Joint inversion of seismic and gravity data for lunar composition and thermal state

Abstract

SUMMARY We invert the Apollo lunar seismic data set, together with lunar mass and moment of inertia, directly for the chemical composition and thermal state of the Moon. The lunar mantle and crust are modelled in the chemical system CaO‐FeO‐MgO‐Al2O3‐SiO2. The stable minerals, their seismic properties, and the bulk density are computed by Gibbs free energy minimization. Voigt‐Reuss‐Hill averaging is then used to compute seismic-wave velocity profiles, from which traveltimes are estimated, while mass and moment of inertia are obtained by integration of the density profile. Given this scheme, the data are jointly inverted using a Markov chain Monte Carlo algorithm, from which a range of compositions and temperatures fitting data within uncertainties are obtained. The analysis constrains the range of compositions, thermal states, mineralogy and physical structure of the lunar interior that are consistent with data. Additionally, the analysis provides estimates for the size and density of the lunar core. The inferred lunar compositions have lower Mg#s (∼83) than the Earth’s mantle (∼89), suggesting that the Moon was derived from material other than the Earth’s mantle. This supports giant impact simulations of lunar origin that show that more than 80 per cent of the material making up the Moon is derived from the impactor.