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

## TL;DR

This study uses lunar laser ranging and seismic data to constrain the size and shape of the lunar core-mantle boundary, providing new estimates consistent with hydrostatic equilibrium and refining the core's radius and mass fraction.

## Contribution

It introduces a new dynamical model that determines the lunar core-mantle boundary's radius and oblateness from observational data, resolving previous uncertainties.

## Key findings

- Lunar core oblateness is estimated at (2.2 ± 0.6)×10⁻⁴.
- CMB radius is constrained to 381 ± 12 km.
- Core mass fraction is estimated between 1.59% and 1.77%.

## 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 $\pm55$ 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.