Two different teams report some unexpected heavy metal discoveries in the fuzzy atmosphere of comets. A Belgian team has discovered the presence of iron and nickel in 20 comets across the solar system, suggesting that the presence of such elements in these icy bodies is very common.
And not just in the solar system. A second team spotted in nickel in the atmosphere of interstellar comet 2I/Borisov. Both pieces of research are published in Nature (here and here) and they have important implications for the composition of planetary systems in their formative years.
"It was a big surprise to detect iron and nickel atoms in the atmosphere of all the comets we have observed in the last two decades, about 20 of them, and even in ones far from the Sun in the cold space environment," Jean Manfroid from the University of Liège, Belgium, who lead the new study on Solar System comets, said in a statement.
Previous observations of heavy metal vapors have been confined to hot environments such as the extreme atmosphere of some exoplanets that orbit too close to their host star. Many objects in these two studies are far from having extreme temperatures. Comet Borisov was twice as far from the Sun than Earth when the observations took place.
"At first we had a hard time believing that atomic nickel could really be present in 2I/Borisov that far from the Sun. It took numerous tests and checks before we could finally convince ourselves," explained study author Piotr Guzik from the Jagiellonian University in Poland.
Iron and nickel are believed to be found deep in the nucleus of a comet. How they come to its surface and how they are then freed into its tenuous atmosphere is not exactly clear, though there are some suggestions. There could be conditions on comets where dust grains are heated to hundreds of degrees. Or, maybe these metals are bonded to other compounds and then released when these compounds are broken apart by the harsh ultraviolet light from the Sun.
The team estimates that about 1 gram of iron and roughly the same amount of nickel is released by a comet every second compared to roughly 100 kilograms (220 pounds) of water. The rough equivalence between the two heavy elements is also as puzzling as it is different from what’s been observed in the Sun and in meteorites.
"Usually there is 10 times more iron than nickel, and in those comet atmospheres we found about the same quantity for both elements. We came to the conclusion they might come from a special kind of material on the surface of the comet nucleus, sublimating at a rather low temperature and releasing iron and nickel in about the same proportions," added Damien Hutsemékers, also a member of the Belgian team from the University of Liège.
Comets and asteroids are remnants of the primordial solar system so studying them tells us about our cosmic neighborhood's past. It’s unclear what this discovery means exactly, but the surprising observations will certainly shift some of our assumptions about the origin of planets.