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.
The progress of our adventure so far (current in bold):
1. The SunTitan in true color, as we would see it from nearby:
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)
39. Minor Moons of Saturn
48. The Kuiper Belt & Scattered Disk
50. The Interstellar Neighborhood
52. Overview: Human Destiny Among the Worlds of Sol
53. Test Your Knowledge
Titan is Saturn's 21st moon from the planet, and its sixth of seven major moons. The size of its orbit around Saturn is over three times the average size of the Earth-Moon system, and yet is still somewhat deeper in the planet's gravity well than Luna is in Earth's - but considerably shallower than the major moons of Jupiter. Orbital and gravity well diagrams:
You probably can't see either the Sun or Saturn from Titan's surface, barring rare atmospheric events where the obscuring hazes become relatively clear, so you definitely can't see things like the stars and other planets either. Sunlight probably diffuses throughout the daylit sky, much like people experience on Earth in especially hazy or smoggy weather: You can't see the Sun, but it illuminates the entire sky. On Titan it would be a gloomy orange soup rather than the blinding white we get on Earth, but still illuminated well enough to see the ground. Sadly, this means the illustrations you may have seen showing Saturn in the Titanian sky are purely fanciful. The rule is: If you can't see the ground from space, you can't see things in space from the ground. At least not in visible light.
But if you could see Saturn from Titan, it would cover 5.5° of arc across the sky, or about 11 times larger than the full Moon from Earth. This is a little smaller than Jupiter as seen from Ganymede, and a little bigger than Jupiter from Callisto. So that's another reality that artwork usually fudges - if you were on Titan and could see Saturn through the haze, it would be substantial, but not gargantuan like it's usually depicted. Its apparent size would be like this crude illustration:
But once again, you can't see it. Although maybe some day in the distant future, Saturn and other moons of the system may be visible through an artificially altered sky on Titan.
When forming theories about the history of a rich and varied satellite system like Saturn's, scientists don't just study each individual moon in isolation: They look at everything together, and try to create models that evolve to produce a system like the one we see today. So in trying to understand what produced Titan, three features of the system in general stand out: First, which you can see in the orbital diagram above, is that five of the seven major moons are tightly packed in a relatively small region not much bigger than the Earth-Moon system that is suddenly interrupted by a huge gap before Titan. Second is Titan's relatively high eccentricity compared to these inner moons. And third is the outermost major moon, Iapetus, whose orbit is significantly inclined from Saturn's equator.
An ordinary, uninterrupted process of accretion should have produced a handful of large moons, much like the Galilean moons of Jupiter, with nothing else in the system but small asteroidal objects and debris. But instead what we see is one gigantic moon, two mid-sized moons (Rhea and Iapetus), and a steadily-graded group of smaller ones in a compact region. In other words, the Saturn system is an embarrassment of riches that requires some further tinkering to explain, and whatever explains it bears directly on why Titan is so huge, so lonely in both location and properties, and why its orbit is unusually elliptical.
One model being advanced holds that the system originally was Jovian-like, with a small number of large ice moons bigger than Rhea but smaller than Titan. At some point the system experiences some destabilizing influence - possibly due to the migration of Uranus or Neptune in the early solar system - that perturbs the orbits of the large satellites. What follows is a sequence of huge collisions that merge most of their mass into Titan while the remainder accretes as fragments into smaller moons.
One would think this is highly relevant to discussions of these other moons, but I've only just now seen this theory in reference to Titan, so the pitiful state of cross-referencing in the available information is part of what hopefully makes this series useful. If the model is correct, then everything stated earlier about the formation of the smaller moons is still also correct, but an intermediate step would have to be introduced between formation of the Saturn system and accretion of those moons - one where they are fragments of larger moons that merged to form Titan. Such a dramatic origin for this mysterious world would explain both its own and Iapetus' odd orbital characteristics, and the drastic discontinuity in mass and location between itself and its closest neighbors.
