3I/ATLAS: An interstellar comet hiding its secrets beneath an irradiated crust

Comets within our Solar System are typically composed of a mixture of dust and frozen gases. In other words, they are dusty ice bodies made of material from the early Solar System. As they approach the Sun, these ices sublimate and release gases and dust, producing the characteristic coma and tails that can be studied through in situ measurements (e.g., the Rosetta mission) or by remote observations with space telescopes.

3I/ATLAS is an interstellar comet originating from another stellar system. New research led by the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) shows that 3I/ATLAS does not directly expose the material from which it formed. Instead, the pristine ice of the interstellar comet 3I/ATLAS lies beneath an outer layer of primordial material progressively altered by cosmic rays over billions of years.

3I/ATLAS, a rare visitor from beyond the Solar System

Discovered in July 2025, 3I/ATLAS is the third confirmed interstellar object ever detected, after 1I/Oumuamua and 2I/Borisov. With an estimated diameter of a few kilometres and a composition dominated by ices and dust, it is essentially comet-like. Its hyperbolic orbit confirms that it comes from another stellar system beyond the Solar System. 3I/ATLAS is believed to have formed several billion years ago, based on the estimated interstellar travel time, to which an unknown residence time around its parent star must be added. Over this timescale, 3I/ATLAS has been continuously exposed to galactic cosmic rays, the energetic particles originating from supernovae and other high-energy astrophysical processes, that permeate galaxies and bombard all bodies in space.

Observations by the James Webb Space Telescope reveal exceptionally high CO₂/H₂O ratios, higher than for any comet observed to date, along with an elevated CO abundance and a distinctly red colour. These signatures match expectations for low temperature ices altered by energetic particles. Indeed, laboratory experiments have demonstrated that H₂O and CO ice mixtures irradiated by energetic particles produce CO₂ and a reddish crust.

A frozen time capsule hidden under a crust

To understand how cosmic rays have modified the nucleus of 3I/ATLAS, BIRA-IASB researchers collaborated with Guillaume Gronoff (NASA Langley Research Center) to model the energy deposition by galactic cosmic rays and the resulting chemical and physical evolution of the nucleus. This model shows that the radiation-processed layer likely extends ~15-20 meters into the nucleus. Within this layer, the ice composition is significantly altered, with strong CO₂ enrichment while some CO remains. The physical structure of the ice also evolves, becoming progressively more compact under sustained cosmic-ray irradiation. The BIRA-IASB model relies on results of laboratory experiments using simplified ice mixtures, which may not fully capture the of cosmic ray induced chemistry that involves extremely slow reaction pathways. Moreover, the composition of the original ices in ATLAS is not known. Despite these uncertainties, the existence of an irradiated crust is highly probable.

The nucleus of 3I/ATLAS is thus encased within an irradiated crust that began to sublimate as 3I/ATLAS approached the Sun. Solar heating and associated outgassing erode the nucleus and can potentially expose deeper, less processed material. Most erosion occurs near perihelion, when a comet reaches its minimum distance from the Sun, which 3I/ATLAS reached on 30 October.

Depending on the erosion rate, unaltered pristine ices might be exposed afterwards. It is therefore essential to compare pre- and post-perihelion observations to identify compositional and colour changes that could indicate the exposure of less processed material. However, solar heating also induces compositional evolution through selective sublimation and re-condensation. Thus, even if erosion eventually exposes deeper layers, directly sampling primordial material remains a major challenge.

Romain Maggiolo (BIRA-IASB researcher)

Why it matters: opening a window on another stellar system and preparing Comet Interceptor

ATLAS is an ancient object hiding its secrets beneath an irradiated crust, yet it remains an exceptional scientific target. Interstellar comets are the only known samples of exoplanetary system material that approach Earth closely enough to be observed in detail. They carry both the imprint of their birth environment and the effects of billions of years spent in space. Decoding this dual history is key to understanding their history and origin.

This research also supports ESA’s Comet Interceptor mission (launch in 2029), in which BIRA-IASB is involved. Comet Interceptor will perform a fly-by of a solar system comet entering the inner solar system for the first time. Such dynamically new comets originate in the Oort cloud, and are equally likely to have a processed crust, shaped by continuous cosmic ray bombardment since the solar system formed 4.5 billion years ago. Understanding cosmic ray processing is therefore essential for interpreting the future measurements of Comet Interceptor.

Reference

The paper has been submitted to the Astrophysical Journal and is available on arXiv

Maggiolo, R., F. Dhooghe, G. Gronoff, J. de Keyser and G. Cessateur, (2025), Interstellar Comet 3I/ATLAS: Evidence for Galactic Cosmic Ray Processing, arXiv, https://doi.org/10.48550/arXiv.2510.26308