A little larger than the planet Mercury, Saturn's largest moon, Titan, has a significant nitrogen atmosphere but is cold enough for lakes of liquid methane.

Discovery of Titan

In 1655 Christiaan Huygens observed Saturn with his telescope that was newly improved over Galileo's original telescope. He discovered Saturn's largest moon, Titan, and identified Saturn's rings as a ring structure.

Basic Titan Facts

Titan's 5150 kilometer diameter is about 50% larger than Earth's Moon and a little larger than the planet Mercury. Its density of 1.9 times the density of water is close to the densities of Jupiter's outer Galilean moons, Ganymede and Callisto. Titan's density tells us that like these other moons, Titan is a mixture of ice and rock. An icy mantle surrounds a rocky core.

Space Missions Exploring Titan

The Pioneer missions of the late 1970s followed by the Voyager missions in the early 1980s flew by Saturn and yielded much of our knowledge of Titan. More recently the Cassini mission, which is currently in orbit around Saturn is providing considerably new knowledge of Saturn and its moons, including Titan. As part of the Cassini mission, the Huygens probe landed on Titan's surface.

Titan's Atmosphere

In the early 20th century, Jose Comas Sola, a Spanish astronomer, observed dark areas on Titan's surface which he interpreted as evidence of an atmosphere. In 1944, Gerard Kuiper observed spectral lines of methane from Titan, providing evidence of an atmosphere containing methane. The Voyager missions confirmed this evidence for Titan's atmosphere and provided more knowledge.

Titan is the only moon in the solar system with a significant atmosphere. The atmosphere is thicker than Earth's and contains cloud layers and haze obscuring Titan's surface. The haze consists of hydrocarbon aerosols, giving Titan its orange color. It extends about 600 kilometers above the surface.

Like Earth, Titan has a nitrogen atmosphere. Titan's atmosphere is 95% nitrogen, compared to earth's 78%. The other 5% of Titan's atmosphere is mostly methane with other hydrocarbons, including ethane, propane, and acetylene. This composition may be similar to Earth's atmospheric composition before biological activity started releasing oxygen.

Methane Lakes and Seas

Titan's atmospheric composition may be similar to Earth's, but Titan is not Earthlike. Its -180 degrees C surface temperature is cold enough to liquefy methane and ethane. On Titan it rains methane and ethane rather than water. Titan has a methane cycle similar to Earth's water cycle. Titan's surface is covered with lakes and seas of liquid methane and ethane that were long suspected and finally discovered by Cassini radar images. These lakes range in size from about the size of Lake Superior to the smallest size resolvable by the radar maps, one square kilometer.

Titan's Surface

Other surface features on Titan discovered by the Cassini mission include large dunes up to about 100 kilometers long and about 10 kilometers apart. The sand making up these dunes consists of hydrocarbon grains a few tenths of a millimeter in size.

Erosion causes some rugged terrain on Titan. There are 200 meter tall ridges with erosion channels caused by methane rains.

Titan also has a large volcanic mountain range. that is about 150 by 30 kilometers. The largest mountain is about 1.5 kilometers high. This range seems to have formed in a way similar to the mid-oceanic ridge on Earth

Though it is a moon rather than a planet, Titan is a complete and varied world.

Further Reading

Chaisson, E, and McMillan, S. Astronomy Today 5th ed., Pearson, 2005.

NASA/ESA Cassini Mission