Here's one possible simulation of the model, which progresses just far enough to see the smaller moons accrete from the arc of debris:
What the model shows is a glancing blow by two large moons, which then collide a second time with the smaller moon being stretched out into an arc of material that subsequently divides into multiple arcs and objects. Some screencaps showing this:
As you can see from the timecode in the upper left, the entire process would have unfolded over only a few Earth days. If this is what happened, then much like Earth and its Moon, Titan owes its unique character to a cataclysm. Had the original moons continued as the model envisions them, they would not likely have had the dense atmosphere, hydrological cycle, and complex chemistry Titan does. Which brings us to the subject of how Titan itself specifically formed, whether under this giant impact model or more traditional assumptions. Why does a moon roughly the same size and mass as Jupiter's moon Ganymede have a thick atmosphere while Ganymede has none? The answer is like in real estate: Location, location, location.
The terrestrial planets (Mercury, Venus, Earth, and Mars) are so close to the Sun that almost every common gas other than CO2 can't survive or stick around very long, and even that only persists because carbon-oxygen double bonds are incredibly strong and the CO2 molecule is relatively heavy. Most of the rest are broken down by the UV in sunlight into simpler gases (a process called photolysis), and then blown away into space by the Sun. This includes water, by the way: The vast majority of water that existed in the primordial nebula in this location was never captured by the terrestrial planets because it was too hot - most of it was blown outward.
In fact, the only reason our world has an atmosphere and appreciable surface water is volcanic venting of nitrogen and water trapped in bubbles in rocks called clathrates. If Earth were geologically dead, our location in the solar system guarantees that our water would be broken down, the hydrogen blown away into space (along with the nitrogen that makes up most of the atmosphere), and the oxygen would mostly end up as CO2 and, to a lesser extent, sulfur dioxide (SO2). Basically like Venus, but at a lower temperature. So our region of the solar system is a desert that from the beginning doesn't have a lot of water by proportion, and exists in a solar environment too intense for anything other than strongly-bonded oxide atmospheres to persist unless replenished by volcanism and/or biology.
The Jovian system, on the other hand, formed with a lot more water than the terrestrial region because it was at a lower temperature and didn't just blow away, so the Galilean moons of Jupiter (other than Io) have their characteristic ice shells. But that region of the solar system is kind of in a thermal valley where atmospheres around large moons don't happen because (a)it's cold enough that CO2 and SO2 would freeze to the surface as solids even if they were present in large proportions (which they aren't, except on Io), (b)cold enough that the water ice doesn't release vapor fast enough to form a gaseous H2O atmosphere that would persist under prevailing solar conditions before being blown away, and (c)still too hot to hold on to lighter gases like nitrogen or methane. So if Jupiter were somewhat closer to the Sun, Io could have an SO2 atmosphere and Europa, Ganymede, and Callisto could have water vapor atmospheres, but where they actually are is inconvenient for any significant atmosphere to persist around objects with their mass.
Saturn, on the other hand, formed in a region of the solar system where temperatures were optimum for both the formation of water and the retention of an atmosphere around a large moon. Although Jupiter's major moons have a greater absolute quantity of water because of the higher density of the cloud that formed that system, the moons of Saturn have the highest concentrations of water by proportion. This is because prevailing temperatures resulted in more of the hydrogen ending up in H2O and, to a lesser extent, methane (CH4) and ammonia (NH3) rather than being blown away as simple H2, but conditions were also warm enough that the oxygen needed to form this much water wasn't diverted into a lot of carbon monoxide (CO) like we see in more distant parts of the solar system.
So Titan was cold enough to have a lot of water, methane, and ammonia, but also ideal in the other direction: Warm enough that all three could occur to varying degrees as gases, liquids, and solids, with ammonia and water being overwhelmingly solid and methane being gas and liquid. If it were farther out in the solar system, all three would be frozen to the surface and you'd just have another airless moon; and if it were closer, it would be like Ganymede, without much methane or ammonia (as far as we know).
