Interstellar Comet 3I/ATLAS Found To Have "Extreme Abundance Ratio" Of Iron And Nickel

A new study on interstellar comet 3I/ATLAS has found unusual and as-yet-unexplained metals in the object's coma.

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On July 1, 2025, astronomers spotted an object moving through the Solar System at nearly twice the velocity of previous interstellar visitors ‘Oumuamua and Comet Borisov. The object was confirmed to be an interstellar comet with its own dusty coma, and suspected to be far larger than the previous two, with an estimated nucleus (the rocky part of the comet, excluding its coma) of around 5.6 kilometers (3.5 miles), and a possible "anomalous" mass of over 33 billion tons.

The astronomical community continues to track the object, known to be a comet due to outgassing, as it approaches closer to the Sun on its way through the Solar System. Unfortunately, after potentially being hit by a coronal mass ejection, it has now vanished from our view behind the Sun and won't be back in sight until later this year.

On the upside, we still have plenty of data to pore over. In a new paper, which has not yet been peer reviewed, an international team of researchers analyzed the spectra of the comet, used to determine the elements present on the object and in its coma, at different points of its journey. The team found that compared to Solar System objects and fellow interstellar object 2I/Borisov, 31/ATLAS has an unusual and "extreme" abundance ratio of iron/nickel.

"Emission lines of FeI and NiI are commonly found in the coma of Solar System comets, even at large heliocentric distances. These atoms are most likely released from the surface of the comet’s nucleus or from a short-lived parent," the team explains in their paper. "The presence of these lines in cometary spectra is unexpected because the surface blackbody equilibrium temperature is too low to allow the sublimation of refractory minerals containing these metals."

"Comet 3I exhibits a high production rate of NiI atoms as well as a high NiI/FeI ratio, making it exceptional when compared to solar system comets and 2I/Borisov," the team added. "Additionally, we found that the NiI/FeI ratio decreases rapidly with decreasing heliocentric distance, suggesting that comet 3I could soon become indistinguishable from Solar System comets in this respect."

The main mystery here is how these metals ended up in the object's coma. 

“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 team writes. “Therefore, the presence of nickel and iron atoms in cometary coma is extremely puzzling.”

"The interstellar comet 2I/Borisov showed nickel and iron with an abundance ratio that is similar to that observed in Solar System comets," Harvard astronomer Avi Loeb, who was not involved in the research, explained in a blog post. "On average, the typical cometary ratio is an order of magnitude higher than the solar abundance ratio of nickel to iron."

While this is surprising, this is our first look at an interstellar comet, and maybe it would be more surprising if we found no surprises at all. The object is now behind the Sun from our perspective, but we will soon have more observations to look at thanks to our robotic explorers around Mars.

The study is posted to the pre-print server arXiv.