Telescópio Espacial James Webb identified a radiant crust between 15 and 20 meters deep on the surface of the interstellar comet 3I/Atlas. Essa layer formed after billions of years of exposure to galactic cosmic rays during the comet’s journey through Via Láctea. The object, the third confirmed interstellar comet, was observed in August 2025, while it was 445 million kilometers from Terra.
Researchers from Instituto Real Belga of Aeronomia Espacial coordinate the study, which highlights the conversion of carbon monoxide into carbon dioxide on the altered surface. 3I/Atlas, originating in another star system, reached its perihelion on October 29, 2025, 1.36 astronomical units from Sol, marking the beginning of the sublimation of its outer layer.
The comet’s speed exceeds 210 thousand kilometers per hour, in a hyperbolic orbit, a characteristic that confirms its origin from outside the Sistema Solar. Sua discovery occurred on July 1, 2025 by the ATLAS system, located in Chile, revealing a celestial body with a coma composition dominated by CO2, in a proportion eight times greater than that of water.
Formation of the cosmic crust
Galactic cosmic rays, high-energy particles from outside Sistema Solar, have struck the comet’s ices over billions of years. Esse chemical process transformed CO into CO2, creating the 15 to 20 meter thick surface crust that now characterizes it.
Simulations inspired by comet 67P, studied by the Rosetta mission, indicate that billions of years of exposure are sufficient for the formation of such a structure. The absence of heliospheric protection allowed intense and continuous interactions with these high-energy particles.
The absence of a solar shield
3I/Atlas traversed the galaxy without the protection of the heliosphere, the magnetic bubble that filters radiation for local objects. Essa specific condition exposed the comet to constant collisions with energetic particles, chemically reconfiguring its surface over vast periods.
Solar comets, on the other hand, maintain their primitive compositions thanks to this protective barrier, unlike the interstellar visitor. The crust of 3I/Atlas accumulates the effects of interactions on galactic scales, modifying only the outermost layers of the celestial body.
Discovery and orbital characteristics
Discovered in July 2025 near the constellation of Sagitário, 3I/Atlas follows a hyperbolic orbit, indicating that it will leave Sistema Solar after passing through Sol. Sua’s impressive speed of 210,000 km/h is crucial evidence that validates its extraterrestrial origin, providing valuable data for understanding interstellar objects.
Images captured by the Hubble and James Webb telescopes detail the comet’s coma and tail, with an estimated diameter of between 440 meters and 5.6 kilometers. The reddish surface and ultraviolet emissions were noticed in August 2025, offering clues about its composition and history.
The comet’s initial activity was recorded in May 2025, when it was 6.4 astronomical units from Sol. 3I/Atlas remains visible to professional telescopes, although its luminosity is progressively decreasing as it moves away from Sol.
Atypical chemical composition
James Webb’s NIRSpec instrument detected high concentrations of CO2 in the comet’s coma, suggesting it formed in icy, carbon dioxide-rich regions close to its home star. Essa abundance is an important indication of how the composition of interstellar comets can differ significantly from those formed in our own system.
The modified crust covers much of the surface, but the comet’s nucleus likely preserves primordial materials from Via Láctea, untouched by intense radiation. The bluish hue of the coma, in turn, results from the heating of carbon molecules, a phenomenon observed during its solar approach.
Spectroscopic analysis corroborates cosmic ray processing, with CO/H2O levels at 1.65 ± 0.09. Este data is fundamental to understanding the chemical evolution of celestial bodies that travel for long periods in interstellar space, without the protection of a host star.
Spectroscopic analysis of James Webb
James Webb revealed that the CO2/H2O ratio in 3I/Atlas is 7.6 ± 0.3, the highest ever recorded in comets. Essa discovery suggests a unique formation environment where carbon dioxide was much more abundant than water in the original material.
Emissions of 129 kg of CO2 per second in the nucleus reinforce the dominance of this gas in the comet’s composition. Esse intense flow of carbon dioxide contributes to the formation of coma and the rapid sublimation observed.
The presence of water ice grains smaller than 1 micrometer in the coma indicates that water, although less abundant, is also present, but in a pulverized form, mixed with CO2. Interaction with solar radiation and cosmic rays may have influenced the size and distribution of these grains.
The spectroscopic data allows scientists to reconstruct the comet’s history, from its formation in a distant star system to its journey through interstellar space and its interaction with our own Sol. Essa Detailed analysis is crucial to compare the characteristics of 3I/Atlas with other comets, both interstellar and solar.
Post-perihelion transformations
After perihelion on October 29, 2025, solar heat triggered the release of gas and dust from the irradiated crust. Essa sublimation exposes the comet’s inner layers, which may possibly be preserved and offer insights into its original composition.
Scientists are comparing pre- and post-perihelion spectra to map the comet’s original composition. The rapid increase in brightness and intense bluish color are signs of the release of volatile gases.
The coma exhibits a clear dominance of carbon dioxide, an atypical pattern in solar comets. Jatos of gas has been recorded by satellites such as SOHO and STEREO-A even at considerably cold distances.
Differentiations in relation to solar comets
Interstellar objects such as 3I/Atlas present a predominance of CO2 in the coma in relation to water, contrasting strongly with the majority of comets in our Sistema Solar. Irradiated crust dominates the surface, but deeper layers may preserve primordial galactic chemistry.
Data from August 2025 validates this distinction, emphasizing the role of radiation in external visitors to our system. Telescópios Earthlings continue to monitor the comet’s tail and luminosity rate for more information.
