Observational Constraint on the Radius and Oblateness of the Lunar Core‐Mantle Boundary

Abstract

Lunar laser ranging (LLR) data and Apollo seismic data analyses, revealed independent evidence for the presence of a fluid lunar core. However, the size of the lunar fluid core remained uncertain by ±55 km (encompassing two contrasting 2011 Apollo seismic data analyses). Here we show that a new description of the lunar interior's dynamical model provides a determination of the radius and geometry of the lunar core‐mantle boundary (CMB) from the LLR observations. We compare the present‐day lunar core oblateness obtained from LLR analysis with the expected hydrostatic model values, over a range of previously expected CMB radii. The findings suggest a core oblateness (fc=(2.2±0.6)×10−4) that satisfies the assumption of hydrostatic equilibrium over a tight range of lunar CMB radii ( RCMB=381±12 km). Our estimates of a presently relaxed lunar CMB translates to a core mass fraction in the range of 1.59–1.77% with a present‐day free core nutation within (367±100) years.

Large sulfur isotope fractionation in lunar volcanic glasses reveals the magmatic differentiation and degassing of the Moon

“…The model indicates that if the LMO reaches a range in δ 34 S between −1 and −1.5‰ after core segregation, it will result in an LMO sulfur content between 83 and 33 ppm, a core S content of 0.8 and 1.1 wt %, and an equilibrium sulfur isotope fractionation factor α core-mantle of 1.0016 and 1.0007 for batch equilibrium and open system, respectively. All output results are within values previously estimated or experimentally determined ( 34 , 54 – 56 , 60 , 61 ).…”

“…In the open-system model, the core is treated as a cumulative product. We considered for our model a core mass fraction in the range of 2 mass % ( 54 – 56 ), an initial bulk Moon sulfur content of 250 ppm, with a chondritic δ 34 S = 0 ‰ ( 34 ), and a metal-silicate melt sulfur partition coefficient of ~100 ( 58 , 59 ). The model indicates that if the LMO reaches a range in δ 34 S between −1 and −1.5‰ after core segregation, it will result in an LMO sulfur content between 83 and 33 ppm, a core S content of 0.8 and 1.1 wt %, and an equilibrium sulfur isotope fractionation factor α core-mantle of 1.0016 and 1.0007 for batch equilibrium and open system, respectively.…”

“…We evaluate whether core segregation could produce a light  34 S value in the LMO using a similar approach used to explain the light  34 S of the Earth's mantle (22). Several lines of evidence from geophysical studies (e.g., seismology and core dynamo), to geochemical analyses of the chalcophile and siderophile elements in lunar basalts, to experimental reports on phase equilibria on possible core compositions at the relevant pressure and temperature conditions have been used to define the size and composition of the lunar core (54)(55)(56). These investigations suggest a lunar core of ~1 to 2.5 mass %, with low sulfur content of ~1 to 2 wt %.…”

A Past Lunar Dynamo Thermally Driven by the Precession of Its Inner Core

“…The tilt angle of the inner core is smaller in State A, and hence, Q icb would have dropped significantly after such a transition, though large‐scale flows and a possibly more energetic temporary dynamo may have resulted in the process of the transition. Perhaps offering support for such a scenario, the most recent estimate of the CMB radius from LLR is 381±12 km (Viswanathan et al, 2019): This places the inner core in Cassini State A at present day (see Figures 6e and 6f) but relatively close to State B and consistent with a recent transition.…”

Histoire de la Télémétrie laser terre-lune

“…La présence du noyau fluide a été confirmée en 2011 par deux études indépendantes basées sur la ré-analyse des données des sismomètres des missions Apollo. Cependant, les modèles de noyau sont très différents et l'accumulation des données LLR depuis plus de 50 ans a permis en 2019 de trancher en faveur d'un noyau liquide de 381±12 km de rayon avec une précision inégalée par les autres méthodes géophysiques appliquées à la Lune [7].…”