The core of the moon revealed by synchrotron light

10-04-2015

Seismic recordings obtained by the Apollo space programme (Apollo lunar surface experiments package) provide valuable information about the inner structure of the moon. However, this data is not sufficient to determine the structure and the properties of the lunar core. In order to interpret the different seismic properties, measurements of density and sound velocity in iron under pressure and temperature similar to those of the lunar core have been obtained by a team of scientists from the IMPMC and published in the journal PNAS. This work provides new insight to help in the understanding of the seismic observations, and allows the modelling of the composition and the structure of the moon’s metallic core.

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Iron is the main component of the core of telluric planets (those with a structure similar to that of Earth). It adopts a hexagonal compact structure (hc) under conditions of the inner Earth’s core, whereas a face-centered cubic structure (fcc) is expected under the lower pressures of smaller planetary bodies, such as the moon, Mercury or Mars. Therefore, to model the planetary core, it is essential to determine the physical properties of iron at the appropriate high pressure and high temperature.

Scientists have measured the density and propagation velocity of compressional waves and shear waves in fcc iron at pressures and temperatures expected at the center of small telluric planets. Results indicate that the seismic velocity currently proposed for the inner core of the moon are well below those of fcc iron as well as those of other likely iron based alloys. This data brings significant new constraints for the seismic model of the moon’s core and the cores of small telluric planets and has made it possible to construct a direct model of the composition, structure, density and velocity of the lunar core.

Experiments were carried out on iron samples compressed to 19 GPa and heated to 1150 K in diamond anvil cells at beamline ID28. The sound velocity was determined by inelastic X-ray scattering measurements, whereas the crystalline structure and its density were determined by X-ray diffraction.

The figure shows the proposed model of the lunar core with its structure, its composition, its dimensions and the values of the density (ρ) and the velocity (VP – velocity of compression waves; VS – velocity of shear waves).

model of the lunar core

Schematic view of the interior of the moon and a close-up of the proposed core model. This model results from the seismic observations of the Apollo missions and the measured density and sound propagation velocity, which depended on the pressure and the temperature applied to the solid iron and the liquid iron-sulphur alloys.

The approach, used here to understand the properties of the lunar core, can be extended to other planets such as Mars. The primary objective of the NASA Discovery Program mission “Insight”, which should be launched in March 2016, is the installation of a seismological station to study the interior of Mars. The results presented here, combined with data from other similar studies, are essential in order to interpret the seismic observations.

 

Reference
Toward a mineral physics reference model for the Moon’s core, D. Antonangeli, G. Morard, N.C. Schmerr, T. Komabayashi, M. Krisch, G. Fiquet, and Y. Fei, PNAS 112, 3916-3919 (2015).