Stirring mystery in Titan’s atmosphere

Stirring mystery in Titan’s atmosphere

Read time: 3 minutes

ALMA has found a mystery in the atmosphere of Titan, the largest moon of the planet Saturn. Some organic molecules – molecules that contain atoms of carbon and nitrogen – are distributed unevenly in Titan’s thick layer of air. No one knows why.

If you pour cream in a cup of coffee, and then stir the coffee, the cream will be distributed evenly. The same should happen with organic molecules in the atmosphere of Titan, because the atmosphere is ‘stirred’ by strong winds.


But a brief, three-minute observation by ALMA showed otherwise. ALMA detected the millimeter radiation of two organic molecules (check if you can pronounce their names: they’re called hydrogen isocyanide and cyanoacetylene!). Those molecules are found both in the northern and in the southern hemisphere of Titan.

In deeper layers of the atmosphere, the molecules appear to be more or less evenly distributed in the east-west direction – just what you would expect because of the strong east-west winds. But at higher levels, ALMA found that the distribution is strongly skewed: there are dense ‘pockets’ of molecules on one side of the moon, while the concentration on the opposite side is much lower.


If this would be the case on Earth, the molecules in the northern hemisphere would be concentrated above Europe and not above the United States, while in the southern hemisphere they would be abundant above South America but not above Australia.


No one knows what causes the strange distribution of hydrogen isocyanide and cyanoacetylene. It may be related to unknown wind patterns in Titan’s atmosphere. But some astronomers point out that Titan orbits within the magnetic field of Saturn. The magnetic field may somehow affect the distribution of the organic molecules.

Scientists are keen to learn more about Titan. In many respects, Titan resembles our own Earth as it was a few billion years ago. There are even lakes on the surface of Titan, although they don’t contain water, but liquid methane gas instead. Knowing more about Titan might learn us more about the newborn Earth and the origin of life.

Titan is the largest moon of the planet Saturn, and even the largest in the solar system. It has a diameter of 5,152 kilometers – bigger than the planet Mercury! Titan orbits Saturn every 16 days at a distance of 1.2 million kilometers – just over three times the distance between the Earth and the moon. Because it’s so far from the sun, the surface temperature at Titan is minus 180 degrees Celsius.

The ALMA observations of organic molecules in the atmosphere of Titan were carried out by astrochemist Martin Cordiner of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, together with colleagues from the United States, the United Kingdom, France and Taiwan. They published their results on 22 October 2014 in the online edition of the magazine Astrophysical Journal Letters.