Volatile distribution inversion and rotation analysis of comet 103P/Hartley 2 using nongravitational effects

¹ School of Astronautics, Beihang University, 100191 Beijing, China
² Key Laboratory of Spacecraft Design Optimization and Dynamic Simulation Technologies, Ministry of Education, 100191 Beijing, China
³ Institute of Mechanics, Chinese Academy of Science, 100190 Beijing, China

^★ Corresponding author: xhwang@buaa.edu.cn

Received: 12 March 2025
Accepted: 24 September 2025

Abstract

Aims. The rotational states of comet 103P/Hartley 2 (103P) during its perihelion passage are strongly modulated by nongravitational forces. By fitting its attitude-variation measurements, this study aims to infer the spatial distributions of H₂O and CO₂ ices on the nucleus and to assess how these inhomogeneities drive the nucleus’s rotational behavior.

Methods. We adopted a dust-mantle thermal model of the nucleus that incorporates H₂O and CO₂ ices as the principal volatiles. A coupled attitude-thermal simulation framework was developed, in which a neural-network surrogate greatly accelerates the iterative computations. The nucleus was partitioned into three morphological regions – large end, waist, and small end – and a localized activity model described the uneven sublimation across these zones.

Results. Our model successfully reproduces both the observed rotational variations and the measured volatile production rates of 103P during its 2010 encounter. At the observation instant, in addition to the pronounced CO₂ sublimation at the sunlit small end, the dust mantle covering the large end insulated the underlying CO₂, allowing it to continue sublimating on the shadowed side. We find that the H₂O ice-front depth is on the order of millimeters, whereas the CO₂ ice front lies between centimeters and decimeters, with the large end exhibiting stronger activity. The fitted CO₂ production rate closely matches the observations, while direct H₂O sublimation contributes only a minor fraction. The total H₂O production rate of 103P may primarily originate from H₂O ice ejected by CO₂ sublimation at the nucleus’s ends. Moreover, CO₂ sublimation at the large end produces the majority of the recoil torque, driving the nucleus’s enhanced long-axis excitation following the encounter.