For the first time, astronomers have detected traces of heavy metals in the atmospheres of comets – both those whizzing through our Solar System and those from interstellar space.
Scientists have long known that the dusty, rocky interiors of comets harbour solid forms of heavy metals. But these metals have only previously been observed in hot environments, like evaporating comets soaring to close too the Sun or ultra-hot exoplanet atmospheres.
Since these heavy metals don’t usually turn to gas (sublimate) at low temperatures, scientists didn’t expect them in the icy atmospheres of comets.
Yet that’s exactly what they found.
The results were reported in two separate papers by two separate research teams, both published in Nature.
The first study used ESO’s Very Large Telescope (VLT) in Chile to analyse the spectra of comets in our Solar System. When comets get close to the sun, their materials begin to heat up and sublimate, and astronomers can use a technique called spectroscopy to reveal the comet’s chemical composition.
The team had been observing comets using ESO’s VLT for 20 years but only just noticed the weak spectral lines signalling the presence of iron and nickel in small amounts.
Lead author Jean Manfroid, from the University of Liège in Belgium, says: “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.”
The most far-flung comet they analysed was more than 480 million kilometres from the Sun, which is over triple the distance between the Earth and the Sun.
The team found iron and nickel in about equal amounts, which is notable because most other material in the Solar System contains ten times more iron than nickel.
“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 from the University of Liège – though the team is not yet sure what such a material would be.
The second Nature study looked further afield to analyse a comet not from our own neck of the woods – but from another Solar System entirely.
Another team from Poland spotted a gaseous form of nickel in the icy interstellar comet 2I/Borisov, also using a spectrograph on ESO’s VLT.
“At first we had a hard time believing that atomic nickel could really be present in 2I/Borisov that far from the Sun,” says lead author Piotr Guzik from the Jagiellonian University in Poland.
2I/Borisov was just 300 million kilometres from the Sun and had an estimated temperature of 180 degrees Kelvin, much cooler than the 700 degrees Kelvin necessary to sublimate nickel.
In their paper, the authors suggest a possible origin: “Unbound nickel atoms seem to originate from the photodissociation of a short-lived nickel-containing molecule that sublimates at low temperatures or is otherwise released with major volatile compounds.”
The results show that comets from other star systems have much more in common with comets from our own Solar System than we thought.
In an accompanying opinion piece, astronomers Dennis Bodewits and Steven J. Bromley conclude:
“If we can unravel the origin of iron and nickel in regular comets and this interstellar object, we might uncover a story of organic chemistry between shared different planetary systems.”
The results are also interesting for what they might add to our understanding of the formation of the Solar System, as comets are composed of dust and ice leftover from planetary formation.
“Comets formed around 4.6 billion years ago, in the very young Solar System, and haven’t changed since that time,” says co-author of the first study, Emmanuel Jehin from the University of Liège. “In that sense, they’re like fossils for astronomers.”