Polarization anomaly of 3I/ATLAS reveals origin conditions absent in our Solar System
Interstellar comet 3I/ATLAS is revealing dust unlike any known in the Solar System. Its extreme polarization pattern, a deep negative dip and delayed inversion, suggests dust grains with properties never seen before, pointing to an origin in very different planetary conditions.
A deep image of interstellar Comet 3I/ATLAS captured by the Gemini Multi-Object Spectrograph (GMOS) on Gemini South at Cerro Pachón in Chile on August 26, 2025. Credit: International Gemini Observatory/NOIRLab/NSF/AURA/Shadow the Scientist
Interstellar objects passing through the Solar System are extraordinarily rare. In modern astronomical history, only two had been identified before this decade: 1I/‘Oumuamua in 2017 and 2I/Borisov in 2019. Both stirred global attention, but for very different reasons. While ‘Oumuamua showed no cometary activity yet accelerated in ways not fully explained, Borisov behaved much like a familiar comet from our own planetary system.
Discovered in July 2025 by the Asteroid Terrestrial-impact Last Alert System in Hawaii, 3I/ATLAS is now the third confirmed interstellar visitor. Its designation follows the International Astronomical Union’s naming convention: “3” for the third interstellar object, “I” for interstellar, and “ATLAS” for the survey that found it.
From its earliest observations, however, it was clear that 3I/ATLAS would not simply repeat the story of ‘Oumuamua or Borisov. Instead, it displayed traits that set it apart, from its unusual chemistry to an unprecedented polarization anomaly that revealed dust unlike any seen in the Solar System.
A visitor unlike any other
On its path through the inner Solar System, 3I/ATLAS has already set itself apart from every comet and asteroid ever studied and the difference is not only in its trajectory or chemistry, but also in the way it interacts with sunlight.
When light from the Sun strikes dust particles in a comet’s surrounding cloud, the scattered light usually becomes polarized — its waves aligned in specific directions. This effect, called optical polarization, has long been used by astronomers to infer the size, composition, and surface structure of cometary dust.
In Solar System comets, the pattern is well established: a shallow dip in negative polarization at small angles, followed by a flip into positive values as the viewing angle increases.
However, observations of 3I/ATLAS show an exceptionally deep negative polarization of –2.7 % when viewed at a phase angle of about 7°. Even more striking, the flip to positive polarization, known as the inversion angle, occurs only around 17°. No comet, asteroid, or small body in the Solar System has ever exhibited such a combination.
This anomaly means that the dust particles around 3I/ATLAS must be fundamentally different from the grains shed by familiar comets. The depth of the negative polarization suggests unusual optical properties… perhaps grains with very fine textures, layered structures, or compositions shaped in an environment far removed from our own. The high inversion angle further points to scattering behavior not explained by standard cometary dust models.
For astronomers, polarization is not just a number. It is a diagnostic tool that acts like a fingerprint of surface and dust properties. The fact that 3I/ATLAS shows a fingerprint unlike anything in the Solar System strongly implies that it carries material forged under different physical and chemical conditions, in the disk of another star.
Chemical fingerprint from another world
Observations from the James Webb Space Telescope, Hubble, and NASA’s new SPHEREx mission revealed that carbon dioxide dominates the coma, making up 87 % of detected gases, while water and carbon monoxide play only minor roles.
Astronomers also found nickel vapor without iron, a surprising decoupling, since in Solar System bodies the two typically appear together. In addition, cyanide molecules were detected, producing a faint glow in the coma. These findings point to chemistry shaped under conditions far different from those that shaped our own comets.
In particular, the dominance of carbon dioxide suggests that 3I/ATLAS formed in a region of its home system that was extremely cold, where CO2 ice could be preserved. The absence of iron may reflect unusual primordial chemistry or selective depletion.
The chemistry behind its green glow
Until late August 2025, 3I/ATLAS appeared reddish in telescopic images, a color consistent with dust scattering off a darkened surface. Observations with the Very Large Telescope detected a sudden rise in cyanide (CN) production at that time, accompanied by the release of nickel vapor.
By September 7, deep imaging showed the coma had shifted to a distinct green hue, most likely caused by the rapid increase of cyanide molecules fluorescing under solar radiation.
The outgassing scaled sharply with heliocentric distance, roughly to the ninth power, indicating that even small changes in proximity to the Sun triggered a disproportionate increase in volatile release.
The transition from a dust-dominated, reddish coma to one rich in fluorescing gases is unusual. Most comets maintain relatively stable optical characteristics, whereas 3I/ATLAS is undergoing a notable transformation that sets it apart from known Solar System comets.
Trajectory category of its own
The orbit of 3I/ATLAS is retrograde, moving opposite to the direction of planetary motion, yet it is inclined by only about 5° relative to the Solar System’s ecliptic plane.
This combination is uncommon as most retrograde comets have steeper inclinations, often well outside the planetary plane.
Harvard astrophysicist Avi Loeb has calculated that the chance of an interstellar comet arriving with this particular alignment is roughly one in 500, and has suggested it could point to a non-random or even artificial origin. Most other researchers, however, contend that such geometry can arise naturally, if rarely, through gravitational interactions in the comet’s original star system before it was ejected into interstellar space.
The orbital elements therefore place 3I/ATLAS in a category of its own, with a path that resembles Solar System comets in inclination but defies them in direction of motion.
On October 3, 2025, 3I/ATLAS will make its closest approach to Mars at about 30 million km (19 million miles). This geometry gives orbiters around Mars an excellent opportunity to capture images. The High Resolution Imaging Science Experiment (HiRISE) camera aboard NASA’s Mars Reconnaissance Orbiter is expected to achieve about 30 km (19 miles) per pixel.
Although too coarse to reveal fine details, the images could finally constrain the size of the comet’s nucleus. Current Hubble-based estimates suggest a diameter between 0.3 and 5.6 km (0.2–3.5 miles) but Loeb has proposed a much larger possible size of up to 46 km (29 miles).
As 3I/ATLAS approaches Mars, coordinated observations from orbiters and Earth-based telescopes will refine its size, structure, and chemical makeup. The data will help answer whether its anomalies can be explained by known astrophysical processes or whether they demand new thinking.
References
1 Extreme negative polarization of the interstellar comet 3I/ATLAS – Hadamcik et al. – arXiv – September 2025 – DOI: https://doi.org/10.48550/arXiv.2509.05181 – OPEN ACCESS
2 Extreme Negative Polarization of 3I/ATLAS – Avi Loeb/Medium – September 10, 2025
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