Comets and Asteroids

Dust Colour, Phase Behaviour, And Monte Carlo Modelling Of Interstellar Comet 3I/ATLAS From 4 au Pre- to 4 au Post-Perihelion

By Keith Cowing

Status Report

astro-ph.EP

June 19, 2026

Filed under 3I/ATLAS, astro-ph.EP, astrochemistry, astrogeology, Comet, dust, icy world, Interstellar, Spectroscopy

Dust Colour, Phase Behaviour, And Monte Carlo Modelling Of Interstellar Comet 3I/ATLAS From 4 au Pre- to 4 au Post-Perihelion — Panels (a)–(o) show contour maps of the observed isophotes (black lines) together with the corresponding modelled isophotes (red lines) for the case 𝑟max = 1 cm. For each of these contour panels, a companion plot is displayed to the right, labelled (a′ )–(o′ ), showing a one-dimensional scan extracted along the direction indicated by the straight red line in the corresponding contour map. In these primed panels, the black and red curves represent the observed and modeled brightness profiles, respectively. The 𝑥- and 𝑦-axes in the contour plots are given in pixel units, and the innermost isophote levels as well as the physical dimensions of each image are listed in Tables 1 and 3. In panels (a)–(o), isophote levels increase outward in steps of 1 mag. In the primed panels, the 𝑥-axis is expressed in pixel units and the 𝑦-axis in surface brightness units (mag arcsec⁻²). — astro-ph.EP

We report multi-band photometric imaging observations and Monte Carlo dust tail modelling of the interstellar comet 3I/ATLAS covering a wide range of heliocentric distances, from about 4 au pre-perihelion to 4 au post-perihelion.

The extensive imaging data set allowed us to constrain the dust physical properties, ejection speeds, and production rates as a function of heliocentric distance. The post-perihelion observations, obtained at high cadence in multiple photometric bands (SDSS g, r, i, and luminance filters) and spanning phase angles between approximately 0.7 deg and 30 deg, enabled us to determine the dust color and phase function.

The resulting phase curve exhibits a prominent backscattering enhancement, distinct from those derived for Solar System comets, with an opposition surge of 0.1–0.4 mag, a width of 1–3 deg, and a linear phase coefficient of 0.02-0.04 mag/deg, consistent with independent pre-perihelion estimates.

A possible interpretation of the imaging data, together with independent photometric measurements, indicates a dust size distribution characterized by a power-law index of -3.5, with minimum and maximum particle radii of rmin = 10 micrometer and rmax in the interval 1-10 cm.

The reported water production rate correlates well with the dust production rate post-perihelion, but fails to do so pre-perihelion, an effect possibly linked to the high CO₂/H₂O ratio measured before perihelion. The derived maximum dust-loss rate at perihelion is (0.5-1.8)E4 kg/s.

F. Moreno, M. Serra-Ricart, J. Licandro, P.J. Gutiérrez, L.M. Lara, I. Mariblanca-Escalona, M.R. Alarcón

Comments: Accepted to MNRAS, June 16, 2026
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2606.18751 [astro-ph.EP] (or arXiv:2606.18751v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2606.18751
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Submission history
From: Fernando Moreno
[v1] Wed, 17 Jun 2026 06:54:19 UTC (3,335 KB)
https://arxiv.org/abs/2606.18751

Astrobiology, Astrochemistry,

Keith Cowing

Biologist, Explorers Club Fellow, ex-NASA Space Biologist and Payload integrator, Editor of NASAWatch.com and Astrobiology.com, Lapsed climber, Explorer, Synaesthete, Former Challenger Center board member 🖖🏻

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