Unravelling Anisotropic Sublimation Effects on Comet 103P/Hartley 2 through Coupled Attitude-Thermal Simulations

Non-gravitational effects serve as a critical nexus between comet dynamics and thermodynamics, offering essential insights into cometary activity and evolution. While sublimation-driven recoil forces influence both orbital and rotational motion, these effects simultaneously encode critical information about a comet’s physical properties. Comet 103P/Hartley 2 exhibits pronounced non-gravitational torque-driven rotation during perihelion passage. This study employs observational spin-state variation data to inversely constrain the H₂O and CO₂ ice sublimation depths across surface regions and investigates long-term rotational evolution.

We present a coupled attitude-thermal simulation framework integrating nucleus thermal modeling with rotational dynamics simulations. The dust mantle model is used to simulate the one-dimensional thermal process of the cometary nucleus surface. In this model, volatiles sublimate from a certain depth on the nucleus surface. By setting different ice front depths for various regions, we can model the anisotropic sublimation on the cometary nucleus. To address computational bottlenecks, we implement a neural network proxy model achieving 60-fold acceleration with a relative error of 13%. Based on observation results, the nucleus is divided into three regions: the large end, the waist, and the small end. CO₂ ice sublimates from the ends, while H₂O ice sublimates from the waist. By treating sublimation depth as optimizable variables, we successfully reproduce 103P’s observed spin-state variations and volatile production rates.

Key results demonstrate:

• The angular momentum and rotational energy of the nucleus exhibit a combination of short-term fluctuations and long-term decay (Fig. 1).
• Sublimation patterns at E+7 min show H₂O release from the waist and asymmetric CO₂ outgassing – predominantly from the sunlit small lobe with secondary production on the large lobe's nightside. Extended analysis reveals that the dust mantle layer acts as a thermal insulator, maintaining sustained volatile sublimation throughout the nucleus's rotational cycle. Derived CO₂ ice front depths are 71 mm (large lobe) and 92 mm (small lobe), with the large end showing stronger activity. While modeled CO₂ production (1.77×10²⁷ mole/s) matches observations, waist H₂O production (4.93×10²⁶ mole/s) constitutes merely 5% of observed values, suggesting H₂O production rate mainly originates from CO₂-driven water ice ejection at the ends.
• Long-term simulations predict initial short-axis excitation transitioning to long-axis mode around E-200 days. In the following orbital cycles, the nucleus’s angular momentum gradually shifted toward the minimum inertia axis, and a sun-pointing attitude emerged. Each time the nucleus passes through perihelion, the angular velocity increases. We estimate that 103P may reach its rotational break-up limit after approximately 25 orbital cycles.

Fig. 1: Variations in angular momentum and rotational energy. The upper graph shows the angular momentum variation, and the lower graph shows the rotational energy variation. The black crosses represent Belton’s estimates, and the red curve represents the fitting results from this study.