Inconsistencies in the hypothesis of the moon's origin: how did the Earth's satellite come about?

2024 Author: Seth Attwood | [email protected]. Last modified: 2023-12-16 15:55

We don't know exactly how the moon came about. According to a popular hypothesis, long ago, the Earth collided with a planet the size of Mars, and our satellite was formed from the debris. Only here something does not add up.

The hypothesis of a megacollision between the Earth and the planet Teia was put forward by the Americans Hartman and Davis in 1975. In those distant times, two types of satellites were known in the solar system: those that are radically smaller than their planets (Phobos and Deimos near Mars, satellites of the gas and ice giants), and the Moon. She was the only satellite whose mass was more than one percent of the mass of her planet.

The strangeness of the moon demanded a non-standard explanation of where it came from. Previous guesses were somewhat naive and easily refuted. For example, the son of Charles Darwin assumed that the Earth once revolved faster and a huge chunk fell off from it. This and similar hypotheses poorly explained the fact that the iron core of the Moon is small in comparison with the Earth, and it was believed that there was no water there.

In fact, at that time, water in the lunar rock had already been discovered: it was contained in the soil (regolith) delivered by the Apollo. The find was attributed to terrestrial pollution or meteorites. The readings of the ion detectors, which recorded the water near the Apollo, were also attributed to terrestrial pollution. Scientists rejected empirical facts because they did not fit with the then theories of the origin of the moon.

In all these theories, the moon first melted, because of this, it had to lose water. Science of that time assumed only one option for water to hit the moon - with comets. But in cometary water, there is a different ratio of hydrogen to its heavy species, deuterium, and in water found on the moon by the Americans, the ratio of these isotopes was the same as on Earth. The mismatch was most easily explained by contamination.

However, it remained unclear why the regolith contains less titanium and other relatively heavy elements. It was then that the hypothesis of a mega-impact (mega-impact) was born. According to it, 4, 5 billion years ago, the ancient planet Theia collided with the Earth, and a superpowerful impact threw into space the debris of both planets - from them the Moon was formed over time. The upper layers of the Earth contain few heavy elements, because most of them sank