Cosmic ray particles disturb composition of Titan’s atmosphere
New discoveries

Cosmic ray particles disturb composition of Titan’s atmosphere

Read time: 3 minutes

A sand storm is not the best place to cook a meal or to bake a cake. The sand particles migh contaminate and affect the ingredients, and the end result would taste badly.
In space, too, particles from outside may affect the way molecules are ‘cooked’. Using ALMA, astronomers have found evidence for this process in the atmosphere of Titan, the largest moon of the planet Saturn.
Titan is a large moon: much larger than our own Moon, and even larger than the planet Mercury. Unlike the moon and Mercury, Titan has a thick atmosphere, containing lots of nitrogen gas. In the atmosphere of Titan, nitrogen atoms combine with other atoms to form larger molecules, just like a chef combines various ingredients to cook a meal.
However, there are two types of nitrogen atoms in nature: ‘normal’ nitrogen and ‘heavy’ nitrogen. As a result, there are also different types of nitrogen-bearing molecules.
Astronomers already knew that ultraviolet sunlight can influence the relative amounts of normal and heavy nitrogen. But ultraviolet sunlight only affects the nitrogen-bearing molecules in Titan’s upper atmosphere.
Now, three Japanese astronomers have found evidence for a similar effect at a much deeper level in the atmosphere. They studied molecules of acetonitrile, which consist of three atoms of hydrogen, two of carbon, and one of nitrogen. The number of these molecules containing heavy nitrogen turned out to be lower than expected.
Ultraviolet sunlight can’t be the culprit. Instead, the astronomers believe that the molecules containing heavy nitrogen are destroyed by cosmic rays from outer space. Cosmic rays are fast- moving particles that easily penetrate an atmosphere.
Computer models that simulate the effects of cosmic rays on nitrogen-bearing molecules neatly explain the observed results.
So it’s just like the cake that was baked in the midst of a sand storm: the molecules that are ‘cooked up’ in Titan’s atmosphere are a bit different, because of the effect of particles from outside!
The new result may help astronomers to better understand the effects of cosmic rays on chemical reactions in an atmosphere – not only in the atmosphere of Titan, but also in the atmosphere of other planets.
As for our own Earth: there’s not that much of an effect. Most cosmic ray particles are stopped by the Earth’s protective magnetic field.


Titan is the largest moon of Saturn, the giant planet famous for its beautiful ring system. Titan was discovered in 1655 by Dutch astronomer Christiaan Huygens. In the 20 th century, another Dutch astronomer, Gerard Kuiper, discovered that Titan has a thick atmosphere. The first close-up photos of Titan were shot in 1980 by NASA’s Voyager 1 spacecraft. Between 2004 and 2017, Titan has been studied in detail by Cassini, another NASA spacecraft. A European lander called Huygens (after the discoverer of Titan) even descended down to the surface. The ALMA observations now reveal details about the various nitrogen-bearing molecules in Titan’s thick, hazy atmosphere.


The precise composition of acetonitrile molecules in Titan’s atmosphere was determined by Japanese astronomers Takahiro Iino of the University of Tokyo, Hideo Sagawa of Kyoto Sangyo University, and Takashi Tsukagoshi of the National Astronomical Observatory of Japan. Takahiro, Hideo and Takashi did not need to carry out new ALMA observations of Titan. They just studied existing measurements that had been made before and are now stored in the ALMA archive. By analyzing these older data, they found the unexpected low abundance of acetonitrile molecules containing heavy nitrogen atoms. The three astronomers have published their findings in The Astrophysical Journal.