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

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.

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.

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.

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.

Artist view of the lunar interior structure. Three of the five laser retroreflectors (Apollo 11, 14, 15, Luna 17 and 21) are visible here. The laser beams originating from the Earth stations are symbolized by the green trails. The precise analysis of the lunar rotational and orbital motion has allowed the determination of the radius of the lunar core-mantle boundary (381 ± 12 km) and its oblateness ((2.2 ± 0.6) × 10^− 4) with unprecedented accuracy. Credits Y. Gominet/IMCCE/Observatoire de Paris–PSL

Laser ranging station at the observatoire de la Côte d’Azur, located at Calern. Credits Hervé de Brus

Lunar core radius determination. The lunar core oblateness is plotted versus the radius of the lunar core-mantle boundary. The black dots with the red zone of uncertainty are provided by the analysis of the lunar laser ranging (LLR) data. The blue and pink curves express the constraints given by a hydrostatic model considering two different values of the thickness of the lunar crust (34 or 43 km) with an uncertainty of ± 18 kg/m³ on the average lunar crustal density. The intersection zone corresponds to the determined values of both the radius of the lunar core-mantle boundary and its oblateness. Credits Viswanathan, V *et al.*, *Geophysical Research Letters*, 10 juillet 2019

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”, GRL accepted for publication.

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.