If Titan replaced Earth's Moon, it would turn night to day. Being twice as reflective as the Moon and half again as large in diameter, Titan would shed some five times as much moonlight. But instead of Luna's interesting rocky face, Titan would be almost as blank as Venus because unlike every other moon in the solar system it has a deep, hazy atmosphere.
The atmosphere is peculiar, mostly nitrogen with a big dash of methane. Even though the gravity at Titan's surface is pretty feeble, less than on the Moon, the Titanian air is four times as dense as Earth's and its pressure 1.5 times as high. Titan gets all this gas, apparently, from inside.
Titan has a core of rock and a thick mantle of ice, typical of bodies out in the cold parts of the solar system. The ice is thought to include methane (CH4)as well as ammonia (NH3). This material enters the atmosphere, where the Sun's radiation breaks off the hydrogen atoms. The hydrogen drifts off into space; the nitrogen stays in the air as nitrogen gas N2, the same gas that makes up most of our air.
But the methane pieces find each other and combine into dozens of heavier hydrocarbons including ethane, ethylene and acetylene. These slowly rain downward and form a dark, probably tarry coating of crud on the Titanian ground. The ethane is a liquid in Titan's deep cold, minus 180 degrees C (95 K). So we can picture ethane rain washing this surface, flowing off in streams, and probably draining into permanent lakes and seas. The waves in these bodies of ethane have been modeled: as high as a meter and slow as dreams. The dominant solid would be acetylene and its polymers; these could float on the ethane like ice floes.
Nothing catches the geologist's imagination like the idea of puddles. Bouncing radar signals off Titansomething we've done with other planets and moonsgives hints of liquid bodies there. The Cassini spacecraft has not seen hints of liquid, but it uses a different radar wavelength than the Earth-bound observations. The observations could be explained by liquid hiding within a spongy crust, or covered with acetylene floes or hidden by ethane fog. The Hubble Space Telescope and Cassini made near-infrared images that show slowly changing cloud patterns near the moon's south pole.
There is a large bright feature on Titan's surface, about 2000 kilometers across (half the diameter of Antarctica), that might be several kilometers high. One idea is that it's made mostly of ice, and it would be bright because the ethane rain washes the crud off of it. But until the Huygens probe tells us more, we can't be sure of much. In Titan's profound cold, ice is a rock just as if it were granite. It would not act much like Earth's glaciers, more like its granitic continents.
There may be a liquid layer inside the planet consisting of ammonia-saturated water. Calculations suggest that this could erupt and flow very much like basalt flows on Earth. Radar images show "lava" flows on the surface, and also dome-shaped features similar to the lava domes of Venus.
When Huygens gets its first real closeups of Titan, geologists of many stripes will be very interested.