Pre-perihelion Development of Interstellar Comet 3I/ATLAS (2510.18769v1)

Abstract: We describe pre-perihelion optical observations of interstellar comet 3I/ATLAS taken during July - September 2025 using the Nordic Optical Telescope. Fixed aperture photometry of the comet is well described by a power law function of heliocentric distance, rH, with the exponent (``index") n = 3.8+/-0.3 across the 4.6 au to 1.8 au distance range (phase function 0.04+/-0.02 magnitude/degree assumed). This indicates that the dust production rates vary in proportion to rH*(-1.8+/-0.3). An rH*(-2) variation is expected of a strongly volatile material, and consistent with independent spectroscopic observations showing that carbon dioxide is the primary driver of activity. The measured heliocentric index is unremarkable in the context of solar system comets, for which n is widely dispersed, and provides no basis on which to describe 3I as either dynamically old (thermally processed) or new (pristine). The morphology of the comet changes from a Sun-facing dust fan in the early 2025 July observations, to one dominated by an antisolar dust tail at later dates. We attribute the delayed emergence of the tail to the large size (effective radius 0.1 mm) and slow ejection (5 m/s) of the optically dominant dust particles, and their consequently sluggish response to solar radiation pressure. Small (micron-sized) particles may be present but not in numbers sufficient to dominate the scattering cross-section. Their relative depletion possibly reflects interparticle cohesion, which binds small particles more effectively than large ones. A similar preponderance of 0.1 mm grains was reported in 2I/Borisov. However, 2I differed from 3I in having a much smaller (asteroid-like) heliocentric index, n = 1.9+/-0.1. Dust production rates in 3I are 180 kg/s at 2 au, compared with 70 kg/s in 2I/Borisov at the same distance.

• The paper presents a detailed photometric analysis showing that 3I/ATLAS’s dust production scales as r_H⁻¹.8, consistent with CO₂-driven activity.
• It utilizes syndyne and synchrone modeling to reveal a delayed emergence of an antisolar tail caused by large (~100 µm) slow-moving dust grains.
• Comparative analysis with 2I/Borisov indicates higher dust mass loss and similar grain distributions, challenging the use of brightness indices for comet classification.

Pre-perihelion Evolution and Dust Properties of Interstellar Comet 3I/ATLAS

Introduction

This paper presents a comprehensive analysis of the pre-perihelion activity of the interstellar comet 3I/ATLAS, utilizing optical photometry from the Nordic Optical Telescope (NOT) and synthesizing data from other facilities. 3I/ATLAS is the third confirmed interstellar object to traverse the solar system, following 1I/'Oumuamua and 2I/Borisov. Its highly hyperbolic orbit ($e = 6.145$) and extreme inclination ($i = 175.1^\circ$) distinguish it as the most dynamically energetic interstellar visitor observed to date. The paper focuses on the comet's dust production rates, coma morphology, and the physical properties of the ejected grains, with comparative analysis to 2I/Borisov and solar system comets.

Photometric Analysis and Heliocentric Brightness Index

Fixed-aperture photometry within a projected $10^4$ km radius was performed to mitigate the "delta effect" and ensure consistent measurement of the coma's scattering cross-section. The integrated brightness of 3I/ATLAS as a function of heliocentric distance ($r_H$) is well described by a power law, with an index $n = 3.8 \pm 0.3$ over the range $4.6$ to $1.8$ au. This corresponds to a dust production rate scaling as $r_H^{-1.8}$, which is consistent with CO$_2$-driven activity, as CO$_2$ sublimation rates scale approximately as $r_H^{-2}$ in this regime.

The derived heliocentric index for 3I/ATLAS is statistically indistinguishable from the broad distribution of indices observed in both dynamically new and old solar system comets. This result contradicts claims that the index can be used to infer the thermal history or dynamical age of interstellar comets. In contrast, 2I/Borisov exhibited a much lower index ($n = 1.9 \pm 0.1$), implying a nearly constant coma cross-section with heliocentric distance, a property rarely observed in solar system comets.

Morphological Evolution and Dust Dynamics

Early NOT observations revealed a sunward dust fan with negligible antisolar tail, transitioning over several weeks to a morphology dominated by an antisolar tail. Syndyne and synchrone modeling, combined with surface brightness profile analysis, confirm that the tail's emergence is governed by solar radiation pressure acting on large, slowly ejected dust grains. The effective radius of the optically dominant particles is inferred to be $\sim$100 $\mu$m, with ejection velocities of $\sim$5 m s$^{-1}$. The delayed tail formation is attributed to the sluggish response of these large grains to radiation pressure, in contrast to micron-sized particles which are more rapidly accelerated.

The observed grain size distribution is similar to that inferred for 2I/Borisov, suggesting a possible commonality in the physical processes governing dust ejection in interstellar comets. The relative depletion of small grains may be due to enhanced interparticle cohesion, inhibiting their detachment from the nucleus.

Dust Production Rates and Volatile Drivers

The dust mass loss rate for 3I/ATLAS at $r_H = 2$ au is estimated at $\sim$180 kg s$^{-1}$, substantially higher than the $\sim$70 kg s$^{-1}$ measured for 2I/Borisov at the same distance. Extrapolation to perihelion yields a predicted rate of $405 \pm 10$ kg s$^{-1}$. These rates are consistent with a larger or more active nucleus for 3I/ATLAS. The dust production is tightly coupled to CO$_2$ outgassing, as corroborated by independent spectroscopic measurements. No evidence is found for a transition to H$_2$O-driven activity within the observed pre-perihelion range.

Comparative Context and Theoretical Implications

The analysis demonstrates that the heliocentric brightness index alone is insufficient to distinguish interstellar comets from solar system analogs or to infer their thermal histories. The broad distribution of indices among solar system comets encompasses the values measured for both 3I/ATLAS and 2I/Borisov. The dominance of large grains in the coma and the absence of significant small-particle populations may reflect fundamental differences in the cohesion and ejection mechanisms of interstellar cometary material, possibly shaped by prolonged exposure to interstellar conditions.

The paper also highlights the limitations of using photometric indices for dynamical classification, given the statistical improbability of prior stellar encounters for interstellar objects. The observed dust properties and production rates provide constraints on the physical state and volatile inventory of interstellar comets, with implications for models of protoplanetary disk evolution and the survival of supervolatile species.

Future Directions

Continued monitoring of 3I/ATLAS through perihelion, combined with high-resolution spectroscopy and polarimetry, will be essential to further constrain the volatile composition and grain size distribution. Comparative studies of future interstellar interlopers will help elucidate whether the observed properties of 3I/ATLAS and 2I/Borisov are representative or anomalous. Laboratory experiments on dust cohesion and ejection under simulated interstellar conditions may provide additional insight into the mechanisms governing grain size distributions.

Conclusion

The pre-perihelion development of 3I/ATLAS reveals dust production rates and coma morphology consistent with CO$_2$-driven activity and the dominance of large, slowly ejected grains. The heliocentric brightness index and dust properties are not unique among comets, challenging the use of these metrics for dynamical or thermal classification. The results underscore the need for multi-faceted observational and theoretical approaches to understand the physical nature of interstellar comets and their implications for planetary system formation and evolution.