Titan

Titan
Credit: NASA/JPL/Space Science Institute

Titan is Saturn’s largest moon. It is bigger than the planet Mercury and only slightly smaller than Ganymede. Not only is Titan huge, it more like a planet than a moon in lots of ways.

Titan is the only moon with a thick atmosphere. It is mainly made of nitrogen, like on Earth. It also contains methane gas. This is broken down by the Sun to make a thick smog which covers the planet’s surface. Astronomers use infrared images to cut through the smog and see the surface.

Winds blow across the moon's surface moving clouds of methane. It doesn't rain water on Titan, but liquid methane. This means the surface is covered with huge lakes of liquid methane and ethane. These can evaporate off into the clouds and fall as rain again, just like the water cycle on Earth.

The temperatures on Titan are freezing, averaging -179 °C. This means methane can be either solid, liquid or gas, just like water on Earth. The moon is the only place, other than Earth, to have liquid flow on the surface. There are rivers, gorges and coastlines. The Cassini probe landed on Titan in 2005. It came down on a bed of water ice pebbles which were frozen harder than rock and worn smooth by the flow of liquid methane.

There are more likenesses to Earth too. Large dunes are formed near the equator by high winds. Instead of being made of sand, they are made of hydrocarbons which look like coffee grounds. There are volcanoes that erupt liquid water, which then sets like rock (ice) in the cold. The water comes from an ocean buried under Titan’s surface. Some astronomers think this ocean may be able to support life.