The second Earth
Mass: 1.3 x 1023kg (2.9 x 1023lbs) Diameter: 5,150km (3,200 miles) Parent planet: Saturn Discovered: 1655, Christiaan Huygens
Saturn’s largest moon, Titan is unique in the Solar System as the only satellite with a substantial atmosphere of its own – a discovery that frustrated NASA scientists when images from the Voyager probes revealed only a hazy orange ball. The Cassini orbiter was fitted with infrared and radar instruments that pierced the opaque atmosphere, revealing a softened landscape of rivers and lakes that is unlike any other world in the Solar System except for Earth. Despite being larger than Mercury, Titan can only hold onto its thick atmosphere because of the deep cold. Found some 1.4 billion kilometres (0.9 billion miles) from the Sun, the moon’s average surface temperature is a freezing -179 degrees Celsius (-290 degrees Fahrenheit).
Titan’s atmosphere is dominated by the inert gas nitrogen – also the major component of Earth’s air – but it gets its distinctive colour, opaque haze and clouds from a relatively small proportion of methane. Amazingly, conditions on Titan are just right for methane to shift between its gaseous, liquid and solid forms, generating a ‘methane cycle’ rather similar to the water cycle that shapes Earth’s climate. In cold conditions methane freezes onto the surface as frost and ice. In moderate temperatures it condenses into liquid droplets and falls as rain that erodes and softens the landscape before accumulating in lakes, while in warmer regions it evaporates and returns to the atmosphere.
Titan experiences changing seasons very similar to those on our planet, though its year is 29.5 Earth years. Temperatures at the winter pole seem to favour rainfall, so the lakes migrate from one pole to the other over each Titanian year. With all this activity, Titan is an intriguing target in the search for extraterrestrial life, though most biologists find it hard to envision organisms that could exist in such harsh and chemically limited conditions, and most agree that Titan’s watery inner neighbour Enceladus offers more promising prospects for life.