When observing Earth’s Moon, distinct dark patches are visible to the unaided eye. These features are known as “maria,” a Latin term meaning “seas,” which early astronomers mistakenly believed to be bodies of water. In reality, lunar maria are vast, flat plains composed of solidified volcanic rock. This article will explore the characteristics, formation, and scientific importance of these unique lunar landscapes.
Characteristics of Lunar Maria
Lunar maria appear as large, dark, and relatively smooth areas on the Moon’s surface, contrasting sharply with the brighter, heavily cratered lunar highlands. Their darker appearance is due to their composition, primarily basalt, a dense, iron-rich volcanic rock. This basalt reflects less sunlight than the anorthosite rock found in the highlands, making the maria appear dim.
These expansive plains vary significantly in size, ranging from hundreds to over a thousand kilometers in diameter. Most of the approximately 20 major maria are located on the Moon’s near side. This uneven distribution is related to differences in crustal thickness, with the near side having a thinner crust that allowed magma to reach the surface more easily.
The surfaces of the maria, though appearing smooth from a distance, are not entirely uniform. They contain features such as wrinkle ridges, long, low ridges formed by the compression of lava flows, and rilles, channels likely carved by flowing lava or collapsed lava tubes. The maria also have fewer impact craters compared to the highlands, indicating they are geologically younger.
Formation of Lunar Maria
The formation of lunar maria is a two-step geological process that occurred billions of years ago. It began with massive impacts from asteroids or comets, which struck the Moon’s surface and created enormous depressions known as impact basins, some over 2,000 kilometers in diameter.
Following these immense impacts, volcanic activity within the Moon caused molten rock to rise through fissures in the lunar crust. This basaltic lava then flooded the newly formed basins. Over time, this molten rock cooled and solidified, forming the flat, dark plains observed today.
This period of extensive volcanic flooding occurred primarily between 3.5 and 1.2 billion years ago, with most mare basalts erupting between 3 and 3.5 billion years ago. While the impact events created the initial depressions, subsequent volcanic eruptions filled them, significantly reshaping the lunar landscape.
Prominent Lunar Maria
Several lunar maria are particularly well-known, each with unique characteristics or historical significance. Oceanus Procellarum, or the “Ocean of Storms,” is the largest of all lunar maria, spanning over 2,500 kilometers and covering about 4 million square kilometers on the Moon’s western near side. It is unique in that it does not correspond to a single, distinct impact basin.
Mare Tranquillitatis, known as the “Sea of Tranquility,” is perhaps the most famous mare, serving as the landing site for Apollo 11, the first human mission to the Moon in 1969. Rock samples from this mare have provided valuable insights into its formation around 3.8 to 3.9 billion years ago. Mare Imbrium, or the “Sea of Rains,” formed within a large impact basin with a diameter of about 750 kilometers.
Mare Serenitatis, the “Sea of Serenity,” is a circular mare that often forms the left eye of the “man in the Moon” when viewed from Earth. This mare was visited by Apollo 17 in 1972, the last human mission to the Moon, and also by the Soviet Union’s Lunokhod 2 rover.
Scientific Significance and Exploration
Studying lunar maria provides valuable insights into the Moon’s geological history, internal structure, and the early solar system. Their basaltic composition helps scientists understand the Moon’s volcanic past and the processes that shaped its surface over billions of years. The relatively low number of craters within maria, compared to the highlands, helps in dating different lunar surfaces.
Analysis of mare basalts offers clues about the Moon’s internal formation and evolution, information that can be extended to develop models for other moons and planets. The Apollo missions brought back samples from various maria, allowing for detailed analysis of their composition and age.
Future lunar missions continue to target these regions for research, building upon the knowledge gained from past explorations. Investigations of mare basalt samples, such as those from China’s Chang’e-5 mission, have shown some lunar basalts could be as young as 2.03 billion years old, refining the timeline of lunar volcanism. The ongoing study of maria helps unravel the Moon’s evolution and its place in the broader context of planetary geology.