But if the process of forming Titan's atmosphere had stopped there, it would only be a fraction as thick and composed mainly of methane and other hydrocarbons rather than the dense nitrogen atmosphere we see. So, like in forming Earth's atmosphere, geology must have played a significant role: Frozen ammonia was heated by cryovolcanism (what happens when ice surfaces are heated up enough to become liquid in some places) and vented NH3 into the atmosphere, where it would be broken down by sunlight into its constituent elements, hydrogen and nitrogen.
Meanwhile, the methane in the atmosphere is also being broken down into carbon and hydrogen, so this is where all that interesting chemistry in Titan's atmosphere takes place: The carbon and hydrogen recombine into any number of complex (one might even say, "prebiotic") molecules, while the nitrogen bonds with itself into inert N2 that's much more likely to stick around Titan than around Earth because of far lower temperatures, despite the latter's higher gravity. Thus Titan has a thick atmosphere overwhelmingly dominated by N2 with only a small minority component of methane, and is not only at higher pressure than Earth's, but actually has more total mass. But that raises a final question whose potential implications are staggering: If the methane is constantly being broken down, why is it still there? While there are probably simple chemical explanations, one significant possibility is that biology is replenishing the atmospheric methane.
So like Earth, a number of Goldilocks circumstances and a potential serendipitous cataclysm combined to make Titan possible and produce the exotic conditions we see today. Below is a rough illustration showing the general principle, which I should stress is based on my best amateur understanding of the subject, and is not the output of an expert:
1. Orbital and Rotational Features
The Titanian month is about 16 Earth days, and its day is the same because it's tidally locked to Saturn, rotating only once relative to the Sun for every complete orbit of the planet. However, because of Titan's thick atmosphere, the long days and nights (8 Earth days or so each) are not thermally important on the surface, with only 1-2 K swings in temperature. Unlike other moons of the system, it shows no significant differences in appearance between the leading and trailing hemispheres - the sides facing toward or away from its direction of orbit - because the absorbed materials and different impact rates responsible for these differences on other moons are distributed or eroded by the atmosphere.
Titan's orbit is the most eccentric of the seven major moons, which is part of what motivated the impact hypothesis for its formation, and may contribute some tidal heating to its interior - possibly enough to maintain an internal liquid water layer. However, the eccentricity is still low in absolute terms, and doesn't even approach the kind of elliptical orbits seen among irregular moons, with the size of the orbit varying by only about 5.6% between near and far points from Saturn. This is a lot bigger than you tend to see in a large gas giant moon, but it also occurs in a relatively shallow region of the planetary gravity well, so the tidal consequences should be far less significant than for the eccentricities of Galilean moons. Which is a bit of a shame if it proves out, because with greater tidal heating it could sustain liquid water closer to the surface, possibly enough to interact significantly with the organic chemistry of the atmosphere.
Titan's orbit is within Saturn's main radiation belt - a region where the planet's magnetosphere traps energetic particles coming in from the Sun and the rest of the universe (i.e., cosmic rays). While the surface is completely protected by the soupy atmosphere, the space around Titan presents radiation challenges similar to Earth's Van Allen Belts - hazardous regions that spacecraft headed beyond Earth orbit have to survive on their way out, and would have to survive again on their way back if they're returning.
Space hardening of electronics is more than good enough at this point to deal with Titan's environs - it's nowhere near as bad as being around Europa or Io - but would still be an issue for human transport, particularly because the radiation belt is a lot bigger in size than the Van Allen belts and thus takes longer to cross. Illustrations:
This may prove significant to how humans some day explore and potentially colonize Titan, since you wouldn't want people hanging out in orbit around it where the full brunt of the radiation would be experienced. In other words, the staging area for Titan surface operations would have to be outside the radiation belt, so that increases the likelihood that another moon, such as Rhea or Hyperion, might be utilized. That said, the radiation belts are not homogeneous around a given orbit, and Saturn's magnetic field rotates dozens of times faster than Titan's orbital period, so the radiation environment isn't constant - both higher and lower radiation conditions are constantly sweeping past it like weather. So I would assume you could exploit the timing of the field's rotation to enter and exit Titan's atmosphere at times of lower radiation, although it would still be a big problem. I'll discuss this more in Vol. 3.
The radiation belt also has some significant effects on Titan itself: It accelerates the chemical dynamism in the atmosphere over and above what ambient radiation would cause if it were just floating in space by itself rather than being a gas giant moon. Instead, it's like it exists in front of a firehose spraying it with energetic particles at varying intensities.
One interesting possibility for Titan concerns eclipses: Although you probably can't see the Sun or Saturn through the atmosphere from its surface, you might be able to vaguely see both during a solar eclipse when the arc of the planet is illuminated. During such times, Titan's atmosphere and the side of Saturn facing it would be in shadow, and yet the limb of Saturn and the ring plane would be back-lit with far more intensity than they ever are under other conditions, so you might be able to see a dim, glowing outline through the haze.
Because Titan is so big, it can often be seen in association with other moons of the system despite being far away from most of them, making possible some amazing - and often proportionally deceptive - imagery:
Some of the most amazing views of Titan from space are in phase, particularly crescent, which gives the best opportunity to see its atmosphere from a distance due to the angle of sunlight:
2. Size and Mass Characteristics
The mass of Titan is about 2.3% that of Earth, and about 83% greater than the Moon. It's considerably denser than all the major moons of both Saturn and Uranus, and is on par with Callisto, but isn't even in the same league as the other Galilean moons or bodies inward of Jupiter. Based on the density and volume, we know that its mass is about equally divided between rock and water ice. With such high mass for a moon and moderate density, Titan's surface gravity is substantial at 14% of Earth's - over five times the next highest gravity on a moon of Saturn (Rhea's), and only a couple of percentage points lower than on the Moon.
Lunar-class surface gravity coupled with a thick atmosphere makes it theoretically possible for humans to fly like birds on Titan with the aid of wingsuits, but of course that's if you ignore the weight and volume of insulation and heating apparatus that would be needed to avoid freezing solid almost instantaneously. On the other hand, that's only if you do it outside, at ambient temperatures: People could fly around in large, pressurized enclosures at higher temperature but with the same pressure - a sort of human aviary. More on that in Vol. 3.
So the result is that even with its roughly lunar gravity, you probably wouldn't see Apollo-style moonwalks in spacesuits: It's just too cold for walkable spacesuits to be practical, and being in a thermally extreme atmosphere is much worse for temperature control than being in vacuum at the same temperatures. But you would see the same kind of goofy, semi-buoyant moonwalking by unsuited people inside their habitats. I would guess that tiptoeing would be the most natural, comfortable, and efficient way of walking in 0.14 g, and it might be easier to go on all fours or use handholds on walls if you need speed rather than trying to run. Which has interesting implications for the kind of footwear people would use on such a world, but that's a tangent for another time.
Titan's diameter is about 2/5 (40%) that of Earth, and 48% larger than the Moon - big enough that its cross-section would cover the entire continental United States and Mexico, and much of Canada (see illustration below). Its total surface area would cover 56% of the land area of Earth, although going by its own land area the ratio would fluctuate because Titan's methane seas and lakes drastically change in size seasonally - and we haven't been observing it long enough to know just how drastically. In any case, even its unarguably "dry" surface is larger in area than any continent on Earth: Twice the size of Asia, twice the size of Africa, and three times the size of North America. In fact, North America, South America, Antarctica, Europe, and Australia combined don't even come close. If the Indian Ocean were one solid continent, even that wouldn't equal the area of Titan's surface. Cross-section compared to continental US:
It is the second largest moon in the solar system after Ganymede, which only beats it by 2.3%, and is in fact 6% larger than Mercury (though far less massive). In other words, even after the exclusion of Pluto from the ranks of solar system planets, if Titan were a free object independently orbiting the Sun, it would only be the second smallest planet. Some rough size comparisons - mouse over the image to see the title if you don't recognize something:
That's it for now, but I'm planning for Volume 2 to cover internal structure and surface features, then Volume 3 would cover the atmosphere and hydrosphere, potential for life, human relevance, and physical future.