The Moon presents a striking visual contrast between its bright, heavily cratered regions and its darker, smoother plains. These dark patches are the most prominent features on the lunar surface visible from Earth. They cover approximately 16% of the Moon’s surface and are known collectively as the lunar maria, or simply maria. Understanding the maria involves exploring their historical naming, geological makeup, the processes that created them, and their uneven distribution.
Celestial Seas: Defining Lunar Maria
The name “maria” (pronounced MAH-ree-ah) is the plural of the Latin word mare, meaning “sea”. Early astronomers in the 17th century, observing the Moon through rudimentary telescopes, mistook these large, flat, dark areas for bodies of water. This terminology persisted; the largest dark area on the Moon is still named Oceanus Procellarum, the “Ocean of Storms”.
These features are low-elevation plains that are remarkably smooth compared to the rugged, mountainous lunar highlands. Although devoid of water, the maria are vast, generally circular or irregular basins easily discernible to the unaided eye. The visible contrast between these dark plains and the brighter highlands forms the familiar image of the “Man in the Moon”.
The Basaltic Makeup of Maria
The dark color of the maria is due to their composition of dense volcanic rock called basalt. This is the same type of dark, fine-grained rock that makes up much of Earth’s ocean floor and lava flows. Samples returned by the Apollo missions confirmed that the maria are solidified sheets of ancient lava.
The mare basalts are rich in heavy elements like iron and magnesium, and often contain higher concentrations of titanium dioxide (TiO2). This composition causes them to absorb more sunlight, making them appear dark and less reflective. In contrast, the lighter-colored lunar highlands are composed primarily of anorthosite, a less dense, calcium and aluminum-rich rock that reflects more light.
A History of Impacts and Volcanism
The formation of the maria involved a two-step geological process spanning billions of years. The process began around 3.9 billion years ago with massive impact events from asteroid-sized bodies. These impacts excavated enormous, deep basins into the Moon’s crust, which are the fundamental structures underlying the maria.
Following the impacts, the Moon’s internal heat caused the mantle rock to melt, creating low-viscosity magma. This molten rock then seeped up through large fractures and fissures in the crust. A long sequence of volcanic eruptions flooded the impact basins with enormous sheets of basaltic lava until perhaps 3 billion years ago. The lava cooled and solidified into the flat, dark plains observed today, effectively resurfacing the older, heavily cratered terrain.
The Near Side Mystery
A significant feature of the lunar maria is their uneven distribution across the Moon’s surface. They are overwhelmingly concentrated on the near side—the face that always points toward Earth—covering about 31% of that side. In stark contrast, the far side of the Moon is largely devoid of maria, covering only about 1% of its surface.
The leading scientific explanation for this asymmetry relates to a difference in crustal thickness. The Moon’s crust is nearly twice as thick on the far side as it is on the near side. This thinner crust on the near side made it easier for the deep mantle magma to breach the surface and fill the basins. Additionally, some theories suggest a higher concentration of heat-producing elements like potassium, rare-earth elements, and phosphorus (KREEP) on the near side, which may have contributed to the melting and volcanism.