From Earth, we can look up at the Moon and see several areas that appear darker than the rest. These darker regions make up the features of the “Man on the Moon” that we are all so familiar with seeing. In this article, we discover what these dark spots are called and how they got there in the first place.
What Are the Moon’s Dark Spots Made Of?
While the Moon’s dark spots look like a blanket of shadows, they are actually nothing of the sort. This shadowy illusion is due to the fact that over 80% of the Moon is made up of anorthosite. This gray material plays a big part in reflecting the Sun’s rays toward Earth.
When early scientists observed the Moon, they also wondered what these dark regions were. Believing they were large bodies of water, these early scientists gave them the name maria, which means “sea” in Latin.
The term maria stuck around and is still in use today, along with oceanus (ocean), lacus (lake), palus (marsh), and sinus (bay).
In actuality, these dark areas are nothing of the sort. Modern study of the Moon has revealed these dark spots to be not water but large seas of dried lava. These areas of solid basalt rock cover approximately 16% of the entire lunar surface, with the vast majority being located on the Earth-facing side of the Moon.
Basalt is naturally darker in color than anorthosite but also doesn’t reflect light nearly as well. This leaves it looking the black color we’re used to seeing from Earth.
Where Did the Moon’s Dark Spots Come From?
We know that today the Moon is a cold, dark, and dormant system without any volcanic activity. In order to understand the origin of these lakes of solid lava, we need to jump backward in time a few billion years.
During this time in history, the Moon was at the height of its volcanic activity. The Moon’s hot mantle (the layer under the surface) was full of magma during that time.
This magma would escape the mantle through volcanic eruptions and also through fissures in the crust where it was the thinnest. It’s also possible that meteor impacts on the lunar surface shattered the crust, and lava was able to similarly escape that way.
It’s also believed that the Earth’s gravity played a part in the number of eruptions that occurred on the Earth-facing side of the Moon. This may explain why there are so many more basalt lakes on the near side versus the other.
The Moon’s gravity is much less than that of Earth. This makes it much easier for the magma to flow out of the inner layers of the Moon. The lesser gravity also meant that it didn’t come shooting out of the ground like we sometimes see on our planet. Rather, it flowed out like a cup that runs over.
In some of the lowest valleys of the Moon, this lava pooled, forming large seas of lava. It didn’t take long for the molten basalt to permanently solidify into the dark spots that we’re able to see today.
Perhaps the most incredible realization of observing lunar maria is that you’re looking at rock that has existed in its current condition for upwards of three billion years.
How Were Maria Named?
All in all, there are 22 different maria on the lunar surface, not to mention those designated as lakes, marshes, and bays. Each mare is required to refer to sea features, sea attributes, or a state of mind during the naming process.
When Mare Moscoviense was discovered on the far side of the Moon by the former Soviet Union, they proposed the name Moscoviense after their capital, Moscow. This seemed to go against the standard naming convention, but the International Astronomical Union declared it acceptable because Moscow is a “state of mind.”
Mare Tranquillitatis, also known as the Sea of Tranquility, was the site of the Apollo 11 landing. It would go down in history as the first location that humanity ever stepped foot on the Moon.
In many cases, lava flows left long grooves on the lunar surface as they drained down into maria. These channels were given the name rille after the German word for groove. Lunar rille can be several hundred miles in length and at the same time several miles wide.
Perhaps the most famous is Hadley Rille, where the Apollo 15 spacecraft made its landing. Being the first mission to have a lunar rover, the astronauts were able to explore the trench in some detail.
The Largest Dark Spot on the Moon
Not all lunar dark spots were created equally. Some are quite small, with a diameter of roughly 30 miles (50 kilometers). None, however, compared to the large size of the Ocean of Storms.
Oceanus Procellarum, or the Ocean of Storms, covers a large portion of the western edge of the Moon on the side we see from Earth. This incredible “ocean” covers more than 1,600 miles (2,500 kilometers) from north to south and covers 1,500,000 mi2 (4,000,000 km2). That’s over 10% of the entire surface area of the Moon.
A valley of this size is unique on the Moon. Scientists today believe that the ocean was created by a massive impact of some kind either while lava was still flowing or just after it hardened.
Apollo 12 touched down within the large expanse known as the Ocean of Storms. Several unmanned spacecraft have landed within Oceanus Procellarum. The most recent, Chang’e 5, collected a few pounds of rock samples from the site.
The Moon’s dark spots tell a tale of a time when the Moon was a much more active place than it is today. Today the Moon may be dark and cold, but we’re able to see billions of years of history written all over its surface. Time will tell what other stories the Moon has waiting for us as we continue to explore our closest neighbor.