How Was the Lunar Maria Formed?

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Lunar maria (plural for mare) are smooth and dark basaltic plains on the Moon's surface resulting from lava flows that fill the crater-like basins. This write-up discusses the formation of the lunar maria.
Formation of Lunar maria
Did You Know?
In 1969, the spaceflight, Apollo 11, that assisted the first humans to descend on the Moon, landed in a minor lunar mare, Mare Tranquillitatis, the Sea of Tranquility.
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Lunar maria are smooth volcanic plains on the surface of the Moon. In Latin, the word Maria means seas, and the lunar maria were named as such due to their sea-like appearance. However, although they appear like a watery surface, they are just dark-colored rocks. These dark regions are made up of basalt, which is a type of igneous rock. The maria cover around 17% of the Moon's surface. They contain features like pits and channels. They are less reflective than the highlands, which are the lighter spots on the Moon, due to the high iron content in the area. Hence, they appear dark to us.

The maria cover different parts on the Moon. They are distributed across the Moon's surface in the near and far side. The near side is the part of the Moon that faces towards us and is visible from Earth, while the far side of the Moon is the part that is not visible from the Earth.

The lunar near side include Mare Imbrium, Mare Humorum, Mare Nubium, Mare Frigoris, Mare Serenitatis, Mare Vaporum, Mare Tranquillitatis, Mare Crisium, Mare Anguis, Mare Undarum, Mare Fecunditatis, Mare Nectaris, Mare Cognitum, Mare Insularum, Mare Marginis, Mare Smythii, Mare Orientale, Mare Spumans, Mare Humboldtianum, and Oceanus Procellarum. The lunar maria in the far side of the Moon include Mare Moscoviense and Mare Ingenii. The Mare Australe is located in the overlapping region of the near side and far side of the Moon.

Formation of Lunar Maria

Lunar Maria
Lunar Maria

✦ The Moon has many number of craters (circular depressions in the surface of a planet or Moon) and large flat maria on its surface. At the period of formation of the solar system, there were many materials left over, mostly in the gaseous state or in the form of dust or small and large bodies such as asteroids, meteorites, and comets. It is said that these materials collided with the Moon, thus leading to the formation of basins and craters. These craters measure up to many hundreds of kilometers.

✦ Due to the Moon having no atmosphere, it has no protection from being bombarded by the bodies. Hence, the Moon's temperature is hotter than the Earth as it absorbs more light and heat rather than reflecting it, unlike the Earth. Additionally, there are no erosions and geological changes to wear away these craters.

✦ The lunar maria were formed in the period between 3.9 to 4.5 billion years ago, as measured by radiometric dating, when a long sequence of volcanic eruptions took place on the Moon. Volcanic substances came out from the Moon's interior and flew to the surface through cracks in the lunar surface, which were formed during the bombardment. These lava flows flooded and covered many craters in their path, thus forming the lunar maria. For this reason, the number of craters in the lunar maria are fewer than other places on the Moon. The size of the largest lunar maria is over 1,000 km across. The composition of the lunar maria comprise igneous rocks called basalts that are formed when the lava gets cooled and solidified on the surface.

Lunar Maria Distribution

✦ Most of the maria are formed on the near side of the Moon, while very few are formed on the far side. The reason for this is still unclear. An explanation states that the crust is thicker on the far side, and so the volcanic materials easily rose up on the near side due to the thinner crust in comparison to the far side. Hence, there would be fewer maria on the far side, as we observe.

✦ However, another theory contradictorily states that the thinnest crust on the Moon is present on the far side, thus suggesting the case just opposite to the previous explanation. But, what we observe is that there are more maria on the near side. Thus, the idea failed to explain the distribution of maria on the near and far sides.

✦ Yet another theory was that more radioactive materials are present in the interior on the near side than on the far side. This does not explain the distribution because the bombardment that formed the Moon would have most likely led to an equal mixing of all materials.
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The lunar maria also include one oceanus (ocean) along with features known as lacus (lake), palus (marsh), and sinus (bay).