The Moon undergoes periodic tidal forcing due to its eccentric and oblique orbit around the Earth¹. The response to this tidal interaction drives temporal changes in the lunar gravity field and is sensitive to the satellite’s internal structure^{2, 3–4}. We use data from the NASA GRAIL spacecraft^{5, 6, 7, 8–9} to recover the time-varying lunar gravity field, including a degree-3 gravitational tidal Love number, k₃. Here, we report our estimated value of k₃ = 0.0163 ± 0.0007, which is about 72% higher than that expected for a spherically symmetric moon¹⁰. Such a large k₃ can be explained if the elastic shear modulus of the mantle varies by about 2–3% between the nearside and farside⁴, 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 ago¹¹ and could influence the spatial distribution of deep moonquakes¹².

Juice (JUpiter ICy moons Explorer) 3GM Radio Science Experiment will map the gravitational field of Ganymede with unprecedented accuracy and measure tidally-induced variations. These measurements will allow the characterization of its putative ocean and may resolve lateral variations in internal structure. Lateral variations cause an additional tidal signal that depends on their wavelength and amplitude. We show that shell thickness variations of (Formula presented.) the mean thickness produce an additional tidal signal (Formula presented.) times smaller than the main tidal signal, detectable given the accuracy of Juice. Using a Bayesian framework, we show that measuring differences between (Formula presented.) and (Formula presented.) constrain equator to pole shell thickness differences. Also measuring the degree-3 spherical harmonic signal due to degree 2 forcing constrain degree-1 and degree-3 structure. This demonstrates tidal tomography's potential to map three dimensional structure and supports its consideration for future missions.