Nickel Appeared Before Iron

Metals are not expected to evaporate easily from a cold comet. Yet in 2021, two papers in Nature reported neutral iron and nickel atoms in cometary atmospheres far from the Sun. One survey found both metals in Solar System comets. The other found gaseous nickel in the coma of the interstellar comet 2I/Borisov at a temperature where ordinary metal grains should remain solid.

The third interstellar object sharpened the problem. Early observations of 3I/ATLAS detected numerous nickel lines while iron remained undetected. As the comet moved closer to the Sun and then beyond perihelion, iron appeared and the nickel-to-iron ratio moved toward values seen in Solar System comets. The anomaly was therefore not a permanent absence of iron. It was an evolving release sequence in which nickel entered the coma first and much more efficiently.

The leading natural explanations involve compounds or grains that can release metal at far lower temperatures than metallic material itself. Nickel carbonyl is one candidate because it is more volatile than the corresponding iron compound. Later modelling found that carbonyl sublimation can reproduce much of the observed nickel and iron evolution, possibly with an additional temporary heat source inside the comet. The carrier has not been captured and identified directly.

The observation is no longer “nickel with no natural mechanism.” It is a measured chemical sequence with plausible natural routes whose physical source inside the comet remains under investigation.