Have those former "mysteries of the moon" been solved now?

Chang'e 6 has embarked on a journey to the moon and will land on the back of the moon to solve some ancient mysteries about the moon for humans. So, how many mysteries are there on the moon waiting for humans to solve?

Moon

Are there gods living on the moon?

Why does the moon always face the earth with only one side?

Is there an alien base on the moon?

What is on the far side of the moon?

What causes the moon to wax and wane?

What is a lunar eclipse?

…

For thousands of years, people living on Earth may have had similar doubts every night when they "looked up at the bright moon."

Fortunately, all of the above problems have been solved.

Advances in knowledge of physics and astronomy solved some of the puzzles, and the advent of the probe era solved the rest.

Today we have confirmed that there are neither gods living on the moon nor alien bases. Its back side, like its front side, is a "barren land" covered with craters.

So, have we learned everything about the Moon? Quite the contrary: we have solved many mysteries, but have also discovered countless new ones.

How was the moon formed?

Regarding the origin of the moon, people have proposed several important hypotheses:

The Earth-Moon Common Origin Theory: Both the Earth and the Moon were formed directly in the protoplanetary disk;

Capture theory: The moon was captured by the Earth's gravity;

Differentiation theory: The moon was "thrown" out due to the centrifugal force when the earth rotated too fast in the early days.

Each of these hypotheses has its merits, but there are also many doubts that are difficult to justify. Today, the most popular one is the giant impact hypothesis: one day 4.5 billion years ago, a Mars-sized celestial body fell from the sky and hit the "embryo" Earth, which had not yet fully grown. The violent impact quickly crushed and melted the Mars-sized celestial body, and also knocked out some of the Earth's material. These debris materials were scattered around the Earth, and then gathered again through gravity and collisions, eventually forming today's moon.

Schematic diagram of the giant impact hypothesis

But is the Big Impact Hypothesis perfect? ​​Not really. To make up for some of the problems with the Big Impact Hypothesis, scientists have come up with many "patches" one after another. As for whether other previous hypotheses are definitely impossible, there is still no 100% conclusion.

How are lunar craters formed?

This question, which has been controversial for more than a hundred years, may be the one we are most certain of the answer to.

Many scientists once believed that the craters on the moon were mainly caused by volcanic eruptions... After all, those craters are so round, and there are no meteorites nearby. More importantly, if the bumpy moon was caused by collisions, then why are there almost no craters on our Earth?

But later, scientists gradually discovered that it was impossible for a volcano to form such a huge crater, and there would be no traces of radial sputtering accumulation. More importantly, after witnessing the example of Comet Shoemaker-Levy 9 hitting Jupiter, everyone finally believed that the impact of small celestial bodies could have such a powerful destructive power.

The rock samples near the crater brought back by the Apollo spacecraft showed obvious signs of impact and melting, which can only be caused by impact.

Today, we are completely convinced that the vast majority of craters on the moon were formed by meteorite impacts, as are those on Earth and other solid celestial bodies in the solar system.

However, there are still a few lunar craters that are caused by volcanoes or other unknown causes, such as Hyginus and Ina. The causes of some specific lunar craters are still controversial.

The Crucible: Did the Moon Ever Have a Magma Ocean?

If the giant impact hypothesis is true, the huge energy generated by the impact may have completely molten the early lunar rocks, that is, the early moon may have been covered by a global magma ocean. Then, as the magma ocean slowly cooled, solid rocks slowly formed.

Did the moon ever have a magma ocean? What was it like during that period? This question is related to when and how the lunar crust and mantle were formed, and also to why the chemical composition of the lunar surface is distributed as it is today.

We have found some clues in the rocks of the moon, but these are not conclusive evidence, and related research is ongoing.

Critical Hit: Did the Moon Experience a 'Late Giant Impact'?

From 1969 to 1972, Apollo 11, 12, 14, 15, 16, and 17 successfully landed humans on the moon and completed a series of lunar field explorations that have not been surpassed to this day. The rock samples brought back by the Apollo missions allow us to measure the formation age of different regions of the moon through isotope dating.

From the ages of different rock samples, people found that the moon seemed to have experienced a sudden and violent wave of small body impacts about 3.85 billion years ago. Obviously, if there really was such an event, it would not be just the moon that experienced it - all large rocky bodies in the inner solar system should not be spared. This is the Late Giant Impact (LHB) hypothesis.

But are rock samples really reliable enough? After all, the samples we have are so limited. Did the late giant impact really happen? Or even if it did happen, was it really a short, sudden event? Until recently, scientists were still arguing about this.

The mysterious contrast between the front and back of the moon?

On October 6, 1959, the Soviet Union's Luna 3 transmitted back the first image of the far side of the moon. This was the first time that humans saw what the far side of the moon looked like.

Unlike the numerous dark lunar seas on the near side, the far side of the moon is almost entirely bright highlands, and the density of craters on these highlands is much higher than that on the near side. Moreover, we already know that these dark lunar seas are actually the result of large impact basins filled with dark lava, but there are not so many such large impact basins on the far side - so why is there such a big difference between the front and the far side, since they are both randomly hit by small celestial bodies?

Some people think that because the lunar crust on the front is thinner, dark magma is easier to gush out; others think that because there are more radioactive rocks on the front, the temperature is higher, and the same impact can make the final basin larger...

But none of these have solved the essence of the difference between the front and back sides of the moon, such as why the lunar crust is thinner on the front side, and why radioactive rocks are more concentrated on the front side? We still don't know.

Is there water on the moon?

Decades ago, people believed that there was no water on the moon. After all, it is almost impossible for liquid water to exist on the surface of the moon today, because the moon has no atmosphere and magnetic field to protect it, the gravity is small, the daytime temperature is high, and water vapor will be constantly decomposed by sunlight and then dissipated.

But other forms of water are still possible. For example, water or hydroxyl (-OH) bound to minerals, and water ice in the permanent shadows of the polar regions, have been gradually confirmed by many probe data. It can be said that the step-by-step detection of water on the moon is a great achievement of human lunar exploration.

But there are still many mysteries about the water on the moon: For example, what is the distribution, content and purity of this water? Is there still water deep underground? Where did this water come from? What is the relationship between this water and the geological and climate history of the moon?

These all require a lot of exploration and further research.