The Moon’s surface is a world of stark contrasts, preserving four and a half billion years of solar system history. Unlike Earth, the Moon lacks plate tectonics, wind, or water erosion, leaving its ancient geological record remarkably preserved. The landscape is a mosaic of features reflecting a heavy bombardment history and past volcanic activity, offering a window into the early evolution of rocky bodies.
The Lunar Highlands
The brighter, heavily cratered, and mountainous regions of the Moon are known as the lunar highlands, or terrae. These areas represent the Moon’s original crust, formed early in the satellite’s history through the cooling and solidification of a global magma ocean. The rocks found here are predominantly anorthosite, a light-colored, low-density igneous rock rich in calcium and aluminum.
This high-aluminum composition makes the highlands significantly more reflective, contributing to their bright appearance. The highlands are the oldest geological units on the Moon, with rock samples dating back as far as 4.5 billion years. Their age is confirmed by a dense concentration of overlapping impact craters, which have battered the crust.
The lunar highlands cover most of the far side of the Moon, but they also occupy extensive light-colored areas visible on the near side. The crust in these regions is notably thicker, reaching about 37 miles (60 kilometers) on the far side compared to 25 miles (40 kilometers) on the near side. This asymmetry contributes to the uneven distribution of the Moon’s major surface features.
The Vast Lunar Plains
In stark contrast to the bright highlands are the vast, dark, and relatively smooth areas called the maria, a Latin term meaning “seas.” Early observers mistakenly believed these plains were bodies of water, but they are immense sheets of solidified lava. The maria are composed of dark, iron-rich basalt, a volcanic rock that flowed into and filled large impact basins.
These volcanic flows occurred primarily between 3.9 and 3.0 billion years ago. The iron and titanium content in the basaltic lava absorbs more sunlight, causing the maria to appear much darker than the surrounding highlands. The plains are generally lower in elevation and cover about 16% of the Moon’s surface, with the vast majority concentrated on the Earth-facing side.
Due to their younger age, the maria have been subjected to fewer impacts and are significantly less cratered than the highlands. While smooth on a large scale, they contain surface features like wrinkle ridges, created by the compression and buckling of the heavy basalt layers. They also feature long, winding channels called rilles, which are thought to be the remnants of ancient collapsed lava tubes or channels.
Impact Structures
The most dominant process shaping the lunar landscape is impact cratering from meteoroids and asteroids. The Moon’s lack of atmosphere and geological activity means that craters are preserved almost indefinitely, serving as a record of bombardment throughout solar system history. Impact structures vary significantly in appearance based on their size, which dictates the physics of their formation.
Smaller impacts, typically under 10 kilometers in diameter, form simple craters that are bowl-shaped depressions with sharp rims. As the impact size increases, a transition occurs to complex craters, which feature flat floors, terraced inner walls formed by slumping, and a prominent central peak. This central uplift forms as the rock beneath the crater rebounds upward following the impact shock.
Beyond approximately 200 kilometers in diameter, the largest impacts create multi-ring basins, which are concentric circular features that can span thousands of kilometers. All craters are surrounded by an ejecta blanket, a sheet of material blasted out during the impact event that thins with distance from the rim. The youngest craters are identified by bright rays of ejecta that extend radially outward across the surface.
The Surface Material
The entire visible surface of the Moon, covering both the ancient highlands and the younger maria, is blanketed by a layer of loose, fragmented material called regolith. This powdery, unconsolidated layer is not true soil but is the product of continuous bombardment by micrometeorites and larger impacts. The constant impacts mechanically grind the bedrock into a mixture of rock fragments, mineral grains, and glass spherules.
Regolith thickness varies significantly depending on the age of the underlying surface. In the younger maria, the regolith may only be about 13 to 16 feet (4 to 5 meters) deep. Conversely, in the ancient, heavily bombarded highlands, the regolith layer can reach depths of 33 to 66 feet (10 to 20 meters).
The material itself is highly abrasive and angular, composed of particles ranging from sub-micron dust to house-sized boulders. A unique component of lunar regolith is the presence of agglutinates, which are tiny, glassy beads formed when micrometeorite impacts melt and fuse surrounding mineral and rock fragments. Beneath the regolith lies the megaregolith, a zone of heavily fractured bedrock that extends several kilometers deep into the crust.