Lakes of methane under orange skies

Titan

Moon of Saturn · discovered 1655 by Christiaan Huygens

Titan is Saturn's largest moon and the second-largest in the Solar System, bigger than the planet Mercury. It is the only moon with a thick atmosphere and the only world besides Earth known to have stable liquid on its surface. Under a golden haze, rivers, lakes and seas of liquid methane and ethane shape a landscape that looks strangely familiar.

In 2005 the Huygens probe parachuted through that haze and landed on Titan's surface, still the most distant landing ever made. Next, NASA plans to send a nuclear-powered rotorcraft, Dragonfly, to fly from site to site across this frozen, Earth-like world.

Key facts

Diameter: 5,150 km1.48× Earth's Moon
Mass: 1.35 × 10²³ kg
Surface gravity: 1.352 m/s²0.138 g
Density: 1,880 kg/m³
Distance from Saturn: 1,221,870 km≈ 20.3 Saturn radii
Orbital period: 15.9 daysSame as its rotation (tidally locked)
Discovered: 1655Christiaan Huygens

Atmosphere: About 95% nitrogen, with methane; ~1.5 bar at surface
Surface temperature: About −179 °C (94 K)
Surface: Ice bedrock, organic dunes, methane and ethane seas
Rotation: Tidally locked to Saturn
Named after: The Titans of Greek mythology

Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.

Surface and geology

Titan's north polar region holds large seas of liquid hydrocarbons. Kraken Mare, the largest, covers around 400,000 square kilometres, comparable to the Caspian Sea, while Ligeia Mare is mostly methane and reaches depths of around 160 metres. Smaller lakes dot both poles, including Ontario Lacus in the south.

Around the equator, vast fields of dunes made of dark organic grains stretch for hundreds of kilometres, some rising around 100 metres high. Elsewhere there are mountains of water ice, branching river channels, impact craters such as Selk, and the bright, rugged region Xanadu.

Here water ice plays the role of rock, while methane plays the role of water. It evaporates, forms clouds, falls as rain and carves valleys, in a cycle that mirrors Earth's water cycle at a temperature far colder than anywhere on our planet.

Interior and composition

Beneath its icy crust, Titan is thought to hide a global ocean of salty water, inferred from the way Cassini measured its shape flexing under Saturn's tides. Below that likely lie high-pressure ice and a core of rock mixed with ice.

The atmosphere is mostly nitrogen with a few per cent methane near the surface. Sunlight breaks methane apart high in the atmosphere, producing a smog of complex organic molecules that gives Titan its orange colour and slowly settles onto the surface. Because methane is destroyed this way, it must be replenished, possibly from the interior, though the source is still debated.

Orbit and rotation

Titan orbits Saturn about every 16 days at a distance of roughly 1.2 million kilometres and is tidally locked, so a Titan day lasts as long as its orbit. Because Saturn is tilted, Titan experiences seasons, each lasting about seven Earth years over Saturn's 29-year orbit around the Sun.

Cassini saw these seasons change, with clouds and rainstorms shifting between hemispheres and the lakes of the north coming into sunlight as spring arrived.

Origin

Titan formed from ice and rock in the disc around the young Saturn. Isotope measurements suggest its nitrogen originally arrived as ammonia ice, similar to that in comets, which was later converted into nitrogen gas. How Titan came to be more massive than all of Saturn's other moons combined is still an active research question.

Exploration

Christiaan Huygens discovered Titan in 1655, the first moon found orbiting Saturn. Its opaque haze kept the surface hidden until spacecraft could peer through it with radar and infrared cameras.

Pioneer 11 and Voyager 1 (1979–1980)

Voyager 1 flew past at about 4,000 km, measured the dense nitrogen atmosphere and found the surface hidden beneath thick haze.

Cassini (2004–2017)

Cassini made more than 120 Titan flybys, using radar to map its seas, dunes and rivers and tracking seasonal weather until its Grand Finale in September 2017.

Huygens probe (2005)

ESA's Huygens landed on 14 January 2005 after a parachute descent of about two and a half hours, revealing river channels and rounded ice pebbles.

Dragonfly (launch planned 2028)

NASA's rotorcraft lander is due to arrive in the mid-2030s and fly between sites near the Selk impact crater, studying prebiotic chemistry.

The moons of Saturn

Major moons of Saturn, from the planet outward
Moon	Diameter	Distance	Orbit
Mimas	396 km	185,539 km	22.6 hours
Enceladus	504 km	237,948 km	32.9 hours
Tethys	1,062 km	294,619 km	45.3 hours
Dione	1,123 km	377,396 km	2.74 days
Rhea	1,528 km	527,108 km	4.52 days
Titan	5,150 km	1,221,870 km	15.9 days
Iapetus	1,469 km	3,560,820 km	79.3 days

Surprising facts about Titan

Titan's air is denser than Earth's and its gravity is weak, so a person wearing strapped-on wings could, in principle, fly by flapping their arms.
Titan is larger than Mercury, but has less than half its mass because it contains so much ice.
Huygens touched down on a damp, gravelly plain, and its heated inlet detected methane evaporating from the ground beneath it.
Titan's hydrocarbon seas hold several hundred times more liquid hydrocarbons than all of Earth's known oil and gas reserves, according to some estimates.
Radar images showed 'magic islands' in Ligeia Mare that appeared and vanished, possibly bubbles, waves or floating solids.

Keep exploring

Sources