Active cryovolcanoes discovered on interstellar comet 3I/ATLAS

In late October, as the interstellar comet 3I/ATLAS drew closest to the Sun, something unexpected happened on its frozen surface. Researchers watching through telescopes saw jets of gas and dust erupting from the comet's body—not the gentle sublimation typical of icy visitors from deep space, but active cryovolcanism, the kind of violent ice-volcano behavior scientists usually associate with moons orbiting distant planets. The discovery, detailed in a study uploaded to the scientific repository arXiv on November 24, has rewritten what astronomers thought they knew about this rare visitor from another star system.

The comet 3I/ATLAS is only the third interstellar object ever recorded. It was first spotted in July, and its arrival sparked wild speculation—some wondered aloud if it might be an alien spacecraft. But astronomers quickly settled on a simpler explanation: it was simply a comet, born in some unknown planetary system orbiting a distant star, that had been flung into the void and eventually wandered into our solar neighborhood. What made it remarkable was not its origin story but what it revealed about itself as it approached our Sun.

Josep Trigo-Rodríguez, the lead researcher at Spain's Institute of Space Sciences, and his team used the Joan Oró Telescope at the Montsec Observatory in Catalonia to track the comet as it neared perihelion—its closest approach to the Sun—on October 29. As the comet warmed, its surface ice began to sublimate, a process the team could observe and measure. But the intensity of this sublimation surprised them. When the comet reached about 235 million miles from the Sun, it suddenly grew much brighter, and high-resolution images revealed something striking: distinct jets of material shooting outward from specific points on the surface. These were not random outgassing events. They were cryovolcanic eruptions.

The researchers believe they have identified the mechanism driving these eruptions. Deep inside the comet, ancient materials have been locked away for billions of years. When solar heat penetrated the surface, solid carbon dioxide began to sublime, allowing oxidizing liquids to seep into the comet's core. There, these liquids encountered grains of iron, nickel, and sulfur compounds—and reacted violently with them. The result was the eruptions the team observed. To test this theory, Trigo-Rodríguez's group compared the comet's spectral signature to that of carbonaceous chondrites, primitive meteorites collected by NASA in Antarctica. One of these meteorites contained material from a transneptunian object—a body orbiting beyond Neptune in our own solar system. The match was striking. Despite coming from another star system entirely, 3I/ATLAS appeared to share a fundamental chemical kinship with the icy bodies at the edge of our solar system.

This similarity raises profound questions about planetary formation across the galaxy. If a comet born around a distant star resembles objects formed in our own solar system, it suggests that the processes creating planets and their icy remnants may be universal. The comet's exact size remains uncertain—the Hubble Space Telescope suggests it could be anywhere from 440 meters to 5.6 kilometers wide—but if it is roughly one kilometer across with the rocky composition researchers suspect, its mass would exceed 660 million metric tons. Yet despite these similarities to our solar system's objects, 3I/ATLAS unmistakably belongs elsewhere. It is traveling at 137,000 miles per hour, far too fast for the Sun's gravity to capture it. Its hyperbolic trajectory means it will eventually leave our solar system and return to interstellar space.

Trigo-Rodríguez emphasizes that studying such visitors matters on two fronts. There is the practical concern: objects moving at such speeds could pose a collision risk to Earth. But there is also the scientific treasure they represent. He describes them as "extraordinary objects" and "space capsules containing valuable information about the chemistry occurring elsewhere in our galaxy." The comet is believed to be billions of years older than our solar system itself, and it has spent that entire span exposed to cosmic radiation, making its true origin nearly impossible to trace. Yet the window for studying it remains open. As 3I/ATLAS recedes into the darkness, the observations already gathered offer a rare glimpse into how planets form and evolve around other stars, and what kinds of ancient, violent chemistry might be unfolding in the depths of distant worlds.