Astronomers have detected an unusually high abundance of molecular nitrogen in the interstellar comet 3I/ATLAS. Spectroscopic observations of its plasma tail showed that the ratio of molecular nitrogen to carbon monoxide is significantly higher than what is usually observed in comets of the Solar System. This chemical composition supports the hypothesis that 3I/ATLAS formed at very low temperatures in the distant outer region of another planetary system.

3I/ATLAS became the third confirmed interstellar object after 1I/ʻOumuamua and 2I/Borisov. It was discovered on July 1, 2025, and the very next day the object was classified as a comet. On October 29, 2025, the comet passed perihelion at a distance of approximately 1.36 astronomical units from the Sun. Unlike 1I/ʻOumuamua, 3I/ATLAS displayed pronounced cometary activity, allowing astronomers to study the gas and dust of material that formed beyond the Solar System.

Lea Ferellec of Northumbria University and her colleagues studied 3I/ATLAS after its perihelion passage using the William Herschel Telescope on the island of La Palma. The observations were conducted on November 30 and December 2, 2025, using the WEAVE-LIFU integral-field spectrograph. The instrument simultaneously obtains spectra from numerous regions of an extended object, making it well suited for studying ionized gas in a cometary tail.

In the plasma tail directed away from the Sun, the researchers simultaneously detected five types of ions: molecular nitrogen N₂⁺, carbon monoxide CO⁺, carbon dioxide CO₂⁺, water H₂O⁺, and CH⁺. The simultaneous measurement of several components allowed the researchers to compare their abundances and track how they changed with distance from the comet’s nucleus.

N₂⁺ attracted particular attention. Molecular nitrogen, N₂, is an extremely volatile substance that is difficult to retain in cometary ices. Therefore, its abundance can preserve information about the temperature and other physical conditions that existed when the comet formed.

Using the N₂⁺/CO⁺ ratio and the known fluorescence coefficients of these ions, the authors obtained a lower limit for the ratio of the original neutral molecules of N₂/CO > 0.023 ± 0.001. In other words, for every 100 CO molecules, there are at least approximately 2.3 N₂ molecules.

By this measure, 3I/ATLAS is richer in molecular nitrogen than typical comets of the Solar System. However, the authors emphasize an important limitation. The ratios of the other detected ions cannot be directly converted into the abundances of the corresponding neutral molecules because the chemical and photochemical processes in the plasma tail differ.

Nevertheless, the overall set of measurements is consistent with previous evidence that 3I/ATLAS is rich in highly volatile substances.

The high abundance of N₂ is particularly interesting when considered together with previous studies of 3I/ATLAS. Observations before perihelion showed enhanced abundances of CO₂ and CO, while isotopic measurements revealed strong deuterium enrichment — more than an order of magnitude higher than the values typical of Solar System comets.

These results had previously indicated that the object formed in a cold environment with a temperature no higher than approximately 30 K, or about −243 °C. The new measurement of molecular nitrogen independently supports the scenario of a cold origin.