New Insights from MESSENGER Data on Mercury's Tidal Response and Internal Structure

Abstract

Mercury's unique orbital and rotational dynamics, shaped by its proximity to the Sun and its elliptical orbit, result in periodic variations in tidal forces. These forces induce changes in the planet's shape and gravitational field, described by Tidal Love Numbers (TLNs). TLNs are essential for constraining Mercury’s internal structure, including core size, mantle composition, and crustal properties [1–5]. Accurate estimates of these deformations require precise observations, such as laser and radar altimetry for surface displacement and radio science to detect gravity variations, e.g., [6]. Additional instruments, including the Advanced Pointing Imaging Camera (APIC) and Earth-based repeat-pass Synthetic Aperture Radar (SAR) interferometry, also provide valuable measurements of radial tidal deformation [7–9]. Moreover, the differential flattening of Mercury’s internal layers can influence its tidal response; for example, librations of the inner core may significantly shape these deformations [10]. Mercury's TLNs are particularly sensitive to heterogeneities arising from spatial variations in temperature, composition, and physical properties. These internal variations add complexity to Mercury’s tidal behaviour by affecting the elastic and viscous response of its layers. To investigate these effects, we will employ advanced numerical models that incorporate geophysical and thermodynamic constraints. Our simulations will include variations in mantle composition, temperature distribution, and the rheological properties of the crust, mantle, and core. Similar to icy moons, where localised temperature or mineralogical differences alter mechanical responses [11,12], similar spatial heterogeneities on Mercury may result in non-uniform tidal deformation. Our approach explores a broad parameter space encompassing plausible scenarios for Mercury’s thermal evolution, core–mantle interactions, and lithospheric structure. The modelled TLNs will be compared with observational constraints such as tidal deformations, mass, and moment of inertia to refine our understanding of Mercury’s geodynamic and compositional evolution.

A key outcome of this work will be the generation of predictions to support and interpret upcoming observations from the ESA–JAXA BepiColombo mission [13]. BepiColombo, currently en route to Mercury, will deliver high-precision measurements of the planet’s shape, gravity field, and rotational dynamics. These observations will provide critical tests for our models and enable direct comparison between predicted and observed TLNs, thereby enhancing our understanding of Mercury's internal structure, including core composition, mantle heterogeneities, and lithospheric dynamics. This study underscores the importance of integrating numerical modelling with observational data to probe Mercury’s interior. In addition to leveraging insights from BepiColombo, we will incorporate independent constraints— including libration and tidal Love number h2 measurements derived from the work of H. Xiao and collaborators (EPSC abstract: Mercury’s librations from self-registration of MESSENGER laser profiles) to develop a more comprehensive view of Mercury’s interior. However, it is important to note that the expected precision improvements in obliquity, annual libration amplitude, and tidal h2 from BepiColombo’s BELA instrument, compared to MESSENGER's MLA, may not substantially refine constraints on Mercury’s deep interior. Therefore, a key focus for BELA and related investigations should be the detection of long-period librations (which can constrain inner and outer core sizes), the measurement of tidal phase lag (sensitive to mantle viscosity), and deviations from the Cassini state (related to the dissipation and mantle viscosity). These parameters offer more promising pathways for significantly improving our understanding of Mercury’s internal structure.