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Unexpected discovery of heavy metal vapours in solar system's comets

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A study conducted by researchers from the STAR research unit (Faculty of Science) of the ULiège University reveals the surprising chemical composition - namely traces of free iron and nickel atoms - of comets atmospheres, even at great distances from the Sun. This important discovery, published in the scientific journal Nature, could provide new information on the formation of comets and the birth of our solar system.


 new study by a team of astrophysicists from the University of Liège using data from the European Southern Observatory's (ESO) Very Large Telescope (VLT) shows the presence of iron and nickel in the atmosphere of comets, even those far from the Sun. This is the first time that heavy metals, usually associated with very hot environments, have been discovered in the cold atmosphere of comets. "It was a big surprise to detect iron and nickel atoms in all the comets we have observed over the last two decades, about 20 of them, and even in those far from the Sun, in the cold space environment," explains Jean Manfroid, scientific collaborator at the University of Liège and first author of the study.

Astronomers know that metals are present in the dusty, rocky interiors of comets. But since metals in solid form do not usually 'sublimate' (become gaseous) at low temperatures, they did not expect to find them in the atmospheres of comets far from the Sun. Nickel and iron vapours have even been detected in comets observed more than 500 million kilometres from the Sun, more than three times the distance from the Earth to the Sun.

"Comets formed about 4.6 billion years ago, in the very young solar system, and have undergone almost no transformation since then. In this sense, they are like fossils for astronomers and provide information on the first stages of the solar system, before the formation of planets", explains Emmanuel Jehin, FNRS Senior Research Associate in the STAR research unit (Faculty of Science) at ULiège and co-author of the study.

While the optical spectra of comets have been studied for decades, no one has ever detected the presence of nickel and iron in their atmospheres. "A discovery that went under the radar for many years! "says Emmanuel Jehin.  This discovery is surprising because, prior to this study, heavy metals had only been observed in a gaseous state in very hot environments, for example in the atmosphere of ultra-hot exoplanets or very rare comets eroded by a passage too close to the Sun.

The team used the UVES (Ultraviolet and Visual Echelle Spectrograph) instrument on ESO's VLT, which uses the technique of spectroscopy, to analyse the atmospheres of comets at different distances from the Sun. This technique allows astronomers to reveal the chemical makeup of celestial objects: each element leaves a unique signature - a set of lines - in the spectrum of the light of the objects studied.


Artist’s animation of the heavy metal composition of a cometary atmosphere

This video starts by showing an animation of comet C/2016 R2 (PANSTARRS), which was done using real images taken with one of the SPECULOOS telescope at ESO’s Paranal Observatory. The video then zooms in on the blue comet. In a new study done with the UVES instrument on ESO’s Very Large Telescope, a Belgian team has spotted heavy metal atoms in the inner atmosphere of the comet, a discovery illustrated at the end of the video. There we see the spectrum of the comet and in particular the iron (Fe I, blue) and nickel (Ni I, orange) lines, marking the presence of these two elements in the atmosphere of the comet. Credit : ESO/L. Calçada/M. Kornmesser, SPECULOOS Team/E. Jehin, Manfroid et al.
The Liège team was able to identify iron and nickel among the forest of lines appearing on the UVES spectra. A very small quantity of metals is emitted; the researchers estimate that for every 100 kg of water, there is only 1 g of iron and about the same quantity of nickel.

"Usually there is 10 times more iron than nickel in solar system objects, but in the atmosphere of these comets we found about the same amount of both elements. We concluded that they must come from a particular type of material on the surface of comet nuclei, sublimating at a fairly low temperature and releasing iron and nickel in roughly the same proportions," explains Damien Hutsemékers, astrophysicist and FNRS Research Director at ULiège, who took part in the study.

Although the team is not sure yet of the nature of this material, advances in instrumental astronomy - such as the Mid-infrared ELT Imager and Spectrograph (METIS) on ESO's future Extremely Large Telescope (ELT) - should allow researchers to confirm the source of the iron and nickel atoms in the atmosphere of these comets.

The team of ULiège scientists hopes that this discovery will pave the way for future research. "Now others will look for these lines and those of other metals in their archival data obtained with other telescopes," says Emmanuel Jehin. "We believe that this will also trigger new theoretical and laboratory studies on the subject.


Detection of heavy metals in the atmosphere of comet C/2016 R2

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 UVES spectrum of light of C/2016 R2 (PANSTARRS) on the top left superimposed to a real image of the comet taken with one of the SPECULOOS telescopes at ESO’s Paranal Observatory. Each set of peaks in the spectrum represents a different element, with those for iron and nickel indicated by blue and orange dashes, respectively. Spectra like these are possible thanks to the UVES instrument on ESO’s VLT, a high-resolution spectrograph that spreads the light so much that individual peaks can be individually identified. In addition, UVES remains sensitive down to wavelengths around 300nm, a region where most of the important iron and nickel lines appear, meaning that the capabilities of UVES were essential in making this discovery.
Credit: ESO/L. Calçada, SPECULOOS Team/E. Jehin, Manfroid et al.

Scientific references

Manfroid J., Hutsemékers D. and Jehin E., Iron and nickel atoms in cometary atmospheres even far from the Sun , Nature, May 2021. (https://doi.org/10.1038/s41586-021-03435-0)


Jean Manfroid

Damien Hutsemékers

Emmanuel Jehin

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