  Parameterised by the Love number $k_2$ and the tidal quality factor $Q$, and
inferred from lunar laser ranging (LLR), tidal dissipation in the Moon follows
an unexpected frequency dependence often interpreted as evidence for a highly
dissipative, melt-bearing layer encompassing the core-mantle boundary. Within
this, more or less standard interpretation, the basal layer's viscosity is
required to be of order $10^{15}$ to $10^{16}$ Pa s, and its outer radius is
predicted to extend to the zone of deep moonquakes. While the reconciliation of
those predictions with the mechanical properties of rocks might be challenging,
alternative lunar interior models without the basal layer are said to be unable
to fit the frequency dependence of tidal $Q$.
  The purpose of our paper is to illustrate under what conditions the
frequency-dependence of lunar tidal $Q$ can be interpreted without the need for
deep-seated partial melt. Devising a simplified lunar model, in which the
mantle is described by the Sundberg-Cooper rheology, we predict the relaxation
strength and characteristic timescale of elastically-accommodated grain
boundary sliding in the mantle that would give rise to the desired frequency
dependence. Along with developing this alternative model, we test the
traditional model with a basal partial melt; and we show that the two models
cannot be distinguished from each other by the available selenodetic
measurements. Additional insight into the nature of lunar tidal dissipation can
be gained either by measurements of higher-degree Love numbers and quality
factors or by farside lunar seismology.
