Titan is one of the largest moons in the Solar System. In fact, for many years, it was thought to be the largest of all the moons. However, it has a thick atmosphere, and that atmosphere makes it appear a little larger than it actually is. The way in which radio signals from Voyager 1 were cut off as it passed behind Titan in 1980 shows that Titan is actually slighter smaller than Ganymede, making it only the second largest moon in the Solar System.
      Despite this demotion, if Titan were orbiting the Sun on its own, it would be a respectable planet. With a diameter of 3200 miles, it is over twice the size and ten times the mass of Pluto. In fact, it is even larger than Mercury, although only 40% as massive, because it is made of ice and rock, whereas Mercury is made of iron and rock.
      Titan is unique among the moons of the Solar System in having a thick atmosphere. Its atmosphere contains about ten times as much gas per square foot of surface as the Earth's atmosphere, and even though its surface gravity is only about one-seventh of the Earth's gravity, the surface pressure (which is equal to the weight of the atmosphere, or the amount of gas in the atmosphere multiplied by the surface gravity) is about one and a half times greater than that of the Earth's atmosphere. Because of the low gravity and the large amount of gas, the atmosphere extends about ten times further into space than ours.

Thin blue haze lies above thick orange haze. (Voyager Project, JPL, NASA, apod020905) The atmosphere and haze layers extend more than 300 miles above the surface. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod040810)

      Titan's atmosphere is over 90% nitrogen, but also contains a few percent of methane, and some argon. Chemical reactions driven by absorption of ultraviolet radiation change the nitrogen and methane into various smog-like hydrocarbons which completely block our view of the surface, at least in visible light. Most of the orangish photochemical haze lies a little under 200 miles above the surface of Titan, but there is a thinner layer of blue haze another 100 miles out, and droplets of hydrocarbon compounds probably drip from the lower haze layer toward the surface, possibly covering it with a tarry residue of organic compounds. There are also clouds, most likely of methane or ethane, a half dozen miles above the surface, and there may be ethane or methane oceans on the surface, although the glint characteristic of sunlight falling on lakes and oceans has not been observed in infrared surveys of the surface by the Cassini spacecraft. Infrared observations indicate that there are lighter and darker areas on the surface, so it is not likely that the entire surface is covered by hydrocarbons.

Infrared map of Titan based on 1994 observations with the Hubble Space Telescope. Lighter and darker features are presumably mostly actual surface features (lighter areas being icy continents, and darker areas hydrocarbon-rich basins). The map covers latitudes within about 45 degrees of Titan's equator; the area shown is approximately 9000 miles by 2300 miles.

The surface of Titan, at the Huygens landing site. Ice boulders lie scattered across a surface of wet-sand or soft clay consistency. (ESA, NASA, Descent Imager/Spectral Radiometer Team (LPL), apod050117)

      The atmosphere of Titan is thought to be very similar to the early atmosphere of the Earth, and the presence of large amounts of complex organic compounds, including hydrogen cyanide, which is a precursor of life, makes scientists wonder whether Titan may harbor primitive lifeforms. Even though its very low temperature (discussed below) makes that somewhat unlikely, that is one of the exciting possibilities that will be studied by the Huygens lander, which will parachute into Titan's atmosphere around the end of the year (2004). Hopefully, radar imagining by the Cassini spacecraft, and information from the lander, will resolve a number of questions about the atmosphere and surface of Titan (particularly in the area where Huygens "lands").
      Titan is able to have an atmosphere because it is relatively large and relatively cold, with surface temperatures around 290 degrees below zero Fahrenheit, and temperatures in the upper haze layers around 150 degrees below zero (the haze layers are warmer because of the heat that they absorb during the photochemical processes that produce the smog-like compounds). However, Ganymede and Callisto, the two largest moons of Jupiter, are about the same size as Titan, and only about 30% warmer, so they are theoretically just as capable of holding onto an atmosphere, and neither of them has any substantial amount of gases. This makes us wonder why Titan has an atmosphere, while the other two moons don't.
      The most likely explanation of this difference is probably due to a difference in the composition of ices inside the moons. Because Jupiter formed closer to the Sun, where it was somewhat warmer, volatile ices would have been relatively rare, and Ganymede and Callisto's ices are probably mostly water ice. Titan, forming around Saturn, which is twice as far from the Sun, in a region which was somewhat colder, although also containing substantial amounts of water ice, would have had a chance to accumulate large amounts of more volatile ices, as well. During the early history of the Solar System, as radioactive materials contained within the rocky materials of various bodies began to heat them up and melt them, the evaporation of ammonia and methane ices would have produced a thick atmosphere of such gases. Over a period of time, absorption of ultraviolet radiation would have split off the hydrogen gas (ammonia is made of hydrogen and nitrogen, while methane is made of hydrogen and carbon), and as the hydrogen escaped into space (because anything smaller than a Jovian planet doesn't have enough gravity to hold onto such a light gas), what was left behind was the present atmosphere of nitrogen and argon, and a residue of carbon and hydrocarbon compounds. Since Ganymede and Callisto wouldn't have had nearly as much of the more volatile ices, they probably just didn't have the chance to form much of an atmosphere, and the fact that they could hold onto an atmosphere was irrelevant.
      One question about Titan's atmosphere is how it manages to maintain its concentration of methane. Theoretical calculations indicate that the loss of hydrogen gas to space, and the chemical reactions which convert methane and nitrogen into heavier hydrocarbons which sink to the surface of the moon, should remove all of the methane now in the atmosphere in a relatively short period of time (about ten million years, which seems like a long time, but is very short, compared to the age of the Solar System). As a result, there must be a continual replacement of the methane, presumably by the evaporation of methane ice from the interior.
      Titan's internal structure is unknown, but its density is almost twice that of water ice, which would imply a nearly equal mixture of ice and rocky materials. Heat generated by radioactivity within the rocky materials is probably responsible for the replenishment of methane gas in the atmosphere, but whether the heat was adequate to allow Titan to differentiate, like Ganymede, or whether Titan is more uniformly mixed, like Callisto, is not known. The Cassini spacecraft should resolve this uncertainty, but in any event, Titan is not likely to have a substantially liquid region within its icy interior, because it does not have a magnetic field.
      As is true of many of the moons in the Solar System, Titan rotates synchronously, meaning that its rotation period is the same as its orbital period around Saturn (just under 16 Earth days), and it always keeps the same face towards Saturn.