Image credit: Jessie Eastland
On Earth's night, the Moon is undoubtedly the brightest celestial body in the sky, and it is of great significance to life on Earth. However, when we go to understand the moon, we will find that it has many special places.
The Moon is the fifth largest natural satellite in the solar system's more than 200 planetary moons, its size is only a little smaller than Mercury, for the Earth it is simply a giant, how can the Earth have such a natural satellite?
Comparison of the Moon and Mercury, Source: NASA/JHU-APL
Even if the moon itself is very large, if you look at the moon on Earth, it looks almost as big as the sun, how big a coincidence can this be?
In addition, there are many people who say that the moon is older than the Earth, is there any basis for this? After all, if the moon is older than the earth, it is really terrible.
In fact, all the answers to why the moon is so special lie in how the moon formed.
Is the Moon really older than Earth?
It's hard to figure out where the idea that the moon is older than Earth came from, but I've read books about it since I was a kid.
For now, one thing is certain, and that is that the oldest known material on Earth is at least 100 million years younger than material on the moon.
Because the Earth's geological activity is very active, it is very difficult for scientists to understand the Earth in the past, but there is one material that is an exception, that is, zircon crystals.
Zircon crystal, Source: Rob Lavinsky
The early Earth was very hot, the entire planet was in a state of molten, heavier material sank into the Earth's core during this period, and lighter material was pushed to the Earth's surface and became the later crust.
However, the strong heat from the early Earth's core drove rapid and intense mantle convection, causing the crust to circulate rapidly into the mantle.
Such things are still happening on earth, but they are more "gentle" than the early earth.
Because of this, it is difficult to retain traces or information about the early Earth, and scientists are not even sure what the earliest crust was, but the largest may be basalt.
Illustration: Structure of the Earth
The strong resistance and high-pressure resistance of zircon crystals makes it an important source of information for the study of early Earth, but the oldest known zircon crystals are only about 4.4 billion years old, which is the oldest material found on Earth.
And in contrast, the lunar rocks brought back by Apollo, as well as the lunar meteorites that fell on Earth, some of the oldest of them reached more than 4.5 billion years.
Many authors may use this to infer that the moon is older than Earth, especially in the previous unsolved mysteries of mankind.
In fact, it's just that the Earth is more geologically active than the Moon, which ceased to be geologically active between 150 and 200 million years after its formation.
Note: Lunar basalt from Apollo 17
How did the Moon form?
No one has ever experienced the formation of the moon, and now to understand how the moon formed, the only way to "make up stories" is through known facts.
In the solar system, gas giants tend to have many natural moons, such as Saturn, which is now known to have 12 larger natural moons, because gas giants themselves are so massive that they can capture these huge moons.
However, Earth is a rocky planet, and it doesn't have enough mass to capture an object the size of the Moon, so it would be very difficult to do so, to say the least, especially as they are so close to each other.
Note: Earth photographed by the Apollo 8 mission
In fact, the earth's natural satellites are far more than the moon, but many are very, very small, but also very far, most of these are captured by the earth, and their time around the earth is generally not very long.
In the past, it was more commonly believed that the Earth and the Moon formed together, a hypothesis that could explain the Moon's current position, but it can be known through simple calculations that the Moon is much less dense than the Earth, and if the two are co-formed, then it means that they should have a similar density, or both have a core of heavy elements, and it is obvious that the Moon does not.
The fact that the Moon is large, close to the host planet, and less dense, can rule out the two main sources of the planet's natural satellites — capture and co-formation.
Early Solar System, Source: NASA/JPL-Caltech
We can collate the other data mentioned earlier:
First, the moon's ancient sample is 4.5 billion years old, which could mean that it is about the same age as the solar system.
Second, it can also be known from the lunar sample that the composition of the moon is very similar to that of the Earth; a key piece of information is also obtained - the oxygen isotope ratios of the Earth and the Moon indicate that they originated in the same part of the solar system.
Third, scientists have long discovered that the Earth's rotation rate is relatively fast at this distance (the distance from the Sun).
Combining all of this, the story should look like this:
Lunar impact hypothesis, source: NASA/JPL-Calte
About 4.5 billion years ago, about 70 million years after earth's formation, it was very common for planetary bodies to be hit by various asteroids, but Earth caused a super-sized planet impact — now called Theia, which is about the size of Mars today.
This epic impact caused lighter material to fly over Earth, and over thousands of years, this material formed what is now the moon.
This impact hypothesis (let's call it the impact hypothesis) explains why the moon is made up of lighter materials, as well as almost all other strange facts.
But with one exception, most of the existing models in the impact hypothesis have the premise that the earliest lunar composition required at least 60% of the original Material of Theia, but from the existing lunar samples, the composition of the Earth and the Moon is at most a few parts per million.
But in any case, the impact hypothesis is now the most mainstream explanation for the formation of the moon, and it is normal to have some difficulties in explaining, after all, the lunar samples are very limited now, and the region of origin is very single.
Finally: Why does the Moon look as big as the Sun?
The reason the Moon looks so huge is that it's so close to Earth, and as to why it looks as big as the Sun, it's about 400 times larger than the Moon, and they're just 400 times farther apart.
If we dig a little deeper, we can only say that this is really a coincidence, and we are in the stage where the moon and the sun look as big.
Note: Comparison of the Moon and the Sun from different perspectives
Due to the moon's gravitational pull and the interaction of Earth's oceans, the moon has actually been moving away from us, stretching about 3.8 centimeters farther away each year.
In the distant past, the moon was close enough to Earth, when the moon looked much larger than the sun, so that the creatures of the time could not see the beautiful corona during the total solar eclipse.
In about 50 million years, the moon will be far enough away that our descendants will only see an annular eclipses, not total solar eclipses.