Scientists have unexpectedly found heavy metal vapors in the atmosphere of comets throughout our solar system. And beyond.

A Belgian team has shown that iron and nickel exist in the atmospheres of local and interstellar comets. The study came thanks to data from the European Southern Observatory’s Very Large Telescope (ESO’s VLT). Another study by Polish researchers, who also used ESO data, reported that nickel vapor is also present in the icy interstellar comet 2I/Borisov.

This is the first time scientists have seen such vapor in the cooler atmospheres of comets far from a star. The finding could indicate some previously unknown mechanism or material on the surface of comets.

“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,” says Jean Manfroid from the University of Liège, Belgium, who lead the new study on Solar System comets published today in Nature.

Previously, astronomers have found heavy metal vapors in hot environments such as the extreme atmosphere of some exoplanets that orbit too close to their parent star. The objects in the new study, however, are far from having extreme temperatures.

Now, researchers have detected nickel and iron vapors in comets more than 480 million kilometers from the Sun. That’s more than three times the Earth-Sun distance.

Comet 2I/Borisov was twice as far from the Sun as Earth when the observations took place.

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More interesting is the fact that the Belgian team found these heavy metals in about equal amounts. Generally, iron is about ten times more abundant in the solar system than nickel. And as far as we know, comets are leftovers from the formation of planetary bodies within the solar system. That means it’s a mystery why the comets the team observed have such a relatively large abundance of nickel. This may even change our understanding of the formation of the solar system.

“Comets formed around 4.6 billion years ago, in the very young Solar System, and haven’t changed since that time. In that sense, they’re like fossils for astronomers,” says study co-author Emmanuel Jehin, also from the University of Liège.

The team of researchers has been studying these “fossils” for nearly 20 years. However, it hadn’t detected the presence of nickel and iron in their atmosphere until now.

Typically, iron and nickel lie deep in the nucleus of a comet. But it’s a head-scratcher how these heavy metals come to the surface and rise into the atmosphere. There are some suggestions though. Scientists believe there could be conditions on comets where dust grains reach temperatures of hundreds of degrees. Or, maybe these metals are bonded to other compounds and then freed when the harsh ultraviolet light from the Sun breaks these compounds apart.

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The team used data from the Ultraviolet and Visual Echelle Spectrograph (UVES) instrument on ESO’s VLT. The instrument uses a technique called spectroscopy, to analyze the atmospheres of comets at different distances from the Sun. Spectroscopy is the technique of using the spectrum of light to study matter. It allows scientists to reveal the chemical makeup of cosmic objects. Each chemical element leaves a unique signature—a set of lines—in the spectrum of light from the objects.

The scarcity of these heavy elements made it difficult to detect them. The team estimates that a comet releases about 1 gram of iron and roughly the same amount of nickel every second compared to roughly 100 kilograms (220 pounds) of water. The rough equivalence between the two heavy elements is also puzzling as it differs from what astronomers have observed in the Sun and meteorites.

Comets are packed with primordial ice and dust that emerged when the planets were still coalescing. Water ice and other volatiles will sublimate (convert directly from a solid to a gas) even when a comet is far from the Sun. Thus, creating a ball of gas around the nucleus. As for pumping out gaseous nickel or other metals, that’s not something comets normally do—at least comets that are far from the Sun.

The Belgian astronomers believe that the equal amounts of iron and nickel together with the sublimation at low temperatures means there is something undiscovered at the surface of the comets they studied.

However, telescopes such as the Mid-infrared ELT Imager and Spectrograph (METIS) on ESO’s upcoming Extremely Large Telescope (ELT), could help discover what this material is. The ELT is currently under construction in the Atacama Desert region of Northern Chile.

2I/Borisov: The Interstellar Intruder

Another study by a Polish team also found traces of nickel vapor in the atmosphere of the interstellar visitor 2I/Borisov. If this comet sounds familiar to you, it’s because it made headlines in 2019 when it became only the second known object within the solar system which originated from outside our solar system.

The Polish team observed the interstellar object using the X-shooter spectrograph on ESO’s VLT, as the object passed through the solar system.

This is another surprise for astronomers, as again it details the discovery of sublimated heavy metals in a cold atmosphere.

Studying interstellar objects is fundamental to our understanding of the alien planetary systems they originate from.

“All of a sudden we understood that gaseous nickel is present in cometary atmospheres in other corners of the Galaxy,” says co-author Micha? Drahus, also from the Jagiellonian University.

The Polish and Belgian studies show that 2I/Borisov and the comets in our Solar System have even more in common than previously thought.

Astronomers published both pieces of research in the journal Nature (here and here).

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