Was our moon "spitted out" by the earth?

Have you ever wondered why there is a moon next to the earth? How did the moon come about? Today, we will talk to you about these questions.

01

Where does the moon come from?

There have been many hypotheses about the origin of the moon since ancient times: for example, some say that the moon, like the earth, is the earliest planet formed in the solar system. When the earth grew bigger, it became a planet, and when the moon became smaller, it was "captured" by the earth's gravity and became a satellite of the earth.

There is another theory, proposed by Darwin's son, that when the earth was first formed, due to the huge internal energy, a mass of matter was "thrown out" and became the moon. After the moon was thrown out, the earth left a "scar", which is now the Pacific Ocean. However, after testing, the rocks in the Pacific Ocean are very young, and the age of the moon rocks is 4 billion years, so their ages do not match. So this hypothesis is also wrong.

Finally, there is a more reliable theory that the moon is indeed a part of the earth, but it was not thrown out by the earth, but was obtained by impact with other celestial bodies: Our Earth was formed 4.6 billion years ago, and about 4.56 billion years ago, it was hit by a Mars-sized celestial body from the solar system. This impact produced a lot of fragments, and then these fragments slowly combined due to gravity on the edge of the earth to form our moon.

If we want to know whether the above theory is true, we need to do a "paternity test" for the Earth and the Moon. For humans, we can extract the genes of parents and children and compare them to confirm whether the child is the biological child of the parents. For the Earth and the Moon, we can also extract their "genes" - rocks, and determine the age and compare them to confirm whether the Moon is the "child" of the Earth. This is one of the reasons why we want to explore the moon.

02

The Moon's Contribution to the Earth

The birth of the moon from the impact on the Earth was undoubtedly a harm to the Earth. However, this harm brought some benefits to the Earth. At the same time, the birth of the moon also made a lot of contributions to the Earth.

Contribution 1: Let the Earth have four seasons

Let's recall why all the globes we see are crooked? Because our Earth is crooked, it was knocked 23.5 degrees. This is a "disability" left by the impact that "gave birth" to the Moon 4.56 billion years ago.

However, rather than being a "disability", it is more accurate to say that it is a contribution of the moon to the earth.

Imagine if the Earth had not been knocked crooked, and it was rotating on a vertical axis while revolving around the sun, then every area on Earth would be exposed to the sun for the same length of time after one revolution, and there would probably not be distinct four seasons. But "luckily" the Earth was knocked crooked, and while rotating on a tilted axis, it was also revolving around the sun. When it revolved to different positions, the length of time the sun was shining on the Earth varied, and that’s how we got spring, summer, autumn, and winter, and the distinct four seasons. So the four seasons are a special rhythm that the moon brought to the Earth when it was born, and it can be considered a contribution.

In addition to the Earth, other planets in the solar system did not suffer a "good result". For example, Mars was knocked off by 24.5°, which is slightly more tilted than the Earth, so Mars also has spring, summer, autumn and winter; Venus was knocked over, and other planets rotate counterclockwise, only Venus rotates clockwise; Jupiter is too big and not tilted much; Saturn is also tilted; and Uranus is even worse, it was knocked "lying down", so Uranus "lies" around the sun. The eight planets in the solar system are all "upside down", which is the current observation.

Contribution 2: The Moon - Earth's Guardian

Let's take a look at the digital elevation map of the entire moon produced by Chang'e 1 below. You will find that there are many pits of different sizes on the moon, about three to four million pits. How did these pits come about?

In the solar system, there are many small celestial bodies flying in all directions, some of which will fly towards the Earth. Fortunately, the moon revolves outside the Earth day after day, and with its weak body, it has resisted the impact of many small celestial bodies for the Earth. Although the moon itself has been hit by craters everywhere, it has bravely protected its "mother".

Contribution 3: Helping the Earth Slow Down

The third contribution of the moon is that it brings tides to the earth.

According to paleontological research, the early Earth only rotated once in about 6 hours, sometimes daytime and sometimes nighttime. Who can stand this? Imagine if we lived on an Earth that rotated once in only 6 hours, we might not be able to do anything except sleep. But with the help of the moon, the Earth's rotation has been dragged slower and slower, from the original 6 hours a day to the current 24 hours a day.

In fact, the rotation of the Earth is still slowing down. The main reason for the slowdown is the tidal friction of the moon. In addition, it is also affected by climate and geological activities. Since 1972, the International Earth Rotation Service has been adding or subtracting 1 second (this "1 second" is called a "leap second") to eliminate the difference between the precise time (measured using atomic clocks) and the inaccurate observed solar time. As of June 30, 2022, we have added 27 leap seconds. Although these 27 seconds are extremely short for our lives, we can know that our Earth is indeed getting slower and slower.

Therefore, although the birth of the moon caused the earth to be "knocked" crooked, the moon has deep feelings for its "mother" and has always been the guardian of the earth. At the same time, it also has a huge impact on our future human life.

This article is based on the speech given by Ouyang Ziyuan (Researcher and Academician of the Institute of Geochemistry, Chinese Academy of Sciences) at the Starry Sky Forum