An overview and perspective of terrestrial planets' low-degree gravity field, rotation, and interior structure

The low-degree gravity field and rotation parameters of terrestrial planets are functions of the planet's interior structure, providing insights into their composition and evolution, which serves them as crucial targets throughout deep-space exploration. Particularly, the measurement of rotation state can directly reveal the density and size of most untouchable planetary core. With the accumulation of data and the recent high-precision deep-space missions, substantial scientific advances have been achieved in understanding the internal structure of terrestrial planets, especially the core, using the low-degree gravity field and rotation measurements. In this study, we provide a summary of the fundamental theory and method for determining the principal moment of inertia tensor of planets, and then the basic theory of planetary rotation dynamics and rotational normal modes closely related to internal structures, and the methods for solving principal moments of inertia tensors of planets using the low-degree gravitational potential coefficients. We review the up-to-date researches on determining the mean bulk density and mean moment of inertia factor through limited rotation observations and the low-degree gravity field measurements, and the basic approaches to invert interior structures. Finally, we present existing challenges in revealing interior structures from low-degree gravity field and rotation normal modes for terrestrial planets and asteroids with a hope to provide scientific references and theoretical support for Chinese future deep-space missions.