Polish astronomers have identified nickel vapor in the tail of comet 2I / Borisov – an interstellar comet spotted through the solar system in 2019 – even though the temperature was far too low for nickel to sublimate.1 The researchers therefore suspect that the nickel was released through photodissociation of compounds in the comet. Independent observations by Belgian astronomers identify similar gaseous nickel in comets of the cold solar system, suggesting a possible organometallic origin.2 The two works together could therefore propose a common organic chemistry between different planetary systems.

When Piotr Guzik and MichaƂ Drahus from the Jagiellonian University in Krakow analyzed ultraviolet spectra from the tail of 2I / Borisov, they did not expect any emission lines from metals, as these remain solid up to high temperatures. “It was far from the sun and the coma was cold,” explains Guzik. “But there were some strange, unidentified lines … like more or less spikes.” The researchers searched the record for evidence of previous comets, but drew a loophole until Guzik came across the spectrum of the 1965 Ikeya-Seki comet, which passed so close to the sun that it was visible in the daytime sky. “I noticed that there are some lines that are very close to ours,” says Guzik. “I found that our brightest line is almost the same wavelength that there is nickel in Ikeya-Seki.” The other lines also turned out to be present. The mathematical modeling convinced the researchers that neutral nickel atoms were indeed vaporized at 200 K.

An image that shows how comets can release metal atoms

The researchers postulate that a short-lived molecule – which is stable in space – can photodissociate when exposed to light even at relatively cold temperatures. They don’t suggest a specific molecule, but researchers at the University of Liege in Belgium observed similar nickel vapors – as well as iron vapors – in luminescence from a number of comets in our own solar system. They suggested that a number of possible organic compounds, such as carbonyls and polycyclic aromatic hydrocarbons, could have generated these through photodissociation. If these were also responsible for the emissions from 2I / Borisov, it would increase the fascinating possibility that organic chemistry in other planetary systems is similar to ours. “At least the conditions at the time comets form should probably be similar in order to form at least the same nickel-containing molecules,” says Guzik.

Physicist Steven Bromley and planetary scientist Dennis Bodewits are impressed by the researchers’ detection of nickel.3‘A comet releases about [100kg] Water per second and roughly the same nickel content as a nickel coin, ”says Bromley. Bodewits believes that it will only be possible to achieve the work’s true potential if a comet sample return mission shows exactly what compounds are causing the luminescence. “We see things in the gas phase, but we want information about this icy object,” he says. “That’s partly a question of laboratory astrophysics that we’re interested in: what reactions are possible, how can we simulate them, and then how can we try to figure out what that means?”


1 J Manfroid, D Hutsemékers and E Jehin, nature2021, 593372 (DOI: 10.1038 / s41586-021-03485-4)

2 P Guzik and M Drahus, nature2021, 593375 (DOI: 10.1038 / s41586-021-03435-0)

3 D Bodewits and SJ Bromley, nature2021, 593349 (DOI: 10.1038 / d41586-021-01265-8)


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