Astrophysics: Nickel detected in 2I/Borisov and comets from our Solar System

Nickel detected in the coma cloud surrounding the interstellar comet 2I/Borisov could hint towards the origins of this comet and the conditions at the time of its birth. The finding, reported in Nature this week, is unexpected as the temperatures observed are too cold to facilitate the sublimation of metals, suggesting an alternative mechanism. However, the detection of nickel is in line with the detection of this metal in the cold comae of Solar System comets, also reported in Nature, demonstrating similarities between the interstellar object and comets in our planetary system.

Comets, made up of dust and ice left over from the time of planet formation, may offer insights to the chemistry of early solar systems. The composition of comets is inferred by observations of the coma — a cloud of dust and gas surrounding the comet. Metals are usually notably absent as the typical temperatures of comets are too low to facilitate the release of metal ions into the coma; exceptions to this rule are comets passing close to the Sun, where temperatures are expected to exceed the 700 degrees kelvin (K) necessary to sublimate nickel.

The atomic nickel detected by Piotr Guzik and Michał Drahus is unexpected as, at the time of observation (January 2020), 2I/Borisov was far from the Sun and had an estimated temperature of 180 K. To confirm the identity of atomic nickel vapours, the authors ran models of nickel emission based on the expected levels of incoming solar energy and the velocity of the comet. The results were a near match to the observations. The authors suggest that at cold temperatures, the metal may be released from a nickel-containing molecule via a process called photodissociation, in which photons break down compounds.

In an independent study, Jean Manfroid and colleagues show that gaseous nickel (and iron) is present in the cold comae of Solar System comets. The similarity between these comets and 2I/Borisov suggests an affinity between the unknown birthplace of 2I/Borisov and our Solar System, Guzik and Drahus propose. “If we can unravel the origin of nickel and iron in regular comets and this interstellar object, we might uncover a shared story of organic chemistry between different planetary systems,” write Dennis Bodewits and Steven Bromley in an accompanying News & Views.

doi: 10.1038/s41586-021-03485-4

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