The Moon’s face presents a striking visual duality, characterized by a contrast between bright and dark areas. Early observers mistook these features for continents and oceans, but we now understand they are vast geological regions defining the lunar landscape. The lighter regions are elevated, rugged, and heavily marked by impacts, representing the true mountainous terrain of our satellite. Scientists use specific Latin terms to distinguish the broad highlands from the specific mountain ranges and low-lying plains.
The Bright Regions Understanding the Lunar Highlands (Terrae)
The lighter, brighter regions that cover the majority of the Moon’s surface are formally known as the lunar highlands, or terrae (Latin for “lands”). These areas are significantly higher in elevation than the surrounding plains, and they represent the oldest crustal material on the Moon, with some rocks dating back over 4 billion years. The rugged appearance of the highlands is a direct result of the intense bombardment they endured during the solar system’s early history. They are saturated with impact craters, many of which overlap, creating the complex and elevated terrain.
The composition of the terrae is key to their bright appearance, as they are primarily made of anorthosite. This type of igneous rock is rich in aluminum and calcium silicates and is lighter in color than the material found in the plains. Because anorthosite reflects more sunlight, the highlands stand out as the luminous, dominant feature. The high elevation and density of ancient, eroded craters make the highlands the Moon’s fundamental mountainous zones.
The Dark Regions Context of the Lunar Plains (Maria)
The dark regions are called maria (Latin for “seas”). These low-lying plains are much smoother and less cratered than the terrae, indicating that they formed later in lunar history. The maria are large, flat expanses that cover about 16% of the lunar surface, mostly concentrated on the near side visible from Earth.
The formation of these dark areas resulted from ancient volcanic activity that occurred primarily between 3.3 and 3.8 billion years ago. Massive impact events created deep basins, and subsequent eruptions flooded these depressions with highly fluid basaltic lava. This lava solidified into dense, iron-rich basalt rock, which is less reflective than the anorthosite of the highlands, causing the maria to appear dark.
Naming Specific Peaks and Ranges (Montes)
While terrae is the general term for the Moon’s elevated, mountainous regions, the specific, named mountain ranges and isolated peaks are called Montes (plural) or Mons (singular). These features are typically found along the rims of large impact basins later filled by the maria, such as the prominent Montes Apenninus. The International Astronomical Union (IAU) governs the formal naming of these structures, often naming them after terrestrial mountain ranges, nearby craters, or notable scientists.
A related feature is the Dorsum (plural dorsa), which refers to a system of wrinkle ridges commonly found on the surface of the maria. These features are not true mountains but rather tectonic compressional landforms created as the dense basaltic lava flows cooled and contracted. While the entirety of the terrae constitutes the Moon’s mountainous terrain, Montes is the specific nomenclature used for an individual mountain or mountain chain.