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Heavy metal vapours have been found inside comets in the solar system and beyond by astronomers who say the discovery ‘went under the radar’ for 20 years.
Experts from the University of Liège in Belgium studied data from the European Southern Observatory Very Large Telescope to look into the atmosphere of comets.
They found that iron and nickel exist in the atmospheres of comets throughout our Solar System, even those that exist far from the Sun, and another study using ESO data revealed interstellar comet Borisov had nickel vapour in its atmosphere.
This is the first time heavy metals, usually associated with hot environments, have been found in the cold atmospheres of distant comets.
‘It was a 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 lead author Jean Manfroid.
The detection of the heavy metals iron (Fe) and nickel (Ni) in the fuzzy atmosphere of a comet are illustrated in this image, which features the spectrum of light of C/2016 R2 (PANSTARRS) on the top left superimposed to a real image of the comet taken with the SPECULOOS telescope at ESO’s Paranal Observatory
Astronomers know that heavy metals exist in comets’ dusty and rocky interiors, but solid metals don’t usually become gaseous (sublimate) at low temperatures, so they are not expected to be found in the atmospheres of frozen comets.
Nickel and iron vapours have now even been detected in comets observed at more than 300 million miles from the Sun, more than three times the Earth-Sun distance.
The Belgian team found iron and nickel in comets’ atmospheres in approximately equal amounts.
Material in our Solar System, for example that found in the Sun and in meteorites, usually contains about ten times more iron than nickel.
This new result therefore has implications for astronomers’ understanding of the early Solar System, though the team is still decoding what these are.
‘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.
While the Belgian team has been studying these ‘fossil’ objects with ESO’s VLT for nearly 20 years, they had not spotted the presence of nickel and iron in their atmospheres until now.
‘This discovery went under the radar for many years,’ Jehin says.
The team used data from the Ultraviolet and Visual Echelle Spectrograph (UVES) instrument on ESO’s VLT, which uses a technique called spectroscopy, to analyse the atmospheres of comets at different distances from the Sun.
This technique allows astronomers to reveal the chemical makeup of cosmic objects: each chemical element leaves a unique signature – a set of lines – in the spectrum of the light from the objects.
The Belgian team had spotted weak, unidentified spectral lines in their UVES data and on closer inspection noticed that they were signalling the presence of neutral atoms of iron and nickel.
372.5 SEI*79591374 This image was taken with the FORS2 instrument on ESO’s Very Large Telescope in late 2019, when comet 2I/Borisov passed near the Sun
This image features a comet located in the outer reaches of the Solar System: comet C/2016 R2 (PANSTARRS). As its name suggests, the comet was discovered in 2016 by the Pan-STARRS telescopes in Hawai’i
A reason why the heavy elements were difficult to identify is that they exist in very small amounts: the team estimates that for each 100kg of water in the comets’ atmospheres there is only 1g of iron, and about the same amount of nickel.
‘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,’ said co-author Damien Hutsemékers.
Although the team aren’t sure yet what material this might be, ESO’s upcoming Extremely Large Telescope will allow researchers to confirm the source of the iron and nickel atoms found in the atmospheres of these comets.
The Belgian team hope their study will pave the way for future research.
‘Now people will search for those lines in their archival data from other telescopes,’ Jehin says. ‘We think this will also trigger new work on the subject.’
Another remarkable study published today in Nature shows that heavy metals are also present in the atmosphere of the interstellar comet 2I/Borisov.
A team in Poland observed this object, the first alien comet to visit our Solar System, using the X-shooter spectrograph on ESO’s VLT when the comet flew by about a year and a half ago. They found that 2I/Borisov’s atmosphere contains gaseous nickel.
‘At first we had a hard time believing that atomic nickel could really be present in 2I/Borisov that far from the Sun,’ says study author Piotr Guzik from the Jagiellonian University in Poland.
‘It took numerous tests and checks before we could finally convince ourselves,’
The finding is surprising because, before the two studies gases with heavy metal atoms had only been observed in hot environments, such as in the atmospheres of ultra-hot exoplanets or evaporating comets that passed too close to the Sun.
Borisov was observed when it was some 300 million kilometres away from the Sun, or about twice the Earth-Sun distance.
Studying interstellar bodies in detail is fundamental to science because they carry invaluable information about the alien planetary systems they originate from.
The detection of nickel (Ni) in the fuzzy atmosphere of the interstellar comet 2I/Borisov is illustrated in this image, which shows the spectrum of light of the comet on the bottom right superimposed to a real image of the comet taken with ESO’s Very Large Telescope (VLT) in late 2019. The lines of nickel are indicated by orange dashes
‘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, from the Jagiellonian University.
The Polish and Belgian studies show that 2I/Borisov and Solar System comets have even more in common than previously thought.
‘Now imagine that our Solar System’s comets have their true analogues in other planetary systems – how cool is that?,’ Drahus concludes.
This research was presented in two papers to appear in Nature, one called ‘Iron and nickel atoms in cometary atmospheres even far from the Sun, and the other dubbed ‘Gaseous atomic nickel in the coma of interstellar comet 2I/Borisov’.
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