Scientists study the Moon’s subsurface by measuring how gravity, radar signals and seismic waves behave, then using physical models to infer what lies below. These methods do not produce a direct image of the entire lunar interior: each constrains different properties, and researchers compare the results to distinguish plausible explanations.
What can scientists measure without drilling?
Remote and surface instruments measure signals affected by material beneath the Moon’s visible surface. Orbiters can map gravity and elevation; radar instruments analyze returned radio energy; seismometers record ground motion. Researchers interpret those observations with models to estimate hidden structure. Surface composition and temperature maps provide geological context, but they are not, by themselves, images of buried layers.
That distinction matters: an instrument records a signal, not a labeled picture of rock types underground. The subsurface conclusions are inferences, and more than one structure can sometimes produce similar measurements.
How gravity mapping reveals hidden mass
NASA’s GRAIL (Gravity Recovery and Interior Laboratory) mission used two spacecraft flying in formation. As the Moon’s gravity varied along their paths, the distance between the spacecraft changed slightly. Scientists used those changes in separation, along with spacecraft tracking, to build models of the lunar gravity field. NASA Ames says the work used 7 million tracking observations to calculate precise spacecraft orbits and lunar geodetic characteristics (NASA Ames Research Center); the figure describes the mission’s analysis, not a count of direct underground images.
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Gravity variations indicate differences in mass distribution, but gravity alone cannot identify a unique rock type or buried structure. Surface relief also affects the gravity signal. NASA’s explanation of Bouguer gravity mapping describes subtracting the expected gravity effect of the uneven surface, using topographic measurements such as those from the Lunar Reconnaissance Orbiter (LRO), so remaining anomalies can be interpreted in relation to subsurface mass (NASA Science).
NASA reports that GRAIL results support a view of the Moon’s crust as less dense, more porous and more fractured than previously thought. The mission also found gravity anomalies around Oceanus Procellarum that NASA interprets as a rectangular pattern of ancient rift valleys flooded by lava and buried. These are interpretations of gravity and topographic evidence, not photographs of the structures (NASA Ames Research Center; NASA Science).
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What radar adds
Radar instruments transmit radio energy and analyze the signal that returns. NASA describes LRO’s Mini-RF as a radar technology demonstration intended to image polar regions and search for water ice (NASA Astrobiology). Radar therefore provides a different kind of evidence from gravity mapping: it is based on returned radar energy rather than changes in spacecraft motion caused by mass distribution.
The cited NASA description does not establish a single general penetration depth for lunar radar or show that radar sees through all lunar regolith. Its role should be understood in terms of its documented polar imaging and ice-search objectives, not as a universal way to see through the Moon.
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How moonquakes help constrain structure
Seismometers measure ground motion. Scientists study the arrival times and behavior of seismic waves and compare them with modeled paths to estimate seismic velocities and infer structure at depth. The measurement is the vibration; the interpretation of the interior depends on how waves are modeled.
Apollo missions deployed four seismometers between 1969 and 1972, and the network operated until 1977. The stations were clustered on the near side, leaving sparse near-surface ray coverage across the Moon. A NASA Technical Reports Server abstract describes combining Apollo seismic-delay data with GRAIL gravity data to constrain velocity and density with depth; using the two datasets together can reduce ambiguity, but does not remove dependence on models (NASA Technical Reports Server, 2014 abstract).
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Why researchers combine methods
Each method responds to a different property, covers the Moon differently and has its own interpretive limits. Gravity is sensitive to mass differences, radar analyzes returned radio energy, and seismology records waves traveling through the ground. Topography and surface-composition observations help establish geological context. Comparing these forms of evidence can rule out some explanations that would remain plausible if only one signal were considered.
| Method | What is measured | What it helps constrain | Key limitation |
|---|---|---|---|
| GRAIL gravity mapping | Small changes in the separation of two orbiting spacecraft as lunar gravity varies | Mass distribution and crustal-scale gravity anomalies | Gravity does not uniquely identify the material or structure causing an anomaly; topography must be considered. |
| LRO Mini-RF radar | Returned radio energy | Polar-region imaging and searches for water ice, as described by NASA | The cited source does not give a general depth limit or support a claim that radar sees through all regolith. |
| Apollo seismometers | Ground motion and seismic-wave arrivals | Wave velocities and internal structure inferred through models | Four stations were clustered on the near side, limiting geographic and near-surface ray coverage. |
| LRO elevation and surface observations | Surface elevation and measurements including temperature, composition and radiation environment | Topographic correction and geological context for interpreting other measurements | These observations provide context; the cited overview does not present composition mapping alone as a direct subsurface image. |
The table compares each method’s evidence and limitations, not a shared numerical ranking of resolution or depth. NASA’s cited sources do not provide a common quantified performance comparison across these instruments.
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How this differs from collecting lunar samples
Remote mapping and seismic inference are not the same as physically examining material. NASA says Apollo missions returned 382 kilograms (842 pounds) of lunar rock and soil (NASA Science). Samples provide material collected from specific places; orbital and seismic methods help researchers investigate a broader subsurface without making a borehole. Neither form of evidence should be treated as a complete, direct view of the Moon’s interior.
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