Titan: Moon of Saturn that can host two forms of life

Titan is the largest moon of Saturn and is a unique world with complex chemistry. There are lakes and rivers of liquid hydrocarbons on the surface, blanketed by a thick, hazy atmosphere, creating an environment similar to a primordial Earth. The dense, nitrogen-rich atmosphere and the presence of liquids on the surface makes Titan exceptional amongst all the moons in the Solar System. With a diameter of 5,150 kilometres, Titan is larger than Mercury and is the second-largest natural satellite in the Solar System after Ganymede, an ice moon in orbit around Titan.

The atmosphere of Titan has a density about 1.5 times in excess of the atmosphere on Earth. The pressure on the surface reaches 1.45 bar. 98 per cent of the atmosphere is made up of nitrogen, with the remaining two per cent dominated by methane, along with traces of ethane, diacetylene, cyanoacetylene and other complex organic molecules. High in the atmosphere of Titan, the ultraviolet radiation from the Sun disintegrates the methane molecules, leading to a cascade of chemical reactions that produce complex hydrocarbons and nitriles, that aggregate in thick layers of smog. The surface of Titan is obscured in visible light because of the hazy atmosphere.

NASA’s Cassini spacecraft orbited Saturn between 2004 and 2007, and fundamentally changed our understanding of Titan. The radar instrument on board allowed Cassini to peer through the thick atmosphere, mapping the surface. These maps revealed vast hydrocarbon seas, rivers and drainage channels fed by rain carved into the icy landscape. Cassini deployed a probe from the European Space Agency into Titan in January 2005, that relayed back the first direct images of the landscape, which was shaped by erosion and sediment deposition, similar to fluvial processes on Earth.

Titan is frozen, with an average surface temperature of -179°C. At these temperatures, water ice behaves like rock, while gases on Earth such as methane and ethane exist as liquids. The three largest seas on Titan are called Kraken Mare, Ligeia Mare and Punga Mare. These lakes primarily contain methane and ethane, and are similar to oceans and lakes on the Earth. There are seasonal methane rainstorms that replenish these liquid bodies. Just like the water cycle on the Earth, there is a methane cycle on Titan with evaporation, condensation and precipitation.

There is compelling evidence to indicate that Titan hosts a liquid water subsurface ocean, possibly mixed with ammonia that prevents it from freezing. Precise measurements by Cassini, along with observations of the rotation of Titan and its tidal flexing all suggest that the subsurface ocean exists beneath an ice shell that is between 50 and 100 kilometres in thickness. This ocean of water and ammonia surrounds a rocky core. This subsurface ocean could possibly create a habitable environment for life forms shielded from the extreme cold on the surface. The subsurface ocean can host microbial life if it contains enough heat and chemical nutrients.

The interior of Titan. (Image Credit: NASA). 

Titan has a dynamic geology, with dunes formed by organic particles the size of sand, transported by wind. These dunes are particularly frequent around the equator. Titan also has mountains, possibly uplifted blocks of icy crust, and cryovolcanoes, from where the water-ammonia sludge from the deep interior leaks out on the surface. These cryovolcanic processes could be essential for replenishing the methane in the atmosphere, which would otherwise be destroyed by solar radiation over the course of a few million years.

The atmospheric chemistry of Titan is of profound interest for astrobiology. Scientists have recreated the atmospheric conditions of Titan in the laboratory, which has resulted in the generation of amino acids and other organic precursors to life. The surface of Titan is too cold for carbon-based lifeforms, but the chemistry suggests prebiotic processes could be taking place. Titan could host exotic lifeforms on the surface that are based on a methanogenic biochemistry, using liquid hydrocarbons as a solvent instead of water. In the absence of sunlight, such life would be chemotrophic, using hydrogen and acetylene as an energy source instead of glucose. The metabolism of such life forms would be excruciatingly slow in the frigid environment.

Future probes plan to explore Titan to investigate these possibilities. NASA plans to dispatch the Dragonfly mission in 2028 to Titan, which will arrive in the mid 2030s. The spacecraft takes advantage of the dense atmosphere of Titan and is a rotorcraft that is capable of hopping between sites, analysing surface compositions and atmospheric conditions. The drone will specifically look for biosignatures and study the prebiotic chemistry in situ, allowing scientists to get a better handle on the potential habitability of Titan.

The study of Titan has broader implications for planetary science and the search for life beyond Earth. Titan is a natural laboratory for organic chemistry in frigid, oxygen-poor conditions. There might be exoplanets with similar environments. The landscapes are similar to that of the Earth despite being shaped by methane-based processes rather than water, indicating that these mechanisms can be universal, even when the materials involved are greatly different from those on Earth. Titan is a rich world where geology, meteorology and chemistry converge in a complex and dynamic system. Titan challenges our understanding of where and how life might spring up across the universe.