Titan

Titan is Saturn’s largest moon, 5 150 kilometres across, discovered by Christiaan Huygens in 1655. It is the only moon in the Solar System with a thick atmosphere, and the only world besides Earth known to have rain, rivers and seas on its surface. The liquid is not water. At about 94 K (-179 °C), water is as hard as rock, and what falls from Titan’s clouds and fills its seas is methane and ethane.

A moon you cannot see

Titan’s air is about 95% nitrogen and 5% methane, and the pressure at the ground is about 1.45 times Earth’s. High above, sunlight breaks the methane and nitrogen apart, and the pieces join into complex organic molecules that form a thick orange haze. In visible light it hides the surface completely: when Voyager 1 flew past, it saw nothing but an orange ball. Only from 2004 did the Cassini spacecraft map the ground, through narrow near-infrared windows in the haze and with radar.

Above the orange haze floats a thinner layer that looks blue, probably because its particles are smaller. It sat about 500 kilometres up when Cassini arrived, sank over the following years, vanished in 2012 and had returned by 2016, following Titan’s long seasons.

Rain, rivers and seas

Titan has a weather cycle like Earth’s, with methane in place of water. In September 2010 Cassini saw a large storm over Titan’s equator, and afterwards about 500 000 square kilometres of ground had turned darker, most likely soaked by methane rain. Rivers have cut channels across the surface; one, Vid Flumina, runs for more than 400 kilometres in canyons up to about 570 metres deep.

The seas lie near the north pole. The largest, Kraken Mare, covers about 500 000 square kilometres, a little more than the Caspian Sea. In 2013 Cassini’s radar sounded the second largest, Ligeia Mare, and found it up to about 160 metres deep and made almost entirely of methane and ethane. The radar never reached the bottom of the middle of Kraken Mare, which is probably deeper still.

Landing on Titan

On 14 January 2005 the European Huygens probe, carried by Cassini, parachuted through the haze for about two and a half hours and landed. It is still the only landing ever made in the outer Solar System. On the way down it photographed drainage channels; on the ground it found itself in what looked like a dry riverbed, among rounded pebbles of water ice, on ground with the consistency of loose wet sand. It sent data from the surface for over an hour.

An ocean underneath?

Cassini found that Titan’s surface rises and falls with Saturn’s tides by more than ten metres, far more than a solid moon would. In 2012 this was taken as evidence of a global ocean of water hidden beneath the ice. In 2025 a new analysis of the same data argued that Titan loses too much energy to tidal flexing for a free ocean to exist, and proposed a thick shell of ice over a layer of slush with pockets of liquid water instead. The question is open.

Dragonfly

NASA’s Dragonfly, a rotorcraft that will fly from place to place across Titan’s dunes and craters, is due to launch no earlier than July 2028 and to arrive in late 2034. It will study the organic chemistry of the surface, which may resemble the chemistry that came before life on Earth.