The Moon’s surface displays a dramatic contrast between bright, heavily cratered areas and expansive, darker patches. These dark regions are known as maria (Latin for “seas”). Scientists have determined that maria formed through a complex history of massive impacts and ancient volcanic activity. Understanding the maria provides insight into the geological timeline of the Moon and the early solar system.
Defining Lunar Maria
The term “mare” originated in the 17th century when astronomers, including Galileo, first observed the Moon through telescopes and mistook the dark, flat plains for bodies of water. Although the Moon is devoid of liquid surface water, the Latin nomenclature persisted, resulting in names like Mare Tranquillitatis (Sea of Tranquility).
Lunar maria are vast, relatively smooth plains covering about 16% of the Moon’s surface. They contrast sharply with the lighter, rugged, and heavily cratered lunar highlands. The high concentration of maria on the near side of the Moon forms the familiar “Man in the Moon” pattern visible from Earth.
The maria are recognizable because they are significantly less reflective than the surrounding terrain. Their smooth, low-elevation surfaces appear dark against the brighter, high-altitude highlands. This striking difference in reflectivity is a purely geological phenomenon.
The Geological Process of Formation
The formation of the lunar maria was a two-step process spanning hundreds of millions of years in the Moon’s early history. The first step involved intense bombardment by large asteroids, particularly during the Late Heavy Bombardment (4.1 to 3.8 billion years ago). These massive collisions gouged out enormous, deep circular depressions known as impact basins.
The second step occurred as the Moon’s internal heat caused magma to rise toward the surface. The crust was fractured and thinned by the impacts, allowing low-viscosity lava flows to well up through fissures and vents. This molten material spread out, flooding the vast impact basins and surrounding low-lying areas.
These flood basalt eruptions took place primarily between 3.9 and 3.0 billion years ago. As the highly fluid lava cooled and solidified, it created the expansive, flat plains we observe today. These ancient flows are evidence of a time when the Moon’s interior was hot enough to support widespread volcanism.
Composition and Physical Characteristics
The dark appearance of lunar maria is directly related to their chemical composition. The material that flooded the basins is a dense, fine-grained volcanic rock called basalt, similar to rock found on Earth’s ocean floors. Mare basalt is rich in heavy elements, particularly iron, magnesium, and titanium, giving it a distinctly dark color and low albedo.
Because the basalt is not very reflective, it absorbs more sunlight and appears darker than the rest of the Moon. In contrast, the lighter lunar highlands are composed of anorthosite, an aluminum-rich rock that is highly reflective. The crust under the maria is also thinner than the crust under the highlands.
The solidified lava flows are associated with concentrations of mass, known as “mascons,” which are regions of dense material below the surface. Mascons create gravitational anomalies that affect the orbits of spacecraft. The surface of the maria is covered by regolith, a layer of pulverized rock and dust formed by billions of years of micrometeoroid bombardment.