The dark areas visible on the Moon’s surface are the most recognizable features when observing our nearest celestial neighbor. These large, relatively smooth patches are often mistaken for shadows or water, but they are neither. They represent vast, ancient plains of solidified lava, a testament to the Moon’s tumultuous early history. Understanding their nature requires knowing the specific name given to these dark regions.
The Official Name Lunar Maria
The official astronomical term for the dark spots is maria, the Latin plural for “seas.” The singular form is mare. This aquatic terminology stems from a historical misunderstanding dating back to the 17th century, when early astronomers first viewed the Moon through telescopes. They believed these smooth, dark patches were actual bodies of water, similar to oceans or lakes on Earth.
This misnomer persists in the names of individual features, such as Mare Tranquillitatis (the Sea of Tranquility), the landing site for Apollo 11. Other named features include oceanus (ocean), lacus (lake), and sinus (bay). Although we now know they contain no water, the name is permanently established in lunar geography, covering approximately 16% of the Moon’s surface.
Composition and Appearance
The distinct dark color of the maria results from their geological composition: primarily dense, iron-rich volcanic rock called basalt. This basalt is chemically similar to dark lava found on Earth and solidified from massive flood eruptions billions of years ago. Iron and titanium oxides within the rock give it a low albedo, meaning it absorbs more sunlight and reflects less back to space, thus appearing dark.
This dark basalt contrasts sharply with the Moon’s lighter-colored areas, known as the lunar highlands or terrae. The highlands are composed mainly of anorthosite, an older, less dense, and highly reflective rock. The maria occupy lower elevations and are noticeably smoother than the heavily cratered highlands. These plains have fewer impact craters because they formed later, covering up older scars left by early bombardment events.
The Formation Process
The geological process that created the maria began with intense impact bombardment early in the Moon’s history. Between 3.9 and 4.4 billion years ago, massive asteroid-sized bodies struck the Moon, gouging out immense, deep impact basins. These basins created weak points and low-lying areas in the lunar crust.
Later, the Moon’s interior heated up, causing partial melting of the mantle material deep beneath the surface. This molten rock, or magma, was less dense and began to rise. It eventually seeped up through large fissures and cracks, often associated with the boundaries of the giant impact basins.
Vast floods of low-viscosity basaltic lava subsequently filled these enormous depressions. The primary period for these eruptions is known as the Imbrian period, occurring roughly between 3.3 and 3.8 billion years ago. As the lava cooled and solidified, it created the expansive, flat, dark plains we observe today. The sheer volume of the flows buried nearly all smaller, pre-existing craters within the basins, which is why the maria appear much less pockmarked than the ancient highlands.