Extreme Negative Polarisation of New Interstellar Comet 3I/ATLAS (2509.05181v1)

Abstract: We present the first polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1), obtained pre-perihelion with FORS2/VLT, ALFOSC/NOT, and FoReRo2/RCC, over a phase angle range of 7.7-22.4{\deg}. This marks the second ever polarimetric study of an interstellar object, the first distinguishing 2I/Borisov from most Solar System comets by its higher positive polarisation. Our polarimetric measurements as a function of phase angle reveal that 3I is characterised by an deep and narrow negative polarisation branch, reaching a minimum value of -2.7% at phase angle 7{\deg}, and an inversion angle of 17{\deg} -- a combination unprecedented among asteroids and comets, including 2I/Borisov. At very small phase angles, the extrapolated slope of the polarisation phase curve is consistent with that of certain small trans-Neptunian objects and Centaur Pholus, consistent with independent spectroscopic evidence for a red, possibly water-ice-bearing object. Imaging confirms a diffuse coma present from our earliest observations, though no strong polarimetric features are spatial resolved. These findings may demonstrate that 3I represents a distinct type of comet, expanding the diversity of known interstellar bodies.

• The paper presents unique polarimetric measurements of 3I/ATLAS, identifying an unprecedented deep and narrow negative polarisation branch (Pmin = -2.7% at 7° phase angle).
• It employs multi-instrument observations and rigorous data reduction methods to obtain robust reduced Stokes parameters and mitigate stellar contamination.
• The findings imply a dust composition of large, porous, ice-rich aggregates, challenging existing cometary taxonomies and suggesting an extrasolar origin.

Extreme Negative Polarisation of Interstellar Comet 3I/ATLAS

Introduction and Context

The discovery of 3I/ATLAS (C/2025 N1) as the third known interstellar object (ISO) traversing the Solar System provides a unique opportunity to probe the diversity of small bodies originating from extrasolar environments. Unlike its predecessors—1I/'Oumuamua, which exhibited asteroid-like characteristics with enigmatic non-gravitational accelerations and no detectable activity, and 2I/Borisov, an active comet with polarimetric properties akin to the exceptional Solar System comet Hale-Bopp—3I/ATLAS displays a distinct set of physical and polarimetric properties. The present paper delivers the first polarimetric characterization of 3I/ATLAS, focusing on its negative polarisation branch at small phase angles, and situates its properties within the broader context of Solar System comets, asteroids, and trans-Neptunian objects (TNOs).

Observational Campaign and Methodology

A comprehensive pre-perihelion polarimetric campaign was conducted using FORS2/VLT, ALFOSC/NOT, and FoReRo2/RCC, covering a phase angle range of $7.7^\circ$ to $22.4^\circ$ and heliocentric distances from 3.9 to 2.6 au. Observations were primarily in the R-band, with supplementary V-band data. The polarimetric analysis employed a $\sim2000$ km aperture to mitigate contamination from background stars, particularly critical during early epochs when 3I was projected against a dense stellar field. The data reduction pipeline followed established beam-swapping techniques and instrumental calibrations, ensuring robust measurement of the reduced Stokes parameters and transformation to the scattering plane.

Polarimetric Results: Anomalous Negative Branch

The polarimetric phase curve of 3I/ATLAS reveals a deep and narrow negative polarisation branch, with a minimum of $P_{\rm min} = -2.7\%$ at $\alpha = 7^\circ$ and an inversion angle $\alpha_0 = 17^\circ$. This combination is unprecedented among both Solar System comets and asteroids, including the previously exceptional cases of 2I/Borisov and Hale-Bopp. The polarimetric slope at the inversion angle, $h = 0.42\%$ deg$^{-1}$, is at the upper extreme of the cometary range and comparable to 2I/Borisov, but the depth and narrowness of the negative branch are unique.

Figure 1: Polarisation phase curve of 3I/ATLAS compared to Solar System comets, F-type asteroids, and TNOs, highlighting the extreme negative branch and small inversion angle of 3I.

The phase curve fit using the linear-exponential model of Muinonen et al. (2009) demonstrates that 3I/ATLAS does not conform to the high- or low-polarisation comet classes, nor to the "Hale-Bopp-like" category. The inversion angle is notably smaller than typical cometary values ($20^\circ$–$22^\circ$), and the negative branch is both deeper and shifted to smaller phase angles.

Imaging Polarimetry and Coma Morphology

Deep imaging and polarimetric mapping confirm the presence of a diffuse coma from the earliest epochs, with no significant spatial polarimetric structures detected across the coma or tail. The coma grows steadily as 3I approaches perihelion, but the polarisation remains homogeneous, indicating that the measured polarisation is dominated by the coma dust rather than the nucleus.

Figure 2: Deep imaging and polarimetric maps of 3I/ATLAS at multiple epochs, showing the evolution of the coma and the spatial uniformity of polarisation.

The absence of spatial polarimetric features is consistent with recent studies of other comets, including 2I/Borisov and Jupiter Family Comet 67P/Churyumov-Gerasimenko, suggesting that the dust properties are relatively homogeneous on the spatial scales probed.

Comparative Analysis: Solar System Analogs and Theoretical Implications

The polarimetric behavior of 3I/ATLAS, particularly its low inversion angle and deep negative branch, approaches that of classic F-type asteroids and some cometary nuclei, but with a negative branch depth that is nearly twice as large. Notably, the extrapolated slope at very small phase angles ($\alpha \leq 2^\circ$) aligns with the steep polarisation curves observed for certain small TNOs and Centaur Pholus, which are characterized by moderately red, water-ice-rich surfaces.

The similarity to TNOs is further supported by independent spectroscopic and colorimetric studies, which show that 3I/ATLAS is redder than typical Solar System comets and D-type asteroids, and by direct detections of water ice in the coma. Laboratory and numerical studies indicate that such extreme negative polarisation can arise from mixtures of bright (icy) and dark (organic/silicate) grains, with particle size and porosity playing critical roles. In particular, highly porous aggregates with ice mantles can reproduce deep negative branches and small inversion angles, as observed in 3I/ATLAS.

Instrumental and Observational Considerations

The campaign leveraged the capabilities of three polarimetric instruments, each with distinct optical configurations and field-of-view constraints. The use of a small aperture was essential to avoid stellar contamination, especially during early observations. The data reduction pipeline incorporated standard corrections for instrumental polarisation and wavelength-dependent effects, and the polarimetric maps were constructed using Gaussian filtering to suppress noise while preserving structural information.

Figure 3: Transmission curves of the R- and V-band filters used in the polarimetric observations.

Implications and Future Directions

The polarimetric properties of 3I/ATLAS establish it as a distinct class of interstellar comet, with dust properties not observed in any known Solar System comet or asteroid. The combination of a low inversion angle and extreme negative polarisation suggests a dust population dominated by large, porous, ice-rich aggregates, potentially reflecting formation and evolutionary processes in a different stellar environment. The findings challenge the current taxonomic framework for small bodies and highlight the diversity of ISOs.

Further observations, particularly post-perihelion when 3I will be accessible at smaller phase angles ($\alpha \sim 0$–$30^\circ$), are essential to confirm the extrapolated behavior of the negative branch and to refine constraints on the dust properties. Theoretical modeling, integrating polarimetric, spectroscopic, and photometric data, will be necessary to disentangle the contributions of composition, structure, and size distribution to the observed polarimetric signatures.

Conclusion

The polarimetric paper of 3I/ATLAS reveals a uniquely deep and narrow negative polarisation branch, with a small inversion angle and steep slope, setting it apart from all previously observed comets and asteroids. The results imply a dust population with significant ice content and high porosity, consistent with a red, water-ice-rich surface analogous to certain TNOs. These findings expand the known diversity of interstellar objects and underscore the importance of polarimetry in constraining the physical properties of small bodies from extrasolar origins. Continued monitoring and theoretical analysis will be critical for advancing the understanding of the formation and evolution of ISOs and their relationship to Solar System populations.