Observational constraint on the radius and oblateness of the lunar core-mantle boundary

Vishnu Viswanathan (1, 2) , Nicolas Rambaux (1) , Agnes Fienga (1, 2) , Jacques Laskar (1) , Mickael Gastineau (1)

1 IMCCE - Institut de Mécanique Céleste et de Calcul des Ephémérides
2 GEOAZUR 7329 - Géoazur

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 $\pm55$ 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 ($f_c=(2.2\pm0.6)\times10^{-4}$) that satisfies the assumption of hydrostatic equilibrium over a tight range of lunar CMB radii ($\mathcal{R}_{CMB}=381\pm12$ 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 (FCN) within $(367\pm100)$ years.

Domaines: Physique Générale [physics.gen-ph]; Astrophysique [astro-ph]; Relativité Générale et Cosmologie Quantique [gr-qc]; Planète et Univers [physics]

Vishnu Viswanathan, Nicolas Rambaux, Agnes Fienga, Jacques Laskar, Mickael Gastineau. Observational constraint on the radius and oblateness of the lunar core-mantle boundary. Geophys.Res.Lett., 2019, 46, pp.7295-7303. ⟨10.1029/2019GL082677⟩. ⟨hal-02088588⟩