Cold comets — both natives and the interstellar Comet Borisov — have nickel and iron vapor in their atmospheres, and scientists want to know why.

Interstellar comet 2I/Borisov has brought another surprise to the solar system — nickel atoms that glowed in the coma surrounding the comet's icy core when it was far from the Sun. Even more surprising, comets native to the solar system have been doing the same thing unnoticed.

Comet Borisov's spectrum
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.
ESO / L. Calçada / O. Hainaut, P. Guzik and M. Drahus

The first discovery came when Piotr Guzik and Michał Drahus (both at Jagiellonian University, Poland) analyzed spectra of Borisov, which they had recorded with the X-shooter spectrograph on the European Southern Observatory's Very Large Telescope in Chile. They found a chemical fingerprint — nine near-ultraviolet emission lines — that correspond to vaporized nickel.

The spectrum matched a similar signature seen in the spectrum of Sun-grazing comet C/1965 S1 Ikeya-Seki. Guzik was astonished. That spectrum was recorded when the comet was around a million kilometers from the Sun’s surface. "It was so hot that even the dust in its coma started to evaporate, releasing metal atoms," Guzik says.

Yet Guzik and Drahus had snapped Borisov's spectrum when the comet was nearly 350 million kilometers from the Sun. Ices in a comet's core might vaporize there but not metals like nickel.

Second Surprise

Comet spectrum
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.
ESO / L. Calçada / SPECULOOS Team / E. Jehin, Manfroid et al.

Soon after Guzik and Drahus submitted their study to Nature, an editor told them that a second paper reporting nickel vapor from cold comets had arrived just a week after theirs. Neither group had heard of the other's research before, and neither contacted the other, although they did cite each other’s work. Both studies appear in the May 20th Nature.

The second study came from Jean Manfroid, Damien Hutsemékers, and Emmanuel Jehin (University of Liège, Belgium). Jehin says his group specializes in cometary composition and since 2002 has collected hundreds of spectra with a different instrument on the Very Large Telescope, the Ultraviolet and Visual Echelle Spectrograph. Their new result comes from several years of study revealing iron as well as nickel around some 20 native comets, with observations taken while they were glowing coldly in the inner solar system.

Having kept their project quiet until submitting their paper, Jehin says, “We were very surprised indeed to see there was another paper on the subject in the same issue of Nature.” He says they had also identified iron and nickel lines from observations of Borisov taken shortly after its discovery in 2019, but they had omitted them from their Nature paper. They are now submitting that result in a separate study, which he declined to describe.

Manfroid says it was a big surprise to find emission from both iron and nickel atoms in the spectra of all 20 of the solar system comets. These heavy metals are known to exist in solid forms in comet nuclei along with ices. Iron is typically ten times more abundant than nickel in solar system objects, yet the two metals were present in equal amounts in the comet atmospheres.

How nickel and iron wind up in the comet's atmosphere remains something of a mystery. Ices evaporate as sunlight warms comets approaching the inner solar system. Carbon monoxide is among the first of the ices to vaporize; water ice is among the last. Metal compounds usually remain solid in dust particles unless a comet grazes the Sun. Jehin and his colleagues found only about one atom of iron or nickel for every 100,000 molecules of water in the cometary atmospheres and noted the nickel was distributed unevenly throughout the comas.

They suspect the nickel and iron they find in the comas had originally been part of complex compounds, in which a metal atom clusters with many molecules of carbon monoxide or hydrocarbon ices. Such compounds could vaporize at low temperatures along with other ices. Then, when sunlight splits the compounds apart, the metal atoms are on their own and can emit spectral lines.

How Strange Is Borisov?

Like the enigmatic 1I/‘Oumuamua, it was Borisov’s unusual orbit that initially identified it as an interstellar object. However, early studies found that Borisov seemed to be otherwise much like comets from within the solar system.

Yet further studies revealed some unusual features. Two months ago, two groups reported the coma of Borisov was laden with exceptionally fine dust, a sign the interstellar comet was nearly untouched by other stars' light before it reached the solar system. Interactions with the Sun’s radiation have blown such fine dust away from native comets. The only known exception is Comet Hale-Bopp (C/1995 O1), which is thought to have approached the inner solar system only once before, around 4,000 years ago.

Guzik says the detection of gaseous nickel in both Borisov and solar system comets further confirms their similarity. The presence of cyanide, diatomic carbon, hydrogen cyanide, carbon monoxide and other compounds suggests to him a composition similar to that of the few solar-system comets known to be unusually rich in carbon monoxide. He also looks forward to investigating other interstellar comets, although they may be few and far between.




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