that replenish the cloud layers:
As you can see, it gets really messy close to the surface, which is what makes Titan so interesting. There is a lot of strange, complex chemistry going on there, and with a surface made of water ice, it's at least possible that transient, microscopic, liquid water films might be involved in some of that chemistry at least some of the time under certain conditions. Hell, maybe water could be consumed as a solid mineral by some form of exotic biochemistry? But that's speculation, which we'll get into in more detail in the next entry.
(vii) Climate & Seasons
We don't yet have enough information to develop detailed climate models of Titan: It's a lot of work doing that even for Earth with a constellation of permanent weather satellites in orbit - for Titan all we have are occasional flybys because the Cassini probe is in Saturn orbit, not in orbit around Titan. So as far as I can find there are no detailed descriptions of how Titanian weather evolves - no weather map saying this place is rainy, this place is dry, this place is windy, etc, beyond the broadest terms.
What do know are a few general patterns: In summer the atmosphere rises to much higher altitudes, and in winter it sinks. Clouds and rains are much more prevalent at the poles than in the mid-latitudes and equatorial regions, but still occur there to some extent, so the polar regions have a lot more standing bodies of liquid while the other regions have transient bodies and remain mostly arid. It's thought that in transitional seasons like spring and fall the focus of rain activity might gravitate from pole to pole and thus pass through lower latitudes, giving them a relatively brief rainy season - "relatively" because the Titanian year is on order of 30 Earth years. In general, we can say that Titanian summers are dry and winters are wet. The hotter the summer, the drier; the colder the winter, the wetter. Only ten or so K cooler on average and you could have liquid nitrogen rains at times, but we haven't seen that, although there are nitrogen clouds.
There are also some hypotheses based on climate modeling that storms probably form over the large Northern seas in summer due to evaporation, in a similar way to hurricane formation on Earth - specifically, Ligeia Mare, Kraken Mare, and Punga Mare - with wind speeds of about 70 km/h, and that this is probably strong enough to cause some wave action.
Titan is easily the weirdest and most challenging place in the solar system that human beings can even remotely relate to, and as such it behooves us to invest in its exploration and understanding. Most people have already dismissed space because it hasn't produced a Jetsons lifestyle in the timeframe originally anticipated, but it'll just keep on coming, and eventually it'll just be our everyday reality. Titan has a good shot at being the most exciting locus of that evolution in this solar system over the very long-term.