9.2 The Lunar Surface

Lunar Surface Features and Composition

The Moon has no atmosphere, no weather, and no plate tectonics. That means its surface is essentially a fossil record of billions of years of impacts and volcanic activity. By studying lunar features, you can piece together the history of the early solar system.

Features of the Lunar Surface

Maria are the dark, smooth, low-lying plains visible even with the naked eye. They formed when ancient volcanic eruptions flooded large impact basins with basaltic lava, which then cooled and solidified. Despite looking prominent, maria cover only about 20% of the Moon's surface. Major examples include Mare Tranquillitatis (where Apollo 11 landed), Mare Serenitatis, and Mare Imbrium.

Highlands are the bright, heavily cratered regions that make up roughly 80% of the surface. They're composed of anorthositic rock (a light-colored, aluminum-rich mineral) and represent the oldest surviving parts of the lunar crust. Because they've been exposed to impacts for so long, they're far more cratered than the maria.

Craters are circular depressions formed by the impacts of meteoroids, asteroids, and comets. They range from microscopic pits to basins hundreds of kilometers across. Notable examples include Tycho (with its bright ray system visible from Earth), Copernicus, and Aristarchus. Since there's no erosion from wind or water, craters on the Moon can persist for billions of years.

Rilles are long, narrow depressions on the surface. Some formed from collapsed lava tubes or channels carved by flowing lava, while others are linked to tectonic processes like faulting. Hadley Rille, visited by Apollo 15, is one of the best-known examples.

Evolution of the Moon's Landscape

The Moon's surface formed through a sequence of major stages:

1. Formation (~4.5 billion years ago): A Mars-sized body (sometimes called Theia) collided with the early Earth, ejecting debris that coalesced into the Moon.
2. Magma ocean stage: The young Moon was covered by a global ocean of molten rock. As it cooled, lighter anorthositic minerals floated to the top, forming the highlands crust. Denser minerals like pyroxene and olivine sank to form the mantle.
3. Late Heavy Bombardment (~4.1–3.8 billion years ago): An intense period of impacts scarred the surface with enormous craters and basins.
4. Volcanic flooding (~3.9–3.2 billion years ago): Heat from radioactive decay in the interior melted mantle rock. Basaltic lava erupted and filled many of the large impact basins, creating the dark maria we see today.
5. Gradual weathering (3.2 billion years ago to present): With large-scale volcanism over, the surface has been slowly modified by continued meteoroid impacts, micrometeorite strikes, solar wind bombardment, and cosmic rays. This ongoing process is called space weathering.

Composition of Lunar Regolith

Regolith is the blanket of loose, broken-up rock and dust that covers the Moon's entire surface. It's not soil in the Earth sense because it contains no organic matter or significant water.

How it forms: billions of years of meteoroid impacts, micrometeorite strikes, and solar wind exposure have gradually shattered the bedrock into fragments ranging from fine dust to small rocks.

Key characteristics:

• Composed of minerals including anorthite, pyroxene, olivine, and ilmenite
• Contains tiny beads of impact glass created when micrometeorite strikes melt surface material
• Thickness varies: about 2–3 meters in the younger maria, but 10–20 meters in the older highlands (more time exposed to impacts means thicker regolith)
• Acts as an insulating layer, protecting the lunar interior from the extreme surface temperature swings (roughly +127°C in sunlight to −173°C in shadow)
• Preserves a record of the Moon's bombardment history and surface evolution

Regolith is also a potential resource for future missions. Ilmenite in the regolith contains oxygen that could theoretically be extracted, and the material itself could serve as radiation shielding for habitats.

Lunar Dynamics and Phenomena

Moonquakes are seismic events detected by instruments left by Apollo astronauts. They're generally weaker than earthquakes but can last much longer (up to an hour or more) because the dry, fractured lunar crust doesn't dampen vibrations the way Earth's does. Some are triggered by tidal stresses from Earth's gravity, while others originate deep in the lunar interior.

Mascons (short for "mass concentrations") are regions of unusually dense material beneath the surface, typically found under large impact basins like Mare Imbrium. They create slight gravitational anomalies that affect the orbits of spacecraft around the Moon.

Lunar phases refer to the changing appearance of the Moon as seen from Earth. As the Moon orbits Earth, different portions of its sunlit side face us. This cycle from new moon to full moon and back takes about 29.5 days (a synodic month). The phases aren't caused by Earth's shadow; they result from the changing angle between the Sun, Moon, and Earth.