Gaseous atomic nickel in the coma of interstellar comet 2I/Borisov

On 31 August 2019, an interstellar comet was discovered as it passed through the Solar System (2I/Borisov). On the basis of initial imaging observations, 2I/Borisov seemed to be similar to ordinary Solar System comets^1,2—an unexpected characteristic given the multiple peculiarities of the only known previous interstellar visitor, 1I/`Oumuamua^3-6. Spectroscopic investigations of 2I/Borisov identified the familiar cometary emissions from CN (refs. ^7-9), C₂ (ref. ¹⁰), O I (ref. ¹¹), NH₂ (ref. ¹²), OH (ref. ¹³), HCN (ref. ¹⁴) and CO (refs. ^14,15), revealing a composition similar to that of carbon monoxide-rich Solar System comets. At temperatures greater than 700 kelvin, comets also show metallic vapours that are produced by the sublimation of metal-rich dust grains¹⁶. Observation of gaseous metals had until very recently¹⁷ been limited to bright sunskirting and sungrazing comets^18-20 and giant star-plunging exocomets²¹. Here we report spectroscopic observations of atomic nickel vapour in the cold coma of 2I/Borisov at a heliocentric distance of 2.322 astronomical units—equivalent to an equilibrium temperature of 180 kelvin. Nickel in 2I/Borisov seems to originate from a short-lived nickel-containing molecule with a lifetime of 340₋₂₀₀⁺²⁶⁰ seconds at 1 astronomical unit and is produced at a rate of 0.9 ± 0.3 × 10²² atoms per second, or 0.002 per cent relative to OH and 0.3 per cent relative to CN. The detection of gas-phase nickel in the coma of 2I/Borisov is in line with the recent identification of this atom—as well as iron—in the cold comae of Solar System comets¹⁷.