Does the term “man in the moon” sound familiar?
The face in the Moon that many of us learn to recognize as children is formed by features called lunar maria which are the darker regions on the Moon’s surface.
The term lunar maria means ‘sea on the Moon’. Mare (pronounced MAHR-ay) is the Latin word for sea, the plural of which is maria.
Early humans mistook these dark plains to be real seas, which is where the name comes from. However, we know today that these dark gray zones are formed from iron-rich basalts produced in volcanic eruptions.
The Moon‘s maria cover about 16% of its surface, with the majority found on the Moon’s Earth-facing side. The major seas visible to us are shown in the image below.
Lunar Mare Formation
Around 4.1 billion years ago, the newly-formed Moon was still cooling after its formation. It formed a thin crust on the outermost surface on top of hot bubbling magma.
It was then that the Moon witnessed huge asteroid impacts that cracked the surface, leaving large craters in their wake.
The liquid magma underneath the surface flowed out through the cracks and filled the craters, where it eventually cooled. These are areas that we identify as lunar maria.
They are flatter regions than the surrounding uplands which is a result of the magma flowing and leveling before it cools and hardens to rock.
Why are Lunar Maria Dark?
The magma that flowed out onto the craters contained iron, magnesium and titanium. Mainly due to the high concentration of iron, lunar maria reflect ½ of the light compared to the surrounding lighter regions.
Lower reflectivity means less light reaches our eyes from these areas than from the surrounding uplands, this is why the maria appear darker.
Are There Maria on the Far Side of the Moon?
Following the first Soviet exploration of the Moon’s far side in 1959, scientists noted that it has fewer flat, dark maria than the side locked facing Earth.
Yet, impact craters are uniformly seen on both sides of the Moon, so it took some time and further analysis to discover why there are fewer lunar maria on the dark side of the Moon.
We now believe that the crust is twice as thick on the far side of the Moon, compared to the near side which always faces us. The crust is thicker (scientists think) because it cooled more quickly when the Moon was formed.
The Earth and Moon were both huge lumps of molten rock when the Moon was created. The Moon cooled more quickly than Earth because it is much smaller.
As the Moon cooled, the side facing Earth stayed warmer. This is because the Earth was also still molten and radiated heat towards the side of the Moon facing it. More materials condensed on the far side of the Moon as it cooled more quickly, leaving it with a thicker crust.
The thicker crust reduced the number of Maria because it was harder for meteor impacts to break it open and let lava flow into the craters present on the Moon’s dark side.
How Are Lunar Maria Named?
Like most things in astronomy, there is a naming convention for the Moon’s seas.
Before we look at what the convention is, it’s worth noting the two maria that had their names fixed before the convention was established: Mare Humboldtianum and Mare Smythii. The former is named for the German polymath Alexander von Humboldt, the latter for British astronomer William Henry Smyth.
All of the other lunar maria are named for either sea features, sea attributes, or states of mind.
Sea Attributes
These lunar maria are named for sea attributes:
- Mare Australe – Southern Sea
- Mare Cognitum – Sea of Knowledge
- Mare Marginis – Sea of the Edge
- Mare Orientale – Eastern Sea
Sea Features
These lunar maria are named for sea features:
- Oceanus Procellarum – Ocean of Storms
- Mare Anguis – Serpent Sea
- Mare Frigoris – Sea of Cold
- Mare Humorum – Sea of Moisture
- Mare Imbrium – Sea of Showers
- Mare Insularum – Sea of Islands
- Mare Nectaris – Sea of Nectar
- Mare Nubium – Sea of Clouds
- Mare Spumans – Foaming Sea
- Mare Undarum – Sea of Waves
- Mare Vaporum – Sea of Vapors
States of Mind
These lunar maria are named for states of mind:
- Mare Crisium – Sea of Crises
- Mare Moscoviense – Moscow (seriously named as a state of mind!)
- Mare Ingenii – Sea of Cleverness
- Mare Serenitatis – Sea of Serenity
- Mare Tranquillitatis – Sea of Tranquility
Notable Maria
There are a little more than 20 lunar maria worth knowing about. Let’s take a look at each of these, where they are located, and why they are important.
| Name | Coordinates | Diameter (km/m) | Significance | Image |
| Mare Anguis | 22.6°N 67.7°E | 150/93 | Roughly shaped like an ‘X’. Present within the Crisium basin |  Source |
| Mare Australe | 38.9°S 93°E | 603/375 | Largely present in the Moon’s southeastern hemisphere, but its eastern side overlaps the Moon’s near and far sides. Marked with numerous crater impacts. |  Source |
| Mare Cognitum | 10°S 23.1°E | 376/233 | This mare is banked by a mountain range on its northwest. Exists in the second ring of a larger mare called Oceanus Procellarum. |  Source |
| Mare Crisium | 17°N 59.1°E | 418/259 | Easily identified at the edge of the Moon’s face, marked by wrinkled edges. |  Source |
| Mare Fecunditatis | 7.8°S 51.3°E | 909/564 | Seen on the eastern side of the Moon’s face. |  Source |
| Mare Frigoris | 56°N 1.4°E | 1596/991 | Seen on the Moon’s northern side. |  Source |
| Mare Humboldtianum | 56.8°N 81.5°E | 273/169 | Seen on the Moon’s northeastern side but is occasionally hidden from view when seen from Earth. |  Source |
| Mare Humorum | 24.4°S 38.6°W | 389/241 | Roughly circular, south of Apollo 17’s landing site. |  Source |
| Mare Imbrium | 32.8°N 15.6°W | 1123/69 | Marked by 3 concentric rings of mountains surrounding it. |  Source |
| Mare Insularum | 7.5°N 30.9°W | 513/318 | Sandwiched between three craters: Kepler and Encke on the west and Sinus Aestuum on the east. |  Source |
| Mare Marginis | 13.3°N 86.1°E | 420/260 | Present at the corner of the Moon’s near side. Marked by irregular outline and thin walls. |  Source |
| Mare Nectaris | 15.2°S 35.5°E | 333/206 | Bordered by multiple craters, the biggest one being Fracastorius. |  Source |
| Mare Nubium | 21.3°S 16.6°W | 715/444 | Bordered by features Bullialdus on the west, Pitatus on the south and Guericke on the north. |  Source |
| Mare Orientale | 19.4°S 92.8°W | 327/203 | Looks like a bullseye but is difficult to see because of its location on the western border of the Moon’s near and far sides. |  Source |
| Mare Serenitatis | 28°N 17.5°E | 707/439 | Present to the east of Mare Imbrium. |  Source |
| Mare Smythii | 1.3°N 87.5°E | 373/231 | Present on the Moon’s near side, near the eastern equator. |  Source |
| Mare Spumans | 1.1°N 65.1°E | 139/86 | Present at the south of Mare Undarum. |  Source |
| Mare Tranquillitatis | 8.5°N 31.4°E | 873/542 | Marked by numerous, not well-defined rings. |  Source |
| Mare Undarum | 6.8°N 68.4°E | 243/150 | Located south of Mare Spumans. Also banked by craters Dubyago on the south and Condorcet P on the northeastern side. |  Source |
| Mare Vaporum | 13.3°N 3.6°E | 245/152 | Surrounded by mountain range Montes Apenninus. |  Source |
| Oceanus Procellarum | 18.4°N 57.4°W | 2568/ 1595 | The largest mare on the Moon’s near side. Stretches across its north-south axis. |  Source |
Summary
Although humans first mistook lunar maria to be water-filled seas, these maria are an important part of the moon’s history and tell us a lot about its formation.
After the moon was hit with meteorites, the impact basins that formed as a result were filled with lava that oozed out from the surface (because the moon was still cooling down).
We hope the list of 20 notable maria and their unique features will make it easy for you to find them when you go lunar maria-hunting!
