TY - JOUR A1 - Walterová, Michaela A1 - Běhounková, Marie A1 - Efroimsky, Michael T1 - Is There a Semi‐Molten Layer at the Base of the Lunar Mantle? Y1 - 2023-06-29 VL - 128 IS - 7 SP - EP - JF - Journal of Geophysical Research: Planets DO - 10.1029/2022JE007652 PB - N2 - Abstract

Parameterised by the Love number k2 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 1015–1016 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.

N2 - Plain Language Summary: As the Moon raises ocean tides on the Earth, the Earth itself gives rise to periodic deformation of the Moon. Precise measurements of lunar shape and motion can reveal those deformations and even relate them to our natural satellite's interior structure. In this work, we discuss two interpretations of those measurements. According to the first one, the lunar interior is hot and a small part of it might have melted, forming a thick layer of weak material buried more than 1,000 km deep under the lunar surface. According to the second one, there is no such layer, and the measured deformation can be explained by the behavior of solid rocks at relatively low temperatures. We show that the two possibilities cannot be distinguished from each other by the existing data.

N2 - Key Points: A lunar mantle governed by the Andrade model fits selenodetic constraints only with a very weak frequency dependence of tidal dissipation. We seek the parameters of two more complex models that may explain the anomalous frequency dependence of tidal Q measured by lunar laser ranging. Both a dissipative basal layer and elastically accommodated grain‐boundary sliding in the deep mantle can result in the same tidal response. UR - http://resolver.sub.uni-goettingen.de/purl?gldocs-11858/10931 ER -