Far beyond the sun's warmth, a visitor from another star system carried a faint trace of metal into view, and in doing so unsettled an old assumption about how comets speak. Nickel and iron, which should have stayed locked in frozen rock at such cold, were found drifting in the gas around both an interstellar wanderer and some twenty native comets. The shared traces hint that these icy bodies, born in different places and times, may have been shaped by similar conditions at the dawn of their worlds. Why the metals rise into the chill, and what material hides them, remains a question the sky ha
Metal Atoms Found in Comet Atmospheres, Including Interstellar Visitor 2I/Borisov
Hidden in plain sight, mingled among the light of other molecules
Mark
Why is nickel in a cold comet such a big deal?
Mimi
Because the textbook expectation was that it couldn't be there. Nickel vapor needs about 425 degrees Celsius, and Borisov was at roughly minus 93.
Luke
And that's the confirmed part: nickel was detected at that temperature. The mechanism is not confirmed.
Mark
What are the candidate explanations?
Mimi
Hutsemékers suggests a special surface material on the nucleus that sublimates at low temperature. Another idea is solar ultraviolet light breaking apart nickel-bearing molecules.
Luke
Both are possibilities. The reporting doesn't say which, if either, is right.
Mark
Why was it missed for two decades?
Mimi
Guzik says observers likely never imagined metals in such cold conditions, so the signatures went unidentified in the data.
Luke
That's his explanation, and he says 'probably.' It's an informed guess, not a documented fact.
Mark
And the link between Borisov and our comets?
Mimi
Guzik reads the shared nickel as a hint of similar birth conditions.
Luke
A hint, yes. One shared trace element doesn't prove a common origin, and the article offers nothing beyond that suggestion.
Der Puls
- Nickel turned up in interstellar comet 2I/Borisov while it sat near minus 93 degrees Celsius, far too cold for metal to vaporize.
- Astronomers had been misreading these metal signatures for about 20 years, mistaking them for other molecules hidden in the glow.
- Two independent teams using the Very Large Telescope in Chile published matching findings in the same Nature issue.
- The surface material releasing iron and nickel in equal amounts, unlike the usual tenfold excess of iron, puzzles researchers.
- Ultraviolet light may be breaking apart metal-bearing molecules, but the true source on the nucleus is still unidentified.
Far beyond the sun's warmth, a visitor from another star system carried a faint trace of metal into view, and in doing so unsettled an old assumption about how comets speak. Nickel and iron, which should have stayed locked in frozen rock at such cold, were found drifting in the gas around both an interstellar wanderer and some twenty native comets. The shared traces hint that these icy bodies, born in different places and times, may have been shaped by similar conditions at the dawn of their worlds. Why the metals rise into the chill, and what material hides them, remains a question the sky has not yet answered.
In January, astronomers turned the Very Large Telescope in Chile toward 2I/Borisov, the first comet ever confirmed to have come from interstellar space, and found nickel atoms in its coma. The comet was far from the sun, roughly minus 93 degrees Celsius, a temperature at which metal should not vaporize at all. The finding appeared in Nature on May 20.
Comets are usually read as frozen archives of early planetary chemistry, and their metals normally stay hidden because only a close pass to the sun heats them enough to release nickel. Borisov, discovered in 2019 and the second known interstellar visitor after 1I/'Oumuamua, seemed to break that rule.
A separate team publishing in the same issue found gaseous nickel and iron in about 20 solar system comets as well. The signals had been tangled with other molecules in the spectra for years. Observers, Piotr Guzik suggested, could not imagine metal vapor in such cold and simply left the lines unexplained.
The amounts are small, roughly a gram of metal per second against about 100 kilograms of water. Yet the balance is strange: iron usually outnumbers nickel roughly tenfold, but here they appear in nearly equal measure, suggesting a special material on the nucleus that releases them at low temperature. Whether ultraviolet light breaks down these molecules, and what the source material truly is, remain open. Still, the shared metal signature suggests that comets from different stellar nurseries may have grown under similar early conditions.
In January, astronomers pointed the Very Large Telescope in Chile at 2I/Borisov, the first interstellar comet ever identified, and found something that should not have been there: nickel atoms in its coma. The comet was far from the sun at the time, with an estimated temperature of minus 135 degrees Fahrenheit (minus 93 degrees Celsius). The finding was published on May 20 in the journal Nature.
The surprise comes from how comets are usually read. Made of dust and ice left over from planetary formation, they are studied for clues to the chemistry of early planetary systems, and scientists often infer their makeup from the comas, the clouds of gas and dust around their cores. Metals like nickel normally do not show up in those clouds, because a comet's surface is too cold to vaporize them. Nickel vapor needs roughly 800 degrees Fahrenheit (425 degrees Celsius), a temperature reached only by comets that pass near the sun or plunge into it.
2I/Borisov was discovered in 2019. Its speed and trajectory showed it came from interstellar space, making it the first known interstellar comet and the second known interstellar visitor, after the pancake-shaped rock 1I/'Oumuamua.
A separate team, publishing independently in the same May 20 issue of Nature, found gaseous nickel and iron in the cold comas of about 20 comets native to our solar system, of many different types. They also used the Very Large Telescope. The metals' light signatures had been mingled with the spectra of other molecules in the comas, hidden in plain sight.
The quantities are tiny. These comets released about 1 gram of iron and nickel per second, against roughly 220 pounds (100 kilograms) of water per second. The nickel output per second came out almost exactly the nickel content of a U.S. five-cent coin.
The ratio is odd too. Damien Hutsemékers of the University of Liège, a co-author of that study, said iron is usually about ten times more abundant than nickel, yet in these comet atmospheres the two appeared in about equal amounts. His team concluded that a special kind of material on the nucleus surface might sublimate at a fairly low temperature, releasing iron and nickel in about the same proportions.
Why had no one noticed in about 20 years of spectra? Piotr Guzik, an astronomer at Jagiellonian University in Kraków, Poland, and a co-author of the Borisov study, told Space.com that observers who saw the nickel or iron signatures probably could not imagine gaseous metals in such a cold place, and left them unidentified.
How these comets make metal vapor in the cold is still unknown. One possibility is that harsh solar ultraviolet light breaks apart nickel-containing molecules. Guzik said that even a minor constituent like nickel appearing in both Borisov and solar system comets suggests similar conditions at the time and place of their birth. The open question now is what the source material on the nucleus actually is.
Bemerkenswerte Zitate
Even such a minor constituent as nickel being present in both Borisov and solar system comets suggests similar conditions at the time and place of their birth.— Piotr Guzik, Jagiellonian University
Usually there is ten times more iron than nickel, but in those comet atmospheres we found about the same quantity for both.— Damien Hutsemékers, University of Liège
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