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Podcast Transcript

About 384,000 km or 239,000 miles above the surface of the Earth is our planet’s only natural satellite, The Moon. 

Every culture and civilization on the planet has had the moon play a role in its legends, and they have also used the moon to keep track of time, plant, and harvest. 

Scientists have wondered where the moon came from and how it was formed, and with data gathered over the last several decades, we now have a better understanding of its origin. 

Learn more about the Moon, its origin, composition, and its role in helping life develop on Earth on this episode of Everything Everywhere Daily.

I’ve done many episodes that reference the moon, including episodes on moon rocks and the Apollo missions, but in this episode, I want to focus on the moon itself. 

The Moon is one of the few universal things that every culture and civilization in history has in common. No matter where you are in the world, regardless of the continent or the latitude, everyone is familiar with the moon. 

As such, almost every culture has a myth about the moon. In China, Chang’e is the moon goddess living on the moon with her pet rabbit, the Jade Rabbit. According to the legend, she drank an elixir of immortality and floated up to the moon, where she resides eternally.

In Hindu mythology, Chandra, also known as Soma, is the moon god. He is depicted as a beautiful, youthful deity who rides a chariot across the night sky.

In some West African legends, Anansi the Spider, a trickster figure, is associated with the moon. One story tells how Anansi tried to steal the moon and keep it for himself, but he was caught, and the moon was placed high in the sky, out of his reach, to provide light for everyone.

In various Native American cultures, the moon and the sun are often seen as siblings or spouses. For example, in the Inuit tradition, the moon god Igaluk chases his sister, the sun goddess Malina, across the sky. This chase explains the cyclical nature of day and night. Another story from the Cree involves the moon as a protective figure watching over the Earth.

Over time, we moved from making stories about the Moon to asking serious questions about the Moon and making serious measurements and observations. 

Certain things have been known about the moon since time immemorial. We know that we only see one side of the moon, that it goes through phases over the course of a month, and that it is associated with the tides.

In 1609, Galileo Galilei turned a telescope towards the moon for the first time and made some startling discoveries. He saw that the moon’s surface wasn’t smooth as people had always assumed. It was full of mountains and craters. 

Galileo, like centuries of astronomers after him, assumed that the craters on the moon were formed by volcanoes. 

In the 1870s, the English astronomer Richard Proctor theorized that the craters on the moon were not volcanic but were the result of impacts. 

Knowledge of the moon increased throughout the early 20th century as telescopes improved. We were able to map the surface better and could identify the major features. 

Roughly speaking, two types of features on the moon’s surface can be identified with the naked eye or a telescope. The dark parts of the moon are lunar maria. This comes from the Latin word for sea. The Sea of Tranquility, for example, the landing site for Apollo 11, is formally known as Mare Tranquillitatis. 

The light parts of the moon’s surface are known as highlands.

As much information as could be gleaned from direct observation, there were things about the moon that couldn’t be known unless samples could be studied. The only way to truly know the moon’s chemical composition would be to collect samples and bring them back to Earth for study. 

The Apollo program retrieved the first moon rock samples, which have been studied extensively over the last 50 years. Subsequent Soviet and Chinese unmanned missions have also returned samples to Earth. 

So, with some actual samples available for geologists to study, what have we learned? 

The first big question that geologists had was how old is the Moon? 

Using uranium-lead dating of zircon minerals found in some of the rocks, researchers concluded that the rocks were between 4.1 and 4.4 billion years old, approximately the same age as the oldest rocks found on Earth. 

The rocks are primarily composed of silicate minerals, such as basalt and anorthosite, similar to Earth’s crust but with distinct differences. Notably, the moon lacks volatile elements like water, which are abundant on Earth.

Any water found on the surface of the moon would have been lost billions of years ago through a process known as photodissociation. The harsh, unfiltered light of the sun and the cosmic rays beating down on the surface would have long since split apart water molecules. The Oxygen and hydrogen would have then been lost to space. 

While water isn’t commonly found on the moon, that does not mean that the moon is devoid of water. 

In the last several decades, probes sent to orbit the Moon by NASA and the Indian Space Agency have found evidence of frozen water near the polar regions, which is more abundant near the south pole. 

Why the poles? Those areas don’t get direct sunlight, and water inside a crater located near the poles would never see any direct sunlight. This water might have been there since the formation of the moon or it might have been deposited there by a comet that crashed into the surface.

This is important because a source of water would be critical if humans ever want to establish a permanent base on the moon.

Another discovery is that the moon is mostly geologically inactive today, and it probably has been for the last few billion years.  

However, about 4 billion years ago, the moon went through a period known as the Late Heavy Bombardment. This was a period of extensive bombardment of the lunar surface with projectiles from the Early Solar System. This would have made the surface very hot and possibly molten. 

There was also a period of intense volcanism, which took place about 4 to 3 billion years ago. Evidence of this volcanism can be seen in lava tubes, which are believed to exist on the moon. As with water, lava tubes might be able to help humans establish a base on the moon by providing a ready-made shelter with protection from the sun and cosmic rays.

One of the other discoveries was that some of the moon rocks were weakly magnetic. This implies that at some point, very soon after the creation of the moon, there might have been a short period where the moon had a molten core and a magnetic field. 

This is quite a bit of information that has been deciphered from moon rocks. This really leaves the big question of where the moon comes from and how it was created. 

Our moon is very different from the moons of other planets in our solar system in one big way. Our moon is relatively huge compared to the planet it orbits compared to every other moon. The only pair of objects with a greater ratio is Pluto and its moon Charon. 

Our moon is the 5th largest natural satellite in the solar system, but the other four that are larger all orbit Jupiter or Saturn and are tiny in comparison. Moreover, Jupiter and Saturn are gas giants, and the Earth is a rocky planet that is more similar in chemical composition to the moon. 

There are several theories as to how the moon might have come into being. 

One theory is called the Capture Theory.  

The Capture Theory suggests that the moon formed elsewhere in the solar system and was captured by Earth’s gravity. This theory is less favored because it requires highly specific conditions for a body to be captured by another planet without being destroyed or escaping.  Also, the similar chemical composition and age of the Earth and the moon make this highly improbable. 

Another theory is the Co-formation Theory.  

The Co-formation Theory proposes that the moon formed simultaneously with Earth from the primordial accretion disk of the solar system. However, this theory has limitations, as it does not adequately explain the differences in the composition between the Earth and the moon.

The most widely accepted theory, however, is the Giant Impact Hypothesis. 

This theory suggests that about 4.5 billion years ago, a Mars-sized body, often referred to as Theia, collided with the early Earth. The impact was so powerful that it ejected a significant amount of debris into Earth’s orbit. This debris eventually coalesced to form the moon. This hypothesis is supported by several lines of evidence, including the moon’s composition, which is similar to Earth’s mantle, and the angular momentum of the Earth-moon system.

Another question that many people have is, why can we see only one-half of the moon?  

The other side of the moon we cannot see has been called the dark side of the moon, but it isn’t technically dark. It gets as much light as the other side of the moon. 

We can only see one side of the moon because it has become tidally locked. Tidal locking occurs when the time it takes for an object to revolve about its axis is the same as the time it takes to orbit an object. 

Everything I’ve talked about so far in this episode has been about the moon itself. However, many researchers now believe that the moon might have been critical for the development of life on Earth. 

For starters, the moon and its relative size serve as a giant shield protecting the planet from asteroid impacts. Every crater on the surface of the moon represents a meteor that could have hit the Earth. 

The presence of the moon stabilizes the axial tilt of the Earth. Without the moon, Earth’s tilt could otherwise vary dramatically, leading to extreme changes in climate over short geological timescales. A stable axial tilt ensures relatively stable seasons, which is beneficial for developing and evolving complex life forms.

The moon’s gravity also causes tides in Earth’s oceans. Tides play a crucial role in the mixing of ocean waters, which helps distribute nutrients and gases like oxygen and carbon dioxide. This mixing creates diverse marine environments and may have been essential for the development of early life in tidal pools and coastal areas.

The moon’s influence on tides also affects the circulation of ocean currents, which helps distribute heat around the planet. This process can mitigate extreme temperatures and create a more hospitable climate for life.

Many astronomers now believe that the moon has such an important part to play in the evolution of life that if we search for exoplanets that might have life, we should search for smaller planets with large moons. 

Our knowledge of the moon is far from over. In fact, we really haven’t gotten started.  All of the Apollo and unmanned sample return missions combined have only returned 381 kilograms or 840 pounds of moon rock. Those samples have only been collected in a small number of sites.

There are plans to send humans back to the moon this decade and potentially set up a permanent base on the moon. This would result in an exponential increase in the amount of data we would have on the moon, and would increase our understanding accordingly. 

The moon has been a global constant for humanity throughout all of history, yet until recently, we didn’t really know much about it. Now, we finally have a glimpse into the moon’s secrets, and over the course of the rest of the 21st century, we are poised to learn even more.