At night on Earth, 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 among the more than 200 planetary satellites in the solar systemits size is only a little smaller than Mercury, it is simply a giant to the earth, how can the earth have such a natural satellite?
Comparison of the Moon and Mercury. Credit: NASA/JHU-APL
It doesn’t matter that the moon itself is huge. If you look at the moon from the earth, it looks almost as big as the sun. How big of a coincidence can this be achieved?
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 would be really scary.
In fact, all the answers to why the moon is so special lie in how the moon was formed.
Is the moon really older than the earth?
It is difficult to know where the saying that the moon is older than the earth came from. In short, I have read relevant books since I was a child.
For now, one thing is certain, that isOldest 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 isZircon crystal.
Zircon crystals, credit: Rob Lavinsky
The early Earth was very hot, and the entire planet was molten. Heavier materials sank into the Earth’s core during this period, while lighter materials were pushed to the Earth’s surface to become the later crust.
But the powerful heat from the early Earth’s core drove rapid and intense mantle convection, which caused rapid circulation of the crust into the mantle.
This kind of thing is still happening on the earth, but it is more “gentle” than the early earth.
Because of this, it is difficult to keep traces or information of the early Earth on the earth. Scientists are not even sure what material the earliest crust was, but the biggest one is probably basalt.
Legend: Earth Structure
The strong high temperature and high pressure resistance properties of zircon crystals make it an important source of information for the study of the early earth, but the oldest known zircon crystals are only about 4.4 billion years old, which is already found on the earth. the oldest material.
In contrast, the lunar rocks brought back by Apollo and the lunar meteorites that fell on the earth,Some of their oldest are over 4.5 billion years old.
Many authors may infer that the moon is older than the earth, especially in those unsolved mysteries of human beings in the past, there are often such claims.
In fact, it’s just that the Earth is more geologically active than the Moon, which stopped being geologically active between 150-200 million years after it formed.
Caption: The lunar basalt of 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, we can only “make up stories” through known facts.
In the solar system, gas giant planets tend to have many natural satellites. For example, Saturn is now known to have 12 larger natural satellites. This is because gas giant planets are so massive that they can capture these huge satellites.
But Earth, being a rocky planet, doesn’t have enough mass to capture an object the size of the Moon, which would be very difficult to say the least, especially when they’re so close to each other.
Caption: Earth photographed by the Apollo 8 mission
In fact, the earth’s natural satellites are far more than the moon, but many of them are very, very small and very far away. Most of these are captured by the earth, and they generally do not orbit the earth for a long time.
In the past, it was more generally believed that the earth and the moon were formed together. This hypothesis can explain the current position of the moon, but it can be known from simple calculations that the density of the moon is much lower than that of the earth. If the two formed together, So it means that they should have similar densities, or they all have a heavy element core, obviously the moon does not.
The fact that the Moon is large, relatively close to the host planet, and relatively low density can rule out the two main sources of planetary natural satellites—capture and co-formation.
Early Solar System, Credit: NASA/JPL-Caltech
We can organize the other data mentioned earlier:
For one, the moon’s ancient sample is 4.5 billion years old, which could mean it’s roughly the same age as the solar system.
Second, it was also known from the lunar samples that the composition of the moon is very similar to that of the Earth; at the same time, a key piece of information was obtained—the oxygen isotope ratios of the Earth and the moon suggest that they originated in the same part of the solar system.
Third, scientists have long found that the rotation speed of the earth is relatively fast at this distance (distance from the sun).
Combining all of this, the story should look like this:
The Moon Impact Hypothesis, Credit: NASA/JPL-Calte
About 4.5 billion years ago, or about 70 million years after the formation of the earth, it was very common for planetary bodies to be hit by various asteroids, but the earth attracted a super-asteroid impact – now this asteroid is called As Theia, it was about the size of Mars today.
The epic impact sent lighter material flying above Earth,Over thousands of years, these materials formed the present moon.
This impact hypothesis (let’s call it the impact hypothesis) could explain why the Moon is composed of lighter materials, and pretty much every other weird fact.
But except for one point, most of the existing models in the impact hypothesis have a premise that the earliest composition of the moon requires at least 60% of the original materials of Theia, but from the existing lunar samples, the composition of the earth and the moon differs by at most 100% ten thousandths.
But no matter what, the impact hypothesis is now the most mainstream explanation for the formation of the moon, and it is normal to have some difficult explanations. After all, the lunar samples are very limited now, and the source area is very single.
Finally: Why does the moon appear to be the same size as the sun?
The reason the moon looks huge is because it’s so close to the earth, and as to why it looks as big as the sun, the reason is that the sun is about 400 times bigger than the moon, and they’re exactly 400 times apart.
If we look further, it can only be said that this is really a coincidence, and we happen to be at the stage where the moon and the sun appear to be the same size.
Legend: Comparison of the moon and the sun from different perspectives
Due to the interaction of the Moon’s gravity with the Earth’s oceans, the Moon is actually moving away from us, about 3.8 centimeters a year.
In the distant past, the Moon was close enough to the Earth that it looked much larger than the Sun, so creatures at that time couldn’t see the beautiful corona during a total solar eclipse.
And in about 50 million years, the moon will be far enough away that our descendants will only see an annular eclipse, not a total eclipse.
Hashtag: Earth Moon Astronomy
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