Analysis of the second confirmed interstellar comet to visit our solar system suggests that the alien body could be covered in erupting icy, volcano-like structures called cryovolcanoes. Researchers also discovered that the comet has a metal-rich interior, which could challenge our understanding of how comets formed in our own planetary system.
The scientists tracked Comet 3I/ATLAS from July to November 2025 as it hurtled toward our sun. It presented a rare opportunity to study an object formed around another star in interstellar space. What makes it so valuable is that it is pristine, having never passed close enough to a star to be heated, melted, or otherwise altered by radiation. That means it is almost the same as it was when it formed billions of years ago in its home system.
Massive ice surge
In their study posted on the arXiv preprint server, the team describes how photometric observations showed a sharp and lasting surge in the comet’s brightness when it reached about 2.5 times Earth’s distance from the sun (2.5 astronomical units—au). This wasn’t a sudden burst like an explosion, but a sustained increase in brightness, which the scientists interpreted as the activation of the water-ice layer across the comet’s entire surface.
The massive, sustained activity was most likely due to cryovolcanism (ice-based volcanic activity). Unlike comets in our solar system, Comet 3I/ATLAS lacks a protective dust mantle, making this global activation entirely possible.
When the researchers examined light reflected from the comet’s surface and compared it to the spectrum (colors of light reflected from an object which reveal its chemical makeup) of meteorite samples here on Earth, they found it matched a rare type of meteorite called a carbonaceous chondrite (CR). These are ancient, primitive meteorites rich in metals like iron and nickel. Because these samples have a high metal content, the scientists believe that Comet 3I would have the same composition.
According to the study authors, this metal-rich composition helps to explain the comet’s powerful volcanic activity. They suggest that as the surface warmed and ice turned into water, the liquid began to corrode fine metal grains inside the comet. This chemical process releases additional energy and gases, such as carbon dioxide, which help sustain the cryovolcanism.
Rewriting the rule of comet formation?
This unusual combination of metal-fueled activity and ice volcanoes differs significantly from our current understanding of comet formation. Standard models suggest they are composed of ice, rock, and low concentrations of metals, and fueled by the sun heating surface ice. This new research demonstrates a much greater diversity in how they were formed, as the researchers note: “Interstellar visitors like 3I/ATLAS continue to challenge and refine our understanding of planetary-system formation and the chemical evolution of small bodies.” https://phys.org/news/2025-12-interstellar-icy-volcanoes-rewrite-comets.html
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