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Interstellar visitor 3I/ATLAS has been constantly changing as it makes its way through our solar system. That's to be expected, as, for the first time in potentially billions of years, it's getting close to the energy put out by a star.

Scientists have been keeping a close watch on those changes, both to ensure there's nothing unexplainable by our current understanding, but also to compare 3I/ATLAS to both previous interstellar visitors as well as comets in our own solar system.

A recent from European researchers posted to the arXiv preprint server describes how the changes in a particular material ratio in 3I/ATLAS' coma fit with our current understanding of cometary geology.

That ratio is the nickel to iron (Ni/Fe) abundance ratio. It has been measured for two decades, including on 20 in-system comets as well as 2I/Borisov, the last known interstellar visitor our solar system had. However, the fact that either of those materials is even present at all in the coma has puzzled scientists, as the temperatures on their surfaces typically aren't enough to sublimate the silicates or sulfides that are thought to hold these metals on cometary surfaces.

The Ni/Fe ratio from studying 2I/Borisov was similar to those of in-system comets. But that ratio is also about 10 times higher than the Ni/Fe of the sun itself. So the fact that one of our interstellar visitors and our in-system comets had the same high ratio pointed to some common process in cometary formation that was agnostic to the materials available in the star it formed around.

3I/ATLAS, though, was different, as it has been in so many ways, from both in-system comets and 2I/Borisov. The researchers used the European Southern Observatory's (ESO's) Very Large Telescope (VLT) in Chile to watch the comet between August and September, while it was traveling from 3.14 AU to 2.14 AU and started to get increasingly warmed by the sun.

They noted that the spectral absorption line for nickel captured by the VLT's UV-Visual Echelle Spectrograph (UVES) was present the entire time. However, the absorption lines for iron only appeared once 3I/ATLAS got within 2.64 AU of the sun. This discrepancy caused a Ni/Fe ratio much higher than those seen in previous comets, but it was actively changing dramatically as 3I/ATLAS got closer to the sun. No other study had shown that much of a dramatic change.

It also might explain why there is nickel and iron in the comet's coma in the first place. Researchers had previously developed a theory that the metals they were seeing were tied to carbonyl groups, forming high volatile organometallic compounds. In that scenario, the nickel would be tied up as nickel tetracarbonyl (Ni(CO)₄), and iron would be tied up as iron pentacarbonyl (Fe(CO)₅). Both of those organometallic materials have sublimation points low enough to do so when still far from the sun.

Importantly for this data set, nickel tetracarbonyl has a lower melting point than iron pentacarbonyl, which would explain why there was no iron spectra early in the observational period, but there was later when the comet grew closer to the sun. The temperature gradient over that journey must have crossed the threshold where iron pentacarbonyl started sublimating, causing the dramatic drop in the Ni/Fe ratio seen in the data. The researchers also found that the amount of nickel production in the coma closely matches the theoretical sublimation rate of nickel tetracarbonyl.

The researchers put forth one other important data point about 3I/ATLAS—it can be classified as a "C₂-depleted" comet. That means the ratio of diatomic carbon (C₂) compared to the cyanogen radical (CN) in its coma is very small. This classification points to 3I/ATLAS's "primordialness," as it was given that ratio billions of years ago when it was formed, and hints that it was formed in a different part of its home system protoplanetary disk than other, more common comets.

More data is coming out about our third known interstellar visitor on a daily basis. That would likely include more studies of its Ni/Fe ratio as it gets even closer to the sun, and before it moves out of view for a few months. Expect more papers from as many telescopes and research groups as can find observational time to watch this once in a lifetime cometary visitor.