Heavy metal vapors unexpectedly found in comets

James Guilford
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. Each white peak 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 line so much they can be individually identified. In addition, UVES remains sensitive down to wavelengths of 300nm. Most of the important iron and nickel lines appear at wavelengths of around 350nm, meaning that the capabilities of UVES were essential in making this discovery. Credit:ESO/L. Calçada, SPECULOOS Team/E. Jehin, Manfroid et al.

“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.

Astronomers know that heavy metals exist in comets’ dusty and rocky interiors. But, because solid metals don’t usually sublimate (become gaseous) at low temperatures, they did not expect to find them in the atmospheres of cold comets that travel far from the Sun. Nickel and iron vapors have now even been detected in comets observed at more than 480 million kilometers 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 10 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.