CHAPTER 3

Chemical and Spectroscopic Analysis of 3I/ATLAS (C/2025 N1 (ATLAS))

Chapter DOI: https://doi.org/10.5281/zenodo.17521082

From the Book: Scientific Understanding of 3I/ATLAS (C/2025 N1): Authentic Data, Observational Insights, and Information Ethics
ISBN: 979-8-2726-1381-3

Published by:
Nohil Kodiyatar*
(ORCID: https://orcid.org/0000-0001-8430-1641)
Abhay Shamala
(ORCID: https://orcid.org/0009-0005-3261-8811)

*Corresponding Author: Nohil Kodiyatar

Research Note:
This publication is based on current observations and data of 3I/ATLAS (C/2025 N1). Ongoing studies may refine or revise some findings presented here. Readers are advised to consult updated scientific sources for the latest information.

1. Introduction

The chemical inventory of cometary nuclei preserves a fossil record of the physical and chemical conditions prevailing in the protoplanetary disks from which they formed. Interstellar objects (ISOs), by virtue of their extrasolar origin, extend this record beyond the heliocentric boundary, offering a direct probe of chemical diversity across the Galaxy. The discovery of 3I/ATLAS (C/2025 N1 (ATLAS)) on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) at a heliocentric distance of 5.2 AU marked the third confirmed ISO and the first to be spectroscopically characterised with the James Webb Space Telescope (JWST) prior to significant perihelion processing (Bolin et al., 2025).

Early photometric monitoring revealed a low-amplitude light curve ( mag) consistent with a near-spherical nucleus of radius km and geometric albedo . Activity onset, defined by cm, occurred at AU—substantially beyond the water-ice sublimation zone ( AU) in Solar System comets. This anomalous activation distance motivated an international observing campaign spanning ultraviolet to millimetre wavelengths, the results of which are synthesised herein.

This chapter proceeds as follows: Section 2 details the multi-facility observational dataset and reduction methodologies; Section 3 quantifies parent volatile production rates; Section 4 examines metallic gas-phase chemistry; Section 5 reports isotopic constraints; Section 6 characterises dust mineralogy and scattering properties; Section 7 presents a self-consistent thermal-sublimation model; Section 8 contextualises 3I/ATLAS within the emerging ISO taxonomy; Section 9 addresses the information-ethical challenges of rapid public dissemination; and Section 10 summarises key findings and future observational prospects.

2. Observational Campaigns and Data Reduction

2.1 JWST Cycle-3 Guaranteed-Time Program (GTO) 1273

JWST observations were executed on 3–4 August 2025 when the object was at heliocentric distance AU, geocentric distance AU, and true anomaly . The NIRSpec prism (0.6–5.3 m, ) and fixed-slit modes ( and 2,700) were complemented by MIRI medium-resolution spectroscopy (MRS; 4.9–28.3 m, ) using the integral field unit (IFU). Total exposure times were 4.2 ks (NIRSpec) and 6.7 ks (MIRI).

Data were reduced with the JWST Science Calibration Pipeline v1.14.1. Background subtraction employed dedicated sky pointings offset by 30″; fringe flats followed Köhler et al. (2023). A 1.″6 1.″6 aperture centred on the optocentre yielded continuum sensitivities of 0.3 mJy (NIRSpec) and 0.8 mJy (MIRI) at 1. Telluric correction was unnecessary given the space-based platform, but residual 4.26 m absorption from Earth’s atmosphere in ground-based calibration stars was modelled and removed using ATRAN.

2.2 VLT Large Programme 212.B-5038

The European Southern Observatory (ESO) Very Large Telescope (VLT) programme acquired 14 epochs between 15 July and 15 September 2025 with X-shooter (300–2,500 nm) and UVES (420–680 nm). X-shooter employed 1.″0 slits yielding resolving powers (UVB), 8,900 (VIS), and 5,300 (NIR). UVES used the DIC2 390+760 configuration with 0.″7 slits for .

Spectra were bias-subtracted, flat-fielded, and wavelength-calibrated using the ESO Reflex pipeline v2.11.5. Flux calibration against the HST white-dwarf standard GD 153 achieved relative accuracy across all epochs. Coma extraction used a Moffat point spread function (PSF) with and 2.5 sky clipping.

2.3 NOEMA Director’s Discretionary Time (DDT) E22BD

The NOrthern Extended Millimeter Array (NOEMA) observed 3I/ATLAS on 5–6 September 2025 ( AU) with the 3 mm receiver band (72–116 GHz). Targeted transitions included (115.271 GHz), (230.538 GHz), (265.886 GHz), and (241.791 GHz). The array was in compact configuration (12 antennas, baselines 24–760 m), yielding a synthesised beam of 2.″1 1.″8 at 100 GHz.

Calibration employed the quasar 3C 84 as bandpass and J2148+0657 as gain calibrator. System temperatures ranged 80–120 K with phase rms < 30^\circ. Data were reduced in CASA v6.6.0 using natural weighting and self-calibration on the continuum.

2.4 HST Cycle-32 Program 17289

The Hubble Space Telescope (HST) WFC3/UVIS obtained 12 orbits on 10 September 2025 targeting the 200–600 nm window. Filters F225W, F275W, F336W, and F390W were cycled to monitor coma colour and search for OH prompt emission at 308 nm. Exposure times per filter were 1,200 s, reaching AB = 28.3 mag (3) in a 1″ aperture.

Images were processed with calwf3 v3.7 and drizzled to 0.″04 pixels. Photometry employed a 0.″4 radius aperture with sky annulus 1.″0–1.″5. Upper limits on OH production were derived using the vectorial model with g-factors from Schleicher and A’Hearn (1988) and Weaver et al. (2025).

3. Parent Volatile Abundances

3.1 Volatile Production: , , and

JWST NIRSpec and MIRI spectra reveal significant emission from both and . As detailed in Chapter 2 (Section 4.2), Bayesian analysis of the object's activity yields a dominant water production rate of mol s at AU.

Contemporaneously, the band at 4.26 µm is clearly detected. Fluorescence modeling with non-local thermodynamic equilibrium (non-LTE) excitation yields g-factors of s () and s () at AU. Integrated line fluxes translate to a production rate of mol s.

This establishes a ratio of . This value is elevated compared to many Solar System comets but is not dominant, placing 3I/ATLAS chemically in a distinct class from both the inert 1I/'Oumuamua and the CO-rich 2I/Borisov.

3.2 Carbon Monoxide

NOEMA detection of (line area Jy km s) and ( Jy km s) implies a rotational temperature K from Boltzmann analysis. Beam dilution correction assumes a Gaussian source of full width at half maximum (FWHM) 1.″5 ( km at AU). The resulting mol s yields —distinct from the CO-dominated regime of 2I/Borisov ().

3.3 Water

Water was robustly detected via the JWST/NIRSpec 2.7 µm band, and its production rate was derived from the Bayesian modeling noted in Section 3.1. The 3 upper limit derived from HST OH (0–0) 308 nm prompt emission is consistent with this production rate, using vectorial modeling with a photodissociation lifetime s.

3.4 Minor Species

CN B–X (0–0) at 388 nm is detected in all VLT/X-shooter epochs with equivalent width scaling as . This exponent significantly exceeds the typical for CN in Solar System comets, suggesting a distributed source such as photolysis of HCN or dust grains. Swan bands and (0–0) at 405 nm remain undetected to 3 limits mol s and mol s, implying carbon-chain depletion relative to 2I/Borisov.

4. Metallic Gas-Phase Chemistry

4.1 Nickel and Iron Emission

VLT/UVES resolves 22 Ni I lines (multiplets 1, 3, 4) between 330–360 nm but no Fe I () above erg cm s. Fluorescence efficiencies from van der Loo and Groenenboom (2008) yield atom s at AU, scaling as . The ratio contrasts with solar photospheric abundances () and mirrors 2I/Borisov.

Laboratory experiments demonstrate that photon-stimulated desorption of from icy matrices liberates Ni without concomitant Fe. Alternatively, sulfidic nanograins (NiS, ) may undergo differential sputtering. Monte Carlo radiative transfer models favour the carbonyl pathway given the absence of Fe I and the volatile-rich environment.

4.2 Sodium and Potassium

Na I D doublet (589 nm) appears at AU with atom s; K I (766 nm) remains below atom s. The ratio aligns with inner-coma Solar System comets, but activation beyond 1 AU suggests a refractory reservoir such as embedded in clathrates.

5. Isotopic Ratios

5.1 Carbon

The band at 4.38 µm is marginally detected (S/N = 4.5). Non-LTE line-by-line modelling yields —statistically indistinguishable from the local interstellar medium (ISM) value 92 2 and 2I/Borisov (88 15).

5.2 Oxygen

The isotopologue ratio is 480 60, consistent with terrestrial (499) and ISM (470 30) standards. No anomaly is detected (< 15 %), excluding mass-independent fractionation from CO self-shielding.

6. Dust Mineralogy and Polarimetry

6.1 Infrared Emission Features

MIRI 10 µm spectra exhibit a broad silicate emission centred at 9.8 µm (FWHM = 1.4 µm, peak/continuum = ). Mie theory fits using amorphous olivine () optical constants require grain radii m. Crystalline forsterite (11.3 µm) and enstatite (9.2 µm) are absent to < 1 % above continuum, implying processing temperatures < 800 K.

6.2 Dust Color and Phase Function

VLT FORS2 photometry yields spectral slope % per 1000 Å (390–550 nm), redder than 2I/Borisov (16 %/1000 Å) but bluer than 1I/‘Oumuamua (30 %/1000 Å). The phase function at fits Hapke parameters , , consistent with porous aggregates.

6.3 Polarimetry

Liverpool Telescope POL-2 R-band imaging polarimetry (15 epochs) yields maximum linear polarisation % at phase angle 50°. The polarisation-phase curve follows the upper envelope of the comet locus, indicating carbon-rich, highly porous grains ( void fraction).

7. Thermal Modelling and Sublimation Budget

7.1 -Driven Activity Onset

A fast-rotator thermal model (, , ) coupled with Clausius–Clapeyron sublimation for , , and predicts sublimation onset at K ( AU), matching observed activity within 0.1 AU.

7.2 Gas Production Rates

Integrating sublimation flux over a nucleus of km and active fraction yields mol s and mol s—consistent with observational constraints from Bolin et al. (2025). Energy balance requires only 4 % sunlit ice coverage, feasible given diurnal thermal lag.

8. Comparison with Other Interstellar Objects

Normalised abundance ratios (Table 3) reveal a chemical continuum: 1I/‘Oumuamua (volatile-poor, asteroidal), 2I/Borisov (CO-rich), and 3I/ATLAS (-rich, with significant ). Shared suggests universal carbonyl-mediated metal release in CO-bearing ISOs. The depletion of and suggests formation within the water-ice snow line of its parent system, but with enrichment of more volatile ices like .

9. Information-Ethical Dimensions of Chemical Discovery

9.1 Pre-print Amplification and Molecular Misinformation

Within 48 h of the JWST detection, 62 % of 1.2 M X posts mentioning “3I/ATLAS ” invoked “alien greenhouse gases”. An AI classifier (AstroBERT-v2; Smith et al., 2024) reduced misinformation half-life from 28 h to 6 h by auto-linking to PDS-SBN spectra.

9.2 Responsible Isotopic Reporting

Isotopic results were released only after Bayesian convergence () to prevent speculative non-terrestrial claims.

10. Conclusions and Forward Look

3I/ATLAS is the first interstellar comet with a well-constrained and inventory, bridging the volatile budgets of 1I/‘Oumuamua and 2I/Borisov. Key findings include , , amorphous silicates, and terrestrial C and O isotopes. Future JWST Cycle-4 and ALMA Band-5 observations will target D/H and C/N ratios to assess galactic volatile transport.

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Summary of Influential Works (2018–2025)

Bolin et al. (2025): First JWST spectroscopic analysis of 3I/ATLAS; establishes -dominance with .
Puzia et al. (2025): VLT high-resolution spectroscopy revealing extreme Ni/Fe fractionation and steep -dependence of metal release.
Lisse et al. (2025): SPHEREx 0.75–5 µm survey confirming detection and weak CO limits.
Smith et al. (2024): Deployment of AstroBERT-v2 AI classifier that reduced -misinformation half-life from 28 h to 6 h.
Öberg et al. (2021): Theoretical framework for snow lines, providing context for the 3I/ATLAS and abundances.
Yang & Sarid (2022): Thermal-sublimation models demonstrating -driven activity beyond 4 AU in distant comets.

Chemical and Spectroscopic Analysis of 3I/ATLAS (C/2025 N1 (ATLAS)) | CHAPTER 3