Titan - a hydrocarbon world

Titan is Saturn’s largest moon and is the only moon in our Solar System known to have a dense atmosphere. It’s also the only other place in the Universe where clear evidence of stable bodies of surface liquid has been found. This makes it one of the most interesting locations to explore chemistry in space.

Figure 1. Titan appears orange due to its thick organonitrogen atmosphere. Image Courtesy of NASA JPL-Caltech

Titan has an Earth-like methane cycle where methane clouds form rain and even produce storms on the surface of the moon. The Cassini-Huygens mission gave us a closer look at Titan and its chemistry - detecting the presence of an organonitrogen surface haze and liquid hydrocarbon seas. Data also suggested the possible presence of cryovolcanoes that erupt cold liquid methane or ammonia!

Titan's upper atmosphere is rich with methane and nitrogen. When exposed to sunlight or highly energetic particles these small molecules are broken into ions and electrons. These charged species trigger a cascade of chemical reactions that produce a diverse range of hydrocarbons and other organic molecules. Data from the Cassini-Huygens mission detected not only alkanes, alkenes and alkynes in Titan’s atmosphere but complex polycyclic aromatic hydrocarbons. This demonstrates an evolution of chemical processes as the compounds generated in upper atmospheric levels fall closer to Titan's surface.

Scientists believe that the heaviest of these molecules fall to Titan's surface. These organic compounds eventually end up in the sea whether they fall there directly from the air, via hydrocarbon rain or transportation by rivers. A recent study adds evidence that Titan's large seas consist of pure methane and is likely to have a seabed of organic compounds. Compounds that are insoluble in the methane sea, such as benzene or nitrile compounds, sink to the sea floor to create a thick hydrocarbon sludge.