Using data from the Atacama Large Millimeter/submillimeter Array (ALMA), a team of Japanese researchers has found that Galactic cosmic rays affect the chemical reactions involved in the formation of nitrogen-bearing organic molecules in the atmosphere of Titan, the largest moon of Saturn.

This view of Titan is among the last images NASAs Cassini spacecraft sent to Earth before it plunged into the giant planets atmosphere. Image credit: NASA / JPL-Caltech / Space Science Institute.

Titan is a carbon-rich, oxygen-poor world with a wide range of organic and inorganic compounds, atmospheric energy sources, and liquid hydrocarbon seas and lakes.

This moon is the only planetary body in the Solar System, except Earth, where rainfall and seasonally flowing liquids erode the landscape.

In addition to a hazy mixture of nitrogen and hydrocarbons like methane and ethane, the atmosphere of this strangely Earth-like world also contains an array of more complex organic molecules.

Planetary scientists think that this chemical make-up is similar to Earths primordial atmosphere.

In a new study, University of Tokyos Dr. Takahiro Iino and colleagues used ALMA to study the chemical processes in Titans atmosphere.

The researchers detected faint but firm signals of two organic compounds acetonitrile (CH3CN) and its rare isotopomer CH3C15N in the ALMA data.

We found that the abundance of 14N in acetonitrile is higher than those in other nitrogen bearing species such as HCN and HC3N, Dr. Iino said.

It well matches the recent computer simulation of chemical processes with high energy cosmic rays.

There are two important players in the chemical processes of the atmosphere: ultraviolet (UV) light from the Sun and cosmic rays from beyond the Solar System.

In the upper atmosphere, UV light selectively destroys nitrogen molecules containing 15N because the UV light with the specific wavelength that interacts with 14N is easily absorbed at that altitude, the study authors explained.

Thus, nitrogen-bearing species produced at that altitude tend to exhibit a high abundance of 15N.

On the other hand, cosmic rays penetrate deeper and interact with nitrogen molecules containing 14N.

As a result, there is a difference in the abundance of molecules with 14N and 15N.

The team revealed that acetonitrile in the stratosphere is more abundant in 14N than those of other previously measured nitrogen-bearing molecules.

We suppose that galactic cosmic rays play an important role in the atmospheres of other solar system bodies, said Dr. Hideo Sagawa, a researcher at Kyoto Sangyo University.

The process could be universal, so understanding the role of cosmic rays in Titan is crucial in overall planetary science.

A paper on the findings will be published in the Astrophysical Journal.

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Takahiro Iino et al. 2020. 14N/15N isotopic ratio in CH3CN of Titans atmosphere measured with ALMA. ApJ, in press; arXiv: 2001.01484

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Cosmic Rays from beyond Solar System Affect Atmosphere of Titan | Astronomy - Sci-News.com