Illustration par défaut

Laser Moon telemetry sheds light on the internal structure of the Moon

Press release | Observatoire de Paris

Using Laser-Moon telemetry data, the longest experience of the Apollo era, a team from Paris Observatory and Observatory of the Côte d’Azur (OCA) manages to determine a Moon core size of 381 km with an accuracy of +/- 12 km, which is three times better than previous individual estimates.

Our natural satellite, the Moon, has a fluid core. Its presence was identified indirectly by Lunar-laser ranging data in the 1980s, and confirmed by magnetic and seismic data. Nevertheless, estimates of the size of the fluid core varied by +/- 55 km.

Vue d’artiste de la structure interne de la Lune.
Elle montre l’emplacement des cinq panneaux de réflecteurs laser (Apollo 11, 14, 15, Luna 17 et 21) et les faisceaux laser provenant des stations à la surface de la Terre, symbolisés par des traits verts. L’analyse précise des mouvements de rotation de la Lune et de son orbite a permis de déterminer avec une précision inégalée le rayon de la limite noyau-manteau lunaire à 381 km (± 12 km) et son aplatissement ((2,2 ± 0,6) x 10-4).
© Y. Gominet / IMCCE / Observatoire de Paris – PSL

July 21, 1969, with Neil Armstrong’s and Buzz Aldrin first steps on the surface of the Moon, also began an unprecedented scientific experiment. The astronauts have settled a reflective panel that has been used for 50 years to measure the Earth-Moon distance by timing the travel time of photons emitted by laser stations on the surface of the Earth. Five of these reflectors are now present on the lunar surface, including two reflectors from the Russian missions.

Station de télémétrie laser Lune de l’Observatoire de la Côte d’Azur, implantée sur le plateau de Calern
© Hervé de Brus

These Lunar Laser Ranging (LLR) telemetry observations are regularly processed and exploited in the INPOP planetary and lunar ephemerides developed since 2003 by the joint IMCCE / Geoazur (OCA) team. The lunar ephemeris makes it possible to calculate the orbital and rotational motion of the Moon with a precision of the order of one centimeter over 10 years. The introduction of a dynamic fluid core model into INPOP significantly improved the
residuals and probed the interior of the Moon.

The core model introduced in the 1980s into the US ephemerides model assumed that the core-mantle interface is spherical. Today, researchers from the INPOP team have introduced a slightly flattened core and it is the adjustment of this flattening from the LLR data that allowed deducing the size of the lunar fluid core. For this, the team compared the values adjusted to the LLR data with the theoretical value of the flattening of the lunar core at equilibrium. In considering the intersection of the two curves (values adjusted to the LLR observations and theoretical values), it is possible to constrain the size of the lunar core and the value of the core-mantle interface flattening. INPOP also uses the latest measurements of the gravity field determined by the GRAIL space mission.

This new measurement of fluid core size is important for models of the Moon evolution. It will help to better understand the mechanisms that allowed the appearance but also the disappearance of the lunar magnetic field. Such mechanisms are crucial for understanding the condition of life appearance.

Today the lunar exploration is in full growth and the addition of new reflectors on its surface would allow to continue the understanding of its interior and to improve relativistic tests.

Détermination du rayon du noyau lunaire.
L’aplatissement du noyau lunaire est tracé en fonction du rayon de la limite noyau-manteau. Les points noirs avec la zone d’incertitude rouge sont obtenus par l’analyse des données de télémétrie laser lunaire. Les courbes bleue et rose expriment les contraintes données par un modèle hydrostatique considérant deux valeurs différentes de l’épaisseur de la croûte lunaire (34 ou 43 km) avec une incertitude de ± 18 kg/m3 sur la densité moyenne de la croûte lunaire. La zone d’intersection correspond aux valeurs déterminées du rayon du noyau fluide lunaire et de son aplatissement.
© Viswanathan, V et al., Geophysical Research Letters, 8 juillet 2019.

This work was supported by the Labex Space Exploration, Planetary Environments, ESEP, PNGRAM and continuous observations of laser stations, especially the MEO station (OCA), France.

References

Viswanathan, V., Rambaux, N., Fienga, A., Laskar, J., Gastineau, M., 2019, "Observational constraint on the radius and oblateness of the lunar core-mantle boundary", Geophysical Research Letters, publié en ligne le 8 juillet 2019.
https://doi.org/10.1029/2019GL082677

Download

Scientific contacts

  • Nicolas Rambaux
    Maître de conférences
    Observatoire de Paris - PSL
    Sorbonne Université / IMCCE
    +33 (0) 1 40 51 22 63
  • Agnès Fienga
    Géoazur
    Observatoire de la Côte d’Azur
    +33 (0) 4 83 61 85 29

Press contact

Actualités