Interstellar comet 3I/Atlas displays 20-meter crust formed by cosmic rays, Webb points out
New observations by Telescópio Espacial James Webb have revealed unprecedented details about the interstellar comet 3I/Atlas. The data indicate the presence of an irradiated crust between 15 and 20 meters deep on its surface, a feature attributed to its long exposure to galactic cosmic rays during its billion-year journey through deep space, long before it entered our Sistema Solar.
The analysis, coordinated by researchers from Instituto Real Belga of Aeronomia Espacial, was conducted in August 2025, when the comet was approximately 445 million kilometers from Terra. The study details how this outer layer was chemically altered, converting carbon monoxide (CO) into carbon dioxide (CO2), a process that explains the atypical composition observed in his coma.
3I/Atlas is the third interstellar object confirmed to visit our system, following a hyperbolic trajectory that will return it to deep space. Sua speed, exceeding 210 thousand kilometers per hour, is unequivocal proof of its extrasolar origin, offering scientists a rare opportunity to study material from another star system.
The formation of the outer layer over eons
The thick crust of 3I/Atlas did not form overnight. Durante its journey through Via Láctea, the comet traveled without the protection of a heliosphere, the magnetic bubble that our Sol creates and that deflects much of the cosmic radiation. Exposto directly to high-energy particles known as galactic cosmic rays, its surface ice has suffered continuous bombardment for billions of years. Simulações based on data from the Rosetta mission, which studied comet 67P, suggest that one billion years of exposure would be enough to create a layer with the observed characteristics. Esse chemical process turned frozen carbon monoxide into carbon dioxide, resulting in the 15 to 20 meter crust that James Webb was able to identify.
Changes observed in perihelion
As it approached the closest point to Sol, perihelion, on October 29, 2025, 3I/Atlas began to suffer the effects of solar heat. The radiation heated its surface, causing the sublimation of the irradiated crust and releasing large amounts of gas and dust. Esse phenomenon created the coma, the glowing atmosphere around the comet’s nucleus.
This heating process is crucial for scientists, as removing the outer layer exposes the inner material, which may be in a more pristine, preserved state. Comparing the spectra before and after perihelion makes it possible to map the original composition of the object, revealing clues about the environment where it formed.
Learn more: James Webb analysis identifies crust formed by cosmic rays in interstellar visitor 3I/Atlas
The comet’s brightness increased rapidly during this phase, displaying an intense blue color, a characteristic sign of the release of volatile gases, especially heated carbon molecules. Satélites like SOHO and STEREO-A recorded jets of gas being expelled from the nucleus even at considerable distances from Sol, indicating intense activity.
Spectra reveal unique composition
James Webb’s NIRSpec instrument was instrumental in unraveling the chemical composition of the comet’s coma. Spectroscopic analysis detected exceptionally high concentrations of carbon dioxide (CO2).
The ratio of CO2 to water (H2O) was measured at 7.6, the highest value ever recorded on a comet. Isso suggests that 3I/Atlas formed in a very cold, carbon dioxide-rich region of its home star system.
More on this story: Interstellar comet 3I/Atlas reveals 20-meter crust formed by cosmic rays, points out James Webb
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The core’s emissions were estimated at 129 kilograms of CO2 per second, an impressive volume. The coma also contained water ice grains smaller than 1 micrometer, providing more data about its structure.
Although the crust has been modified, the comet’s nucleus is believed to retain primitive materials from Via Láctea, making it a galactic time capsule.
Hyperbolic trajectory and dimensions
Discovered on July 1, 2025 by the ATLAS astronomical survey system, at Chile, 3I/Atlas was located near the constellation of Sagitário. Sua hyperbolic orbit confirms that it is not gravitationally bound to our Sol and that, after its passage, it will continue its journey through interstellar space, never to return. The speed of more than 210,000 km/h is one of the main indicators of its external origin, being much higher than the escape velocity of objects from our own system.
On the same topic: James Webb reveals 20-meter crust on interstellar comet 3I/Atlas shaped by cosmic rays
High-resolution images captured by the Hubble and James Webb telescopes allowed details of the comet’s coma and tail, as well as providing an estimate of its size. The diameter of the core is estimated to be between 440 meters and 5.6 kilometers. Observations from August 2025 showed a reddish surface and emissions in the ultraviolet spectrum, with the first sublimation activity being recorded in May 2025, when it was still at a distance of 6.4 astronomical units from Sol.
Chemical differentiations in relation to local comets
The composition of 3I/Atlas clearly distinguishes it from native Sistema Solar comets. In our comets, water is generally the dominant volatile component in the coma, while in the interstellar visitor, carbon dioxide overwhelmingly prevails.
This fundamental difference is explained by their irradiation history. Comets in our system spend most of their lives protected within the heliosphere, while 3I/Atlas faced the hostile environment of the interstellar medium for a much longer time, chemically altering its surface.
A visitor older than Sol
The estimated age of 3I/Atlas, which could be as high as 7 billion years, makes it potentially older than our own Sistema Solar, which is about 4.6 billion years old. Isso means the comet is a relic of a star system that may have formed and evolved very differently from our own, offering a direct window into primordial galactic chemistry.
More on this story: Webb Telescope reveals 20-meter crust on interstellar comet 3I/Atlas formed by cosmic rays
