New questions have emerged as the interstellar object 3I/ATLAS continues its approach, with a new study pointing to the unusual abundance of nickel and iron in its plume of gas, which its authors describe as “extremely puzzling.”
The discovery, detailed in a paper by an international team of researchers, complicates inquiries by astronomers into the unusual object’s chemistry and why its composition appears to differ significantly from past observations of interstellar comets that have made their way into our solar system.
For their research, the team relied on the UVES spectrograph on the European Very Large Telescope in Chile. Their findings came as a surprise, since comets are generally too cold to allow refractory minerals containing nickel and iron to sublimate, the process by which materials transform directly into vaporous forms when heated.
While some similar signatures were detected on 2I/Borisov, the second known interstellar object observed by astronomers, as well as in comets native to our solar system, the recent UVES spectrograph detections reveal an outstanding nickel-to-iron ratio in 3I/ATLAS, suggesting the mysterious object possesses extreme properties unlike anything previously observed.
3I/ATLAS: A Hard Rock, or Heavy Metal?
The team’s recent study is based on six intervals of observation at heliocentric distances, which range from 3.14 to 2.14 astronomical units (AU). Notably, although nickel was detected consistently across all these observational periods, iron was only detected as the object approached within 2.64 AU of the Sun.
Overall, researchers discovered that 3I/ATLAS appears to be producing nickel atoms at a high rate, displaying a nickel-to-iron ratio that significantly exceeds both solar system comets, as well as 3I/ATLAS’s predecessor, the interstellar comet Borisov.
“At the distances at which comets are observed, the temperature is far too low to vaporize silicate, sulfide, and metallic grains that contain nickel and iron atoms,” the study’s authors write. “Therefore, the presence of nickel and iron atoms in cometary coma is extremely puzzling.
Martian Arrival Imminent
Later this week, additional insights about the object may be possible as it passes Mars, where NASA’s Mars Reconnaissance Orbiter (MRO) and the ESA’s Mars Express and the ExoMars Trace Gas Orbiter missions are poised to make the nearest observations yet of 3I/ATLAS.
Passing within just 29 million kilometers of Mars, observations made possible with the MRO’s HiRISE camera and similar visual systems on the ESA spacecraft in Mars orbit could reveal unprecedented new details about 3I/ATLAS, including its size, structure, and composition.
To date, the highest-resolution images currently obtained were captured in late July by the Hubble Space Telescope, at which time 3I/ATLAS was approximately 570 million kilometers from Earth. By contrast, the MRO’s HiRISE camera will capture the object’s passing at 29 million kilometers at a resolution close to 30 kilometers per pixel, which may help to facilitate the most accurate estimates yet about the diameter of the comet’s nucleus.
A Massive Mystery
Recent studies of 3I/ATLAS have also revealed it to be “anomalously” massive, with the latest estimates placing it at close to five kilometers in diameter (slightly more than three miles). That would make the latest interstellar visitor comparable in size to Manhattan Island, which also means that it dwarfs its predecessors 2I/Borisov and 1I/’Oumuamua.
At an estimated mass of 33 billion tons, the apparent size discrepancy between 3I/ATLAS and its cometary predecessors highlights the diversity of interstellar visitors, and researchers say upcoming images may either resolve—or perhaps only deepen—this cosmic mystery.
Some scientists, like theoretical physicist Avi Loeb, say that the unusual abundances of nickel and iron in the object’s cometary coma could point to even more exotic possibilities as far as 3I/ATLAS is concerned.
“The detection of nickel without iron is only known to exist in industrially produced alloys, made through the artificial carbonyl chemical pathway,” Loeb recently told The Debrief. In an article on his Medium page, Loeb further noted that “Altogether, 3I/ATLAS shows a high production rate of nickel atoms as well as a high nickel to iron ratio, making it exceptional when compared to solar system comets and 2I/Borisov.”
Anomalies and Conclusions
Similarly, in their recent paper, the authors conclude that 3I/ATLAS “exhibits extreme properties in the early phases of its activity with regard to the production rates and abundance ratios” of nickel and iron.
They add that the object “remains exceptional compared to solar system comets due to its high, asymmetrical production rate of [nickel and iron].”
“Compared to comets in the solar system, comet 3I is either metal-rich,” they write, “or, more likely, it has enhanced metal production due to chemical anomalies.”
The new paper, “Extreme NiI/FeI abundance ratio in the coma of the interstellar comet 3I/ATLAS,” appeared on the preprint server arXiv.org and can be read online.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.