The most popular hypothesis for the appearance of the moon is that it was born when, at the dawn of the life of the solar system, a body the size of Mars crashed into the young Earth. But until now scientists could not answer a simple question - where did this cosmic "hammer" come from? By the popularity of the hypothesis, we mean that the majority of scientists adhere to it, and unofficially it is recognized as the most plausible and reasonable version of the birth of our night lamp. Although there are other options.
So, there was a terrible blow. The molten iron core of the "hammer planet" sank to the core of the Earth (or proto-earth, considering what happened 4.5 billion years ago, when our planet had just formed and was not the same as it is now). Lighter stone fragments of the mantles of the two planets formed a ring that eventually merged into the Moon, which apparently gathered around the largest such fragment. The moon, by the way, was originally 20 times closer to the Earth than it is now, and gradually moved away to its current position.
This Big Splash or Big Impact hypothesis was proposed by American astrophysicists Al Cameron, William Ward, William Hartmann and Donald Davis more in 1975. Since then, scientists have found a lot of evidence to support this scenario. For example, this perfectly explains why the Moon contains essentially no iron. There is only one problem - the body that collided with the Earth. Where did it come from?
Richard Gott and Edward Belbruno of Princeton University have solved the mystery that has plagued scientists for a quarter of a century, while also providing a curious clue to the problem of finding extraterrestrial life. But first things first.
The found "keys" suggest, apparently, the impossible location of the mysterious "hammer". One such "key" is comparing the composition of the Earth and the Moon. Cosmologists are sure that the dusty disk from which the planets were formed had a different composition at different distances from the Sun. Another young planet the size of Mars, it seems, would have had a different composition compared to the then Earth. On impact, everything would be mixed and, examining the earth and lunar rocks, we should see traces of fundamentally different rocks. But that is not the case, says Mr. Gott.
Take oxygen, for example. There are isotopes oxygen-16, -17 and -18. Their mutual proportion is like a unique “fingerprint” of the planet. Big Burst simulations predict that Earth's oxygen fingerprint will be quite different from the lunar one. And they are pretty much close. This leads scientists to conclude that the body that hit the Earth and created the Moon formed exactly the same distance from the Sun as the Earth.
This is also seen from the computer simulation of the birth of the Moon, which shows that the "hammer" hit our planet at a relatively low speed and not exactly head-on, but somewhat tangentially. This is where the problem arises - where did this planet manage to "sit out" when the solar system was created in order to grow to the size of Mars?
After all, the accepted theory of the birth of planets says that they gradually "grew together" from dust and debris attracted by gravity. And this is a process in which the "rich" become "richer" and the "poor" become "poorer", that is, the "hammer" had to be "swallowed" by the proto-earth before it could reach a significant mass.
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The answer is ingeniously simple. There are two places in the solar system that fit this theory. These are the points "Lagrange-4" and "Lagrange-5", the existence of which was calculated by the French mathematician Joseph Louis Lagrange in 1772. They are in orbit around the Earth, but 60 degrees behind and in front of our planet in terms of its movement in a circle. At these points, all forces in the Earth - Sun system balance each other. And any slow stones that happen to get there are trapped, as if in the interplanetary Sargasso Sea.
At one of these points, a planet the size of Mars could once have formed, which would revolve around the Sun in the same orbit as the Earth. When this mysterious planet reached a large mass, gravitational perturbations from other planets (mainly Jupiter), in the end, rocked it and expelled from the Lagrange point. In their computer models, Gott and Belbrano calculated the subsequent course of events. And, surprisingly, they found that virtually nothing could prevent the hammer from colliding with Earth. It is just natural. At the same time, in a quarter of the simulated collisions, a body is formed as a result - exactly - the Moon.
The most interesting implication of the Gott-Belbrano scenario is its enormous implications for our prospects for detecting extraterrestrial life. The fact is that the Earth has the largest moon compared to its own size of all the planets in the solar system (not counting the distant cold Pluto). And such a giant moon was important for the development of life.
Without the Moon, the axis of our planet would experience much larger long-term fluctuations, which would cause serious changes in the climate with sad consequences for life. The moon's gravity smoothes out such fluctuations, making the climate more stable. In addition, the tides created by the Moon (and they are three times larger than those caused by the Sun) played a key role, firstly, for the very origin of life, and secondly, later, for its emergence on land.
And now it turns out that the appearance of a large moon near a planet in some star system is a very likely event, and not an exceptional one, as cosmologists believed earlier. Gott and Belbrano even believe that planetary systems where two or more terrestrial planets have such large moons should be commonplace in the galaxy.
This means that our chances of meeting brothers in mind are increased, moreover, it becomes clear what systems we need to look for. Is there a way to prove Belbrano and Gott's scenario? It seems unlikely that any material unchanged later (at least a stone), a witness of that cataclysm, would have survived to the present day, and even been found by people.
And yet … Gott and Belbrano point to asteroid 2002 AA29, the size of a small boulder. He is currently in orbit, which periodically brings it to a distance of 5.8 million kilometers from Earth. This orbit is very specific. And it is very similar to the one along which the "hammer" could have moved 4.5 billion years ago. It is possible that 2002 AA29 carries the material from which the "hammer", the Earth and, accordingly, the Moon were once created.
Curiously, 2002 AA29 was chosen by planetary physicists as an asteroid, to which, due to the parameters of its orbit, it is relatively easy to send a ship to return rock samples. However, so far such a mission is not planned. But, reflecting on the mystery of the birth of the moon, Gott concludes: "This asteroid may be the most valuable piece of rock in the solar system."