Heavy metals unexpectedly found in comets' atmospheres throughout the Solar System © ESO/L. Calçada, SPECULOOS Team/E. Jehin, Manfroid et al.

Heavy metals unexpectedly found in comets’ atmospheres throughout the Solar System

The presence of iron and nickel vapours in comets' atmospheres reveal a hole in our understanding of early Solar System.

Astronomers have unexpectedly found traces of heavy metals in the atmospheres of all of the comets they have studied from the last 20 years – including 2I/Borisov, the first comet to visit from another solar system.

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Heavy metals like iron and nickel are often found in comets, but only in their dusty and rocky interiors. However, solid metals usually don’t sublimate – turn from solid directly to gas – at the low temperatures found in the atmospheres of distant comets. As a gas, heavy metals had only previously been observed in much hotter environments, such as evaporating comets as they passed by the Sun, or in the atmospheres of ultra-hot exoplanets.

So, a Belgian team of scientists were surprised to find trace amounts of the two heavy metals in comet atmospheres throughout the Solar System, including ones more than three times further from the Sun than the Earth’s orbit.

Jean Manfroid, from the University of Liège, Belgium, lead the study on Solar System comets published in Nature. “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,” he said.

Usually, material from our Solar System contains about 10 times more iron than nickel. However, these comets had roughly equal amounts of iron and nickel.

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“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,” explains Damien Hutsemékers, also a member of the Belgian team from the University of Liège.

Since comets formed so early in the lifetime of the Solar System, they are like “fossils for astronomers”, say the team. So, these discoveries suggest that there is a hole in our understanding of early Solar System.

The researchers hope that future research with the upcoming Extremely Large Telescope can help to answer some of these questions, including what the material on the surface of the comet nucleus might be.

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A second paper, published by a Polish team, describes the discovery of nickel in the atmosphere of 2I/Borisov. “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,” said study author Piotr Guzik from the Jagiellonian University in Poland.

Reader Q&A: Who really discovered Halley’s Comet?

The most famous of all comets was certainly seen by the English astronomer and mathematician Edmond Halley when it flew round the Sun in 1682, but he did not discover it. The credit for that goes back at least another 2,000 years to 240 BC, when unknown Chinese astronomers noted what they called a ‘broom star’ appearing in the eastern sky in May of that year.

Halley’s claim to the name stems from his crucial discovery about the nature of the eponymous object. While studying a list of comet observations over the centuries, he noticed that the years 1531, 1607 and 1682 all featured the appearance of one of these supposedly capricious portents of doom. Was it just a coincidence that they were all about 76 years apart?

Using Newton’s then newly published law of gravity, he showed that they were all the same object, swinging round the Sun on a vast orbit. Halley predicted it would return in 1758, which it duly did. While he didn’t live to see it, his calculations played a key role in showing that supposedly fickle natural phenomena can be understood through the power of science.

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