The American “Big Think” website published an opinion article by Ethan Slegel, in which he talked about the reasons for the great difference between the two sides of the moon near and far from Earth.
In his article, the writer said that the moon is the clearest and largest celestial body that the human eye can see in the night sky.
The moon appears to us as we see it as thirty times the diameter of the planet Venus, and it appears to be about a million times brighter than it, but its features differ from one place to another even from our limited perspective of it on Earth.
To the naked eye, these differences are in the form of bright and dark spots. If you look through a telescope, you will not only see those dark spots on the brighter parts, but you will also see hills, craters and mysterious terrain, known as the moving glass line.
And while these features may be familiar, they all hold clues to the moon’s ancient history, and can help us understand why the “face” of the moon we see isn’t the only perspective that matters.
The writer mentioned that there are two main features about the moon that cannot be ignored. The first is that its surface is covered with craters to a large extent, and that light-colored areas are generally full of craters more than dark areas. Many areas include small craters within medium-sized craters within giant craters, providing evidence that the larger craters are so old that younger and smaller craters formed on top of them.
Second, the dark regions, known as lunar seas, contain relatively few craters and most are smaller. These regions are notable because they have a significantly different color and composition from the majority of the Moon’s surface.
The writer pointed out that the same side of the Moon always faces the Earth, but different parts of the lunar hemisphere light up throughout the month depending on the relative positions of the Earth, Moon and Sun. Because the moon’s orbit is elliptical, it moves faster when it’s closer to Earth and slower when it’s farther away, which explains the ever-slightly visible face of the moon, a phenomenon known as lunar oscillation.
When the Soviet spacecraft Luna 3 managed to reach the far side of the moon, we got the first pictures of it. Although the image was not of high quality, it revealed that the near side of the moon is very different in terms of the number and dimensions of lunar craters and seas than the far side.
This discovery came as a huge shock, for decades, even as the quality of images and our understanding of this side of the moon improved. So, what are the big differences between the near and far side of the moon?
The writer mentioned that the first immediately noticeable difference is the almost complete absence of lunar seas on the dark far side. There is one prominent in the northern hemisphere of the moon but it is small, and there may be a few smaller but no seas as wide or as deep as those on the near side of the moon.
The second difference is that the craters on the far side are more prominent and precise. And with a much larger area devoid of seas, there are more areas full of potholes. Although this was first discovered in 1959, it took longer to get to the reason behind this mystery. Later, scientists discovered that the previous analyzes were wrong.
The writer explained that the solar system is full of dangerous comets and asteroids that periodically dissipate in the interior adjacent to our star. And sometimes resulting from these bodies the emergence of comets and meteors. But when things go wrong, one of those large objects collides with a larger object, creating a disastrous effect. For this reason, it is possible that when these huge space rocks are heading towards the moon from the far side there is nothing at all blocking their path and colliding with it, but when they approach the moon from the near side, the Earth acts as a shield.
It should be noted that the far side is filled with about 30 percent more potholes than the near side, which is an enormous difference.
This interpretation does not present any differences in the abundance of lunar seas that appear on the near side versus the far side. It is believed that the seas emerged as a result of lava flows from the fissures. And it turns out that the answer has to do with collisions in space, but not with comets and asteroids.
About 4.5 billion years ago, when the solar system was still in its infancy, the Earth mostly formed. It was about 90-95 percent of its current mass. But there was another very large planet the size of Mars in an orbit roughly similar to that of Earth. For tens of millions of years, these two bodies orbited precariously away from each other. Finally, about 50 million years after the formation of the solar system, they collided with each other.
The Earth ended up forming, while a large amount of debris was scattered into space. Over time, a large amount of this debris combined with the effect of gravity and Earth’s heat to form the moon, while the rest either headed to Earth or elsewhere in the solar system. There is now evidence that moons orbiting other rocky worlds, such as Mars and Pluto, likely formed from giant impacts.
Among the hypotheses put forward – according to the author – is that it is possible that the location of the moon was much closer to us in the beginning, which indicates that it formed gradually after a very short time, that is, after about 100,000 years or less. These last details are currently unknown, but it remains a strong possibility. If so, it is possible that the Earth’s heat affected the material on both sides of the moon during its formation.
It was only in 2014, 55 years after we caught a peek at the far side of the moon, that a study by Arpita Roy, Jason Wright and Stein Sigurdsson appeared to provide the evidence to back it up. They looked at the event that created the early Earth-Moon system and followed the possible paths of its physical evolution, as the Moon formed from a debris disk orbiting the planets and Earth. And if the Earth is too hot, it depletes certain elements as it gets close to the Earth, such as calcium and aluminum. In other words, the heat from the early Earth creates a chemical gradient inside the Moon, resulting in a different composition for the side of the Moon closest to Earth versus the side farthest from Earth.
The writer added that the very strong tidal forces remind that the Earth is very huge compared to the Moon (about 70 times the mass) and that the Moon was closer to the Earth in the past. If this is the case, the greater abundance of calcium and aluminum in the farthest part of the disk surrounding the planet resulted in thicker crust on the far side of the Moon than on the near side.
The writer pointed out that the lunar seas that we see are evidence of lava flows that occurred much later, as molten rock flowed into large basins and lowlands on the surface of the moon. A thinner crust on the near side and a different composition on the far side explain why the two faces are so different even after billions of years.
Source: Arabic 21