Titan was the first of Saturn's moons to be discovered, found by Dutch astronomer Christiaan Huygens in 1655. It's no surprise that Titan was the first, because it's big – bigger than the planet Mercury. Titan's mass accounts for 96% of the total mass of all of Saturn's five dozen moons.
Although Titan is big, it's also far away. At its closest to Earth, Saturn is some 1.3 billion km (800 million mi) from us and Titan looks rather small at that distance. Until space probes went to the outer Solar System we had only a very sketchy knowledge of the moons of the outer planets.
Astronomers knew that Titan was 1.2 million km (745,000 mi) from Saturn, orbiting the planet in about sixteen Earth days. In the first half of the twentieth century there was evidence that Titan had some kind of atmosphere, and that it included methane. This was intriguing. Voyager 1 did a fly-by of Titan in 1980, and confirmed that Titan had a thick nitrogen atmosphere with methane and various complex molecules. Voyager also learned that Titan had a thick smog that blocks visible light, so the probe couldn't study the surface.
Most of what we know about Titan is from the Cassini-Huygens mission. This is a joint NASA and ESA mission which was named for Huygens, the discoverer of Titan, and Jean Dominique Cassini who discovered four further moons of Saturn.
The Cassini probe, which is well equipped with radar and other instruments that can penetrate the smog, has studied Saturn, its rings and its moons since 2004. It also carried the lander Huygens to Saturn and released it to land on Titan in January 2005.
Titan – just like home?
Planetary scientists consider Titan the most earthlike of the Solar System bodies. Besides being the only moon with a dense atmosphere, it's the only body other than Earth to have an atmosphere that's primarily nitrogen. Titan and Earth are also the sole Solar System objects with standing bodies of surface liquid.
Cassini-Huygens has provided evidence of weather and weathering on Titan. There are clouds in Titan's sky and sometimes it rains. Wind has formed vast fields of sand dunes around the equator. Here is a comparison between dunes in the Namib Desert on Earth and those in Belet on Titan. Elsewhere on Titan are various features shaped by flowing liquid.
But let's not rush to book vacations on Titan. It isn't really much like home. To begin with, although Earth's atmosphere is nearly eighty percent nitrogen, around twenty percent is oxygen. Titan's atmosphere is over 95% nitrogen and most of the rest is methane (natural gas) plus some hydrocarbons. We wouldn't have to worry about the methane catching fire, since there's no oxygen, but we would have to worry about breathing.
The rain, the lakes and the rivers aren't water, they're liquid methane. With a surface temperature of around -178 C (-289 F), all the water on Titan's surface is frozen. The Huygens probe photographed hills which scientists think are mainly ice. The rocks and pebbles at the Huygens landing site are not silicates like earthly rocks, but are made of water ice. At Titan's low temperatures, water ice is rock hard.
A methane cycle on Titan may be analogous to Earth's water cycle, but the details aren't yet understood. We do know that Titan's smog is produced by ultraviolet light breaking up methane in the upper atmosphere. The methane must be replaced somehow, otherwise it would all have been broken down long ago. There are various ideas about the source of the methane, but the most popular theory is that it comes from Titan's interior and is released in volcanic eruptions.
If there are volcanoes on Titan, they would be cryovolcanoes, i.e., cold volcanoes which eject water or other liquid instead of molten rock. Such eruptions have been seen on other outer Solar System moons. Several features on Titan are possible volcanoes, but what would be convincing is an eruption. Unfortunately, we aren't likely to witness one through the thick atmosphere.
Internal structure, surface geography
The internal structure of Titan is layered like that of a planet. Based on our current data, it appears that Titan has a rocky core with surrounding layers of different forms of ice. Cassini data now supports the existence of a liquid ocean below the surface.
Titan has a geologically young surface. In other words, like the Earth, the surface is much younger than the body itself. Earth's moon, by contrast, has a very old surface with lots of impact craters. Titan has very few impact craters. One of the reasons is that the thick atmosphere reduces the number of impacts. However those that exist have been filled in by geological processes.
Most of the lakes are in the polar regions, which tend to have lower altitudes than the equatorial areas. Overall the surface is fairly flat. There are some mountains, including some over a kilometer in height, but nothing spectacular.
Titan's cool chemistry
There is a lot of chemistry happening on Titan that fascinates scientists. When they suggest that Titan resembles Earth, they're thinking of an ancient Earth before life began, and wondering if we can learn something from Titan about the origins of life on Earth. The chemistry is also literally cool, or more accurately, cold. Chemical reactions happen quickly at Earth temperatures, so it's useful to study the results of chemistry at Titan's low temperatures.