Potential Thermal Profiles of The Third Interstellar Object 3I/ATLAS

TL;DR

This study addresses whether the thermal evolution of the interstellar object 3I/ATLAS can reveal its bulk physical properties by linking surface insolation to volatile activity. Using a one-dimensional heat-diffusion framework along a hyperbolic trajectory, the authors explore a range of albedos, densities, and conductivities to compute surface-to-subsurface temperatures and compare them to sublimation thresholds for CO, CO2, and H2O. They find CO-driven activity across most of the orbit, CO2 contributing inbound and near perihelion, and H2O activity confined to shallow layers near perihelion, with the depth of active fronts controlled by the thermal diffusivity and albedo; the analysis yields an albedo upper limit of $A \le 0.2$ consistent with observed activity. The work provides a robust, adaptable framework for interpreting interstellar object composition and thermal history, linking observed volatile outgassing to depth-dependent heat transport and guiding expectations for post-perihelion evolution.

Abstract

We investigate the thermal evolution of 3I/ATLAS, the third macroscopic interstellar object discovered on 2025 July 1. By comparing modeled thermal profiles with observations of volatile activity, it is possible to constrain bulk physical properties of a cometary nucleus. 3I/ATLAS is actively producing a variety of cometary volatiles. In this paper, we calculate one-dimensional thermal profiles of the third interstellar object 3I/ATLAS throughout its trajectory in an attempt to gain insight into its bulk properties based on measurements of its volatiles. Assuming a variety of typical comet and asteroid bulk geophysical properties such as heat capacities, densities, and conductivities, we calculate the radial thermal profile as a function of depth throughout the hyperbolic trajectory. The methods and code to generate the thermal profile are flexible for any hyperbolic or bound orbit. The thermal profiles are benchmarked to the nominal sublimation temperatures of H$_2$O, CO$_2$ and CO, but are still applicable to any volatile. Comparison between the modeled surface temperatures and the observed onset of H$_2$O activity near 3 au indicates that surface temperatures exceeding $\sim$150 K can only be achieved if the albedo is below 0.2. We therefore set the upper limit on the albedo of 3I/ATLAS to be 0.2.

Figures (6)

• Figure 1: The temperature versus depth and time for $\kappa=\qtylist{e-4;e-3;e-2}{W\,m^{-1}\,K^{-1}}$ (top, middle, and bottom panels). The sublimation temperatures of CO (30K, dash), CO$_2$ (80K, dash-dot), and H$_2$O (150K, dot) are shown as white contours. The depth of heat penetration and volatile activation strongly depends on the thermal conductivity $\kappa$. Note that the range of the $y$-axis is different for each panel.
• Figure 2: Zoomed-in version of Figure \ref{['fig:thermal_H1']} closer to the discovery and perihelion dates.
• Figure 3: The maximum depth where a given species will sublimate as a function of time and the thermal diffusivity $\alpha$. The depth of volatile activity depends strongly on thermal diffusivity, with CO and CO$_2$ remaining active over a wide range of depths and times, while H$_2$O sublimation occurs only under warmer conditions near perihelion.
• Figure 4: The maximum depth where $T\geq\qty{150}{K}$ versus the thermal diffusivity. The numerical results (crosses) are shown versus an analytic scaling calculation (dashed line). These results indicate that the temperatures required for H$_2$O sublimation do not reach deeper than about 1 m for any thermal diffusivity. Subsurface water ice will be stable below this depth.
• Figure 5: The temperature at series of depths versus time. The different panels show calculations with different albedoes. The surface reflectivity plays a key role in controlling thermal evolution, with high-albedo surfaces staying cooler and suppressing volatile release while low-albedo surfaces heat more efficiently, leading to deeper thermal penetration and stronger H$_2$O activity near perihelion. Here, we used comet-like constants and fixed thermal conductivity to $\kappa=\qty{e-3}{W\,m^{-1}\,K^{-1}}$.