GRAIL Twin Probes: Mapping the Moon's Hidden Interior

GRAIL twin probes in lunar orbit — The two twin probes of the GRAIL mission orbit the Moon while maintaining a controlled distance to measure gravitational variations.
Image source: NASA/JPL (new window)

A twin mission to probe lunar gravity

The GRAIL (gravity Recovery and Interior Laboratory) mission, launched in 2011 by the NASA (national Aeronautics and Space Administration), aimed to measure variations in the Moon's gravitational field with unprecedented precision. The identical probes, Ebb and Flow, were placed in near-circular polar orbits to detect even the smallest density anomalies inside the Moon. Their operation relied on ultra-precise measurement of the distance between them via a microwave link sensitive to micron-scale variations.

Principle of differential gravimetric measurement

When one probe passes over a denser lunar region, it experiences slightly stronger gravitational attraction and accelerates. This relative variation changes the distance between the two satellites, continuously measured by microwave interferometry. By correlating these distance changes with orbital position, scientists reconstructed a global gravitational map of the Moon, revealing its internal structure with unprecedented resolution. The principle is based on the relation \(\Delta g = \frac{GM}{r^2}\), where \(\Delta g\) represents the local variation of the gravitational field.

Results and geophysical discoveries

The obtained maps allowed estimation of the lunar crust thickness between 34 and 43 km, thinner than expected. They also revealed a complex network of fractures beneath large impact basins, indicating a rigid and fractured crust. These data provided essential insights into mantle differentiation and the crystallization of the initial lunar magma, as well as the satellite's thermal evolution since formation.

Table of main orbital parameters

Orbital and technical parameters of the GRAIL mission
Parameter	Value	Unit	Comment
Average altitude	55	km	Near-circular orbits for optimal gravimetric resolution
Average distance between probes	200	km	Continuously measured via microwaves, sensitive to <1 µm
Orbital period	113	minutes	Fine synchronization to detect local gravitational gradients
Gravitational resolution	≈30	km	Spatial limit of the final map produced by GRAIL
Scientific mission duration	9	months	March to December 2012, before controlled lunar impact

Source: NASA - GRAIL Mission Overview (new window).

Scientific legacy

The principle of differential gravimetric measurement directly derives from the GRACE (gravity Recovery and Climate Experiment) project, developed in the early 2000s by Byron Tapley (1936-) and collaborators. Both missions demonstrated that gravity could become a planetary internal probing tool as powerful as seismology.