Lunar Probe
Fundamentals guide

The Moon: A Scientific Guide to Earth's Natural Satellite

A connected introduction to lunar origin, interior, surface, orbit, environment and exploration.

Key takeaways

  • The Moon is a differentiated rocky world with crust, mantle and a small core.
  • Its surface preserves an unusually long impact and volcanic record because erosion and plate tectonics do not continually recycle the landscape.
  • Near-side maria, far-side highlands, impact basins, polar cold traps and local magnetic anomalies record different parts of lunar history.
  • Modern understanding combines returned samples with orbital imaging, spectroscopy, gravity, laser altimetry, thermal measurements and seismology.
The Moon is best understood by combining physical models with measurements from orbit, the surface and returned samples.

The Moon as a planetary body

The Moon is Earth's natural satellite, but scientifically it is also a small differentiated planetary body with its own geological history. Its average radius is about 1,737 kilometres and its surface gravity is roughly one sixth of Earth's. Those numbers shape everything from crater morphology to spacecraft landing and astronaut movement.

The Moon has no dense atmosphere, no oceans and no active plate-tectonic system comparable with Earth's. As a result, many ancient surfaces remain exposed. Craters that would be erased or buried on Earth can survive for billions of years, making the lunar surface an archive of early Solar System impacts.

Inside the Moon

Apollo seismology, gravity measurements and later geophysical studies indicate that the Moon has a crust, mantle and small metallic core. The crust is not equally thick everywhere, and large impact basins have altered both crustal structure and gravity.

The interior is not completely inactive. Moonquakes, tidal stresses and evidence of a partially molten region deep inside show that the Moon is more complex than a geologically dead rock. The scale of present activity is modest compared with Earth, but it still matters for lunar geophysics and future surface infrastructure.

Why the near side looks different from the far side

The familiar near side contains broad dark maria formed by basaltic lava that flooded large impact basins. The far side is dominated by brighter, thicker highland crust and has far less exposed mare basalt.

This asymmetry is one of the central questions in lunar evolution. Differences in crustal thickness, heat-producing elements, impact history and mantle melting all contribute to the explanation. The far side is not permanently dark; it receives sunlight through the lunar day just as the near side does.

A surface built by impacts and volcanism

Impacts excavated craters from metre scale to enormous multi-ring basins. The largest events reshaped regional crust and created low areas that later became sites for lava flooding. Mare volcanism then produced the dark basalt plains visible to the naked eye.

Smaller volcanic landforms include sinuous rilles, domes and pyroclastic deposits. Their distribution and ages show that lunar volcanism was not one brief episode, and sample-return missions continue to refine how long volcanic activity persisted.

The lunar poles

Near the poles, the Sun remains low on the horizon. Topography therefore creates unusual combinations of extended illumination and permanent shadow. Some crater interiors never receive direct sunlight and can become extremely cold.

These permanently shadowed regions can preserve water ice and other volatile compounds. Their scientific value is high because volatiles may record delivery, migration and loss processes. Their engineering value is more complicated: a detected volatile is not automatically concentrated, accessible or economical to extract.

How we know what we know

Returned samples provide laboratory measurements of mineralogy, chemistry, isotopes and radiometric ages at specific landing sites. Orbiters extend coverage globally through imaging, spectroscopy, radar, gravity, topography and thermal mapping.

No single dataset answers every question. A spectral absorption can indicate composition but may not reveal physical form. A crater-count age depends on calibration. A sample gives excellent ground truth but only for its geological context. Lunar science becomes strongest when independent measurements agree.

Sources and further reading