Metals, in particular iron and nickel, have been found in cometary dust by in-situ experiments on board the Giotto and Rosetta spacecrafts as well as in dust particles collected by the Stardust space probe. They essentially appear in silicate, sulfide and metal grains. Two Sun-grazing comets, the Great Comet of 1882 and C/1965 S1 (Ikeya-Seki), approached the Sun so close that dust grains vaporized, revealing lines of several metals, in particular FeI and NiI emission lines in the coma spectrum. However, it came as a surprise to find numerous FeI and NiI emission lines in high-resolution spectra of comets observed at large heliocentric distances, where the equilibrium temperature is far too low to allow sublimation of silicates and sulfides. Moreover the average NiI/FeI abundance ratio derived using a fluorescence model appears one order of magnitude higher than the Solar value. In this talk, I summarize this discovery and present some hypotheses to explain these unexpected results that indicate that constituents of the cometary nucleus or processes in the coma are still missing.
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