Titan is Saturn’s largest moon, and the second largest in the solar system after Jupiter’s Ganymede which is 2% larger. It has a diameter of 5,150 km and is 1.8 times larger than Earth’s moon. It is also bigger than Pluto and Mercury, although smaller than Mars. Titan takes takes 15 days and 22 hours to orbit Saturn and like Earth’s moon always has the same side facing the planet. Titan’s seasons last the same time as on Saturn, so more than seven earth years each
Titan is the only moon in the solar system that has a dense atmosphere and is the only world apart from Earth that has standing bodies of liquid on its surface such as rivers, lakes and seas. Also like the Earth its atmosphere is predominantly Nitrogen and also contains a small amount of methane. The liquid on Titan acts in a similar way to that on Earth too, precipitating into lakes and seas and then evaporating back into the atmosphere, although it is mostly methane rather than water. It is thought to contain a subsurface ocean of water.
The moon’s internal structure is not entirely known but the current hypothesis is that there is a core of water bearing silicate rock about 4,000 km in diameter surrounded by a shell of water ice. This ice is surrounded by a layer of salty liquid water on top of which is a crust of water ice. The final layer is made up of organic molecules that have rained down from the atmosphere in the form of sand or liquid.
The surface of Titan is one of the most Earth-like places in the solar system, although with average temperatures of 179 degrees C, it is so cold that water ice acts like rock. It may also have volcanic activity with liquid water flowing like molten rock lava. On the surface there are flowing rivers of methane and ethane which feed into lakes of liquid natural gas. The moon also has huge dark “sand” dunes with the sand made up of dark hydrocarbon grains. There are very few visible impact craters which suggests the surface is relatively young.
Most of Titan’s highest peaks occur near its equator in so-called “ridge belts”. One possible mechanism for their formation is tidal forces from Saturn. Titan’s icy mantle is less viscous than Earth’s magma mantle and its icy bedrock is softer than Earth’s granite bedrock, so mountains are unlikely to reach heights as great as those on Earth. In 2016, the Cassini team announced what they believe to be the tallest mountain on Titan. Located in the Mithrim Montes range, it is 3,337 m tall.
At Titan’s surface, the atmospheric pressure is 60% greater than Earth’s and extends ten times higher. From afar the atmosphere looks like a dense orange smog and methane occasionally condenses into clouds which rain down on the surface in storms. Sunlight continuously breaks down the methane so some source must be replenishing it but what exactly this is remains a mystery, although one idea is that it is being spewed into the atmosphere in volcanic eruptions.
The discovery of the liquid ocean beneath the surface means that Titan could potentially contain habitable environments. In addition, the methane rivers on the surface also have the potential to support life, although this would be very different to any on earth. Despite this there is so far no evidence of life on Titan.
Titan is not visible from Earth with the naked eye but it can be observed through strong binoculars or a telescope. It can be somewhat tricky to see, however, due to the brightness of the planet and ring system. Not only has Titan been observed through telescopes from earth and from probes flying past, but we have actually sent a probe to the surface of the moon. In 2005, Huygens touched down to make Titan the most distant world we have landed on. The probe took photos of a plain strewn with rocks made of water ice. In 2026 the Dragonfly mission will launch which will send another probe to Titan, this time to study how far prebiotic chemistry may have progressed in the atmosphere.
