A Low-Viscosity Lower Lunar Mantle Implied by Measured Monthly and Yearly Tides

The Moon's frequency-dependent tidal response, expressed as temporal variations in its gravity field through the Love number k₂ and as dissipation through the quality factor Q, provides information about its interior structure. Lunar laser ranging has provided measurements for Q, but so far no frequency-dependent values for k₂ have been determined. We provide the first spacecraft measurements of k₂ and Q at two frequencies, monthly and yearly, from an analysis of Gravity Recovery and Interior Laboratory and Lunar Reconnaissance Orbiter radio tracking data. Interior modeling indicates that these values can be matched only with a low-viscosity zone at the base of the lunar mantle, even when using complex rheological laws to model the mantle's response. The existence of this zone has profound implications for the Moon's thermal state and evolution.