Thermal asymmetry in the Moon's mantle inferred from monthly tidal response.

The Moon undergoes periodic tidal forcing due to its eccentric and oblique orbit around the Earth. The response to this tidal interaction drives temporal changes in the lunar gravity field and is sensitive to the satellite's internal structure. We use data from the NASA GRAIL spacecraft to recover the time-varying lunar gravity field, including a degree-3 gravitational tidal Love number, k. Here, we report our estimated value of k = 0.0163 ± 0.0007, which is about 72% higher than that expected for a spherically symmetric moon. Such a large k can be explained if the elastic shear modulus of the mantle varies by about 2-3% between the nearside and farside, providing an observational demonstration of lateral heterogeneities in the deep lunar interior. This asymmetric structure suggests preservation of a predominantly thermal anomaly of roughly 100-200 K in the nearside mantle that formed surface mare regions 3-4 billion years ago and could influence the spatial distribution of deep moonquakes.