潮汐の応答係数に基づく月の深部構造の制約

抄録

Tidal heating of a solid planetary body occurs by viscous dissipation, depending on its internal structure, and connected to its thermal and orbital states. Earlier calculations of the response of the Moon to tidal forces had considered lunar interior structure, but had not reproduced the geodetically observed dependence of the dissipation on the lunar tidal period. The attenuation of seismic waves in the deep lunar interior is consistent with the existence of a low-viscosity layer at the core-mantle boundary, which may explain the observed frequency dependence. Here I briefly introduce my/our own studies, especially a series of studies showing constraints on the deep lunar interior based on the tidal response parameters, and mention future studies to be done by myself/ourselves. In the first work, we numerically simulated the viscoelastic tidal response of the Moon that contains the low-viscosity layer at the core-mantle boundary, and compared the model results with geodetic observations available at that time (i.e., LLR, SELENE, and Chang'e 1). In our simulations, a layer with a viscosity of about 2×10¹⁶ Pa s leads to frequency-dependent tidal dissipation that explains tidal dissipation observations at both monthly and annual periods. Compared with the lunar asthenosphere, the inferred viscosity is extremely low, and suggests partial melting at the lunar core-mantle boundary. We also found that tidal dissipation is not evenly distributed over the lunar interior, but localized within the low-viscosity layer. This implies that this layer may act as a thermal blanket on the lunar core and has influenced the lunar thermal evolution. In the subsequent work, we revisited the constraints on the deep lunar interior with a possible low-viscosity zone at the core-mantle boundary obtained from our previous forward modeling. We compared the numerical model with several tidal parameters that have been improved or are newly determined by recent geodetic observations and analyses (i.e., LLR, GRAIL, and LRO). Our results are, in principle, consistent with these data, and suggest a low-viscosity layer with its outer radius of 540 ～560 km, which possibly extends into the region where deep moonquakes occur.