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Most comets contain iron and nickel vapours.

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2021-05-20 03:08:15

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Comets are agglomerates of dust and ice — leftovers from the era of planet formation. For most comets, their distance from the Sun keeps their temperature below a few hundred kelvin, which is still hot enough for water ice and other volatile compounds to sublimate (be converted directly from solid to gas). Comet nuclei are mostly obscured by a surrounding cloud of gas and dust called the coma. Therefore, knowledge of comet surfaces and their composition must be inferred from observations of the coma. Typical telescopic observations of cometary comae do not detect metals, because temperatures at comet surfaces are too low for these elements to sublimate. However, two papers1,2 in Nature this week report the discovery of metal atoms in cometary atmospheres, begging the question of where these atoms come from.

There have been several space missions to comets, including Rosetta, Deep Impact and Stardust. These missions have shown that comets are relatively small (typically, just a few kilometres in radius), and might be responsible for moving volatile materials around in the inner Solar System after the planets formed3. Such missions provided detailed studies of individual comets, but Earth-based observations have determined the chemical composition of larger numbers of these bodies4,5.