Interstellar Comet 3I/ATLAS May Be Erupting 'Ice Volcanoes,' New Images Suggest

New observations suggest the interstellar comet 3I/ATLAS may be producing jets from erupting cryovolcanoes — often described as "ice volcanoes" — as it warmed while passing through the inner solar system. The results appear in a study posted Nov. 24 on the preprint server arXiv; the paper has not yet completed peer review.

What the team observed

Researchers led by Josep Trigo-Rodríguez of the Institute of Space Sciences (CSIC/IEEC) in Spain used the Joan Oró Telescope at Montsec Observatory and complementary regional observations to monitor the comet as it approached perihelion on Oct. 29. The team reports a marked increase in activity when the comet came within roughly 235,000,000 miles (378 million kilometers) of the Sun, and they captured high-resolution images showing narrow gas-and-dust jets streaming from the nucleus.

Interpreting the jets: cryovolcanism as a hypothesis

The researchers interpret these jets as signs of cryovolcanic activity. On icy bodies, cryovolcanism can occur when internal heat or chemical reactions mobilize volatiles, producing eruptions that vent vapor and entrained dust. For 3I/ATLAS, the proposed mechanism begins with solar heating that causes near-surface solid carbon dioxide (dry ice) to sublimate. That sublimation could allow an oxidizing liquid to penetrate and react with reactive metallic grains such as nickel and iron sulfides, driving pressurized jets.

"We were all surprised," said study lead author Josep Trigo-Rodríguez. "Although the comet formed around another star, the mixture of materials forming the surface of the body has resemblance with trans-Neptunian objects."

Composition clues from meteorites

To test their composition hypothesis, the team compared the comet's spectral signatures with those of primitive carbonaceous chondrite meteorites recovered from Antarctica. One Antarctic sample examined may be a fragment of a trans-Neptunian body; the spectral similarities led the researchers to suggest that 3I/ATLAS resembles remnants from the earliest days of our solar system and could be relatively rich in native metals. These comparisons are suggestive but not definitive.

Size, trajectory and uncertainties

Hubble Space Telescope observations constrain the nucleus size to roughly 1,400 feet (440 meters) to 3.5 miles (5.6 kilometers) in diameter. The team estimates that if the nucleus is about 0.6 miles (1 kilometer) across and metal-rich, its mass could exceed roughly 660 million tons (600 million metric tons). Importantly, 3I/ATLAS is unambiguously extrasolar: its hyperbolic trajectory and an observed incoming speed near 137,000 miles per hour (221,000 kilometers per hour) show it is not bound to the Sun.

The study's cryovolcanism explanation is plausible but preliminary. The paper is a preprint and the proposed chemical reactions and mass estimates depend on assumptions about composition and interior structure. Long-term exposure to interstellar radiation could also have altered the comet's surface, complicating attempts to trace its origin.

Why this matters

As only the third confirmed interstellar object observed in our neighborhood, 3I/ATLAS provides a rare window into material formed around another star. Studying such objects can reveal chemistry and processes from distant planetary systems and improve our understanding of how icy bodies behave when exposed to a new star. Continued observations and spectroscopic study — and eventual peer review — will be essential to confirm whether cryovolcanism is driving the comet's activity.