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A Physical Model for the Ice Coma of 3I/ATLAS

Eric Keto, Abraham Loeb

TL;DR

This work addresses the origin and evolution of the sunward anti-tail observed in 3I/ATLAS by developing a physical model in which H2O ice grains are entrained in a CO2 outflow and contribute to coma scattering. The model computes the total scattering cross-section via C_sca(rh; rho_ap) = int (dM_d/da) t_res(a,rh; rho_ap) (sigma_sca/m_g)(a) da and translates it into a flux ratio F/F_sun that governs the apparent magnitude, incorporating aperture effects and phase darkening. It predicts a peak in ice-grain scattering near rh ~ 3–4 AU due to the exponential temperature dependence of sublimation, followed by a transition to larger, longer-lived grains that can form a conventional tail as rh decreases further. The results align with the July–August 2025 observations and provide a quantitative framework for interpreting ice comae and anti-tail phenomena in interstellar objects as a function of heliocentric distance.

Abstract

High-resolution imaging of interstellar comet 3I/ATLAS with the Hubble Space Telescope on July 21, 2025 revealed a pronounced sunward anti-tail with a projected 2:1 elongation that our earlier study suggests is dominated by scattering off grains of H$_2$O ice ejected from the nucleus by CO$_2$ sublimation. Subsequent observations with the Keck and Gemini South observatories showed a reduction in the anti-tail and the growth of a conventional tail in a direction away from the sun. In this study we explain the physics behind this evolution. As a function of heliocentric distance, we model the apparent visual brightness of scattering in the ice coma. As the comet approaches the Sun, the exponential temperature dependence of the sublimation rate causes a continuous increase in the production rate of ice grains and a sharp decline in their residence time within the observing aperture. The combined effects produce a peak in total scattering cross-section due to H$_2$O ice grains at 3-4 AU. At closer distances, the scattering becomes dominated by longer-lived refractory and larger volatile grains with survival times long enough to form a conventional tail.

A Physical Model for the Ice Coma of 3I/ATLAS

TL;DR

This work addresses the origin and evolution of the sunward anti-tail observed in 3I/ATLAS by developing a physical model in which H2O ice grains are entrained in a CO2 outflow and contribute to coma scattering. The model computes the total scattering cross-section via C_sca(rh; rho_ap) = int (dM_d/da) t_res(a,rh; rho_ap) (sigma_sca/m_g)(a) da and translates it into a flux ratio F/F_sun that governs the apparent magnitude, incorporating aperture effects and phase darkening. It predicts a peak in ice-grain scattering near rh ~ 3–4 AU due to the exponential temperature dependence of sublimation, followed by a transition to larger, longer-lived grains that can form a conventional tail as rh decreases further. The results align with the July–August 2025 observations and provide a quantitative framework for interpreting ice comae and anti-tail phenomena in interstellar objects as a function of heliocentric distance.

Abstract

High-resolution imaging of interstellar comet 3I/ATLAS with the Hubble Space Telescope on July 21, 2025 revealed a pronounced sunward anti-tail with a projected 2:1 elongation that our earlier study suggests is dominated by scattering off grains of HO ice ejected from the nucleus by CO sublimation. Subsequent observations with the Keck and Gemini South observatories showed a reduction in the anti-tail and the growth of a conventional tail in a direction away from the sun. In this study we explain the physics behind this evolution. As a function of heliocentric distance, we model the apparent visual brightness of scattering in the ice coma. As the comet approaches the Sun, the exponential temperature dependence of the sublimation rate causes a continuous increase in the production rate of ice grains and a sharp decline in their residence time within the observing aperture. The combined effects produce a peak in total scattering cross-section due to HO ice grains at 3-4 AU. At closer distances, the scattering becomes dominated by longer-lived refractory and larger volatile grains with survival times long enough to form a conventional tail.
Paper Structure (8 sections, 9 equations, 3 figures, 1 table)

This paper contains 8 sections, 9 equations, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. Total scattering cross-section and apparent magnitude
  3. Sublimation mass flux
  4. Per-grain scattering cross-section
  5. Albedo
  6. Apparent magnitude
  7. Discussion
  8. Conclusions

Figures (3)

  • Figure 1: Equilibrium temperatures of pure H$_2$O ice grains (upper, blue curve), and the surface of the nucleus (lower, orange curve) with a composition of 80% H$_2$O and 20% CO$_2$ as a function of heliocentric distance.
  • Figure 2: The log of the lifetime (s) of an H$_2$O ice grain (steeper, blue curve) and the log of the CO$_2$ sublimation mass flux (kg m$^{-2}$ s$^{-1}$) off the nucleus (flatter, orange curve) as functions of heliocentric distance. The lifetime is plotted for a grain size of 1 $\mu$m. A constant of 7 has been added to the sublimation mass flux to bring the curves to the same magnitude in value.
  • Figure 3: Apparent magnitude of the contribution of ice grains in the coma of 3I/ATLAS as a function of heliocentric distance.