Methane, ethane, and tholins on Titan

Tholin is the term given to the heteropolymers produced by the action of the ultra-violet irradiation of simple organic molecules like methane and ethane — a process called copolymerization. A heteropolymer is a polymer constructed of more than one type of these simple molecules, as opposed to a homopolymer. The word tholin is derived from the Greek word for muddy. A tholin molecule is small enough to linger in an atmosphere and is therefore considered to be an organic aerosol. It has a reddish brown appearance. Carl Sagan first coined the word tholin in 1979 to describe the organic molecules of a prebiotic Earth. Spectrometer readings by astronomical observatories such as the Hubble space telescope has revealed the presence of tholins in great abundance in the solar systems of stars hundreds of light years away.

The atmosphere of the moon called Titan, around the planet Saturn, is 98.4% nitrogen and 1.6% of mostly methane with trace amounts of other gases. Titan's atmosphere is orange-red in color, thought to be caused by the presence of tholins which are composed of carbon, hydrogen and nitrogen. The presence of Oxygen in Earth's atmosphere predisposes the presence of tholins in it here as they would oxidize rapidly on formation. The Cassini mission has mapped about 20% of the surface of Titan and this has revealed the presence of hundreds of liquid hydro-carbon lakes, any one of which may contain as much potential energy as all the fossil fuel reserves on Earth. Methane is liquid at the -178 °C temperature at Titan's surface. The dunes on Titan are thought to be comprised of tholins which precipitate along with the liquid methane, which occurs as part of the cycle of condensation of lake methane forming clouds which rain down on the surface after cooling in the atmosphere.

In addition to the nitrogon and methane that are present in the upper atomosphere of Titan Cassini has also discovered benzyne, which is the key nucleus in the aromatic hydrocarbons, and large positive and negative ions. These ions along with ultraviolet radiation play a catalytic role in the formation of tholins in Titan's upper atmosphere, or ionosphere.

It is probable that conditions on Titan are very similar to those of a very early Earth atmosphere. Tholins can provide the sole source of carbon for many varieties of soil bacteria and for this reason their presence is thought to be the precursor to life. Tholins won't break down in the methane lakes found on Titan. However, they will dissolve in the presence of water or ammonia. Dissolving tholins in water yields ammonia. This means that there is a slim chance that there are amino acids in Titan's water ice or presumed subterranean liquid ocean. If there is ammonia (NH3) it would act as anti-freeze and it might be keeping water liquid for up to a thousand years in spite of the low temperature. Oxygen would be released slowly in water flows and abruptly during volcanic eruptions which melt the water ice. This means that periodically there might be all the ingredients necessary for the emergence of some form of life.

The relative abundance of tholins now present on Titan suggest that the age of our solar system is much younger than previously thought, since there would be far smaller amounts of tholin on Titan if that moon were billions of years old. Data from Cassini will continue to provide scientists with new insight for a few more years as more of the surface of Titan is mapped and analysed with every fly-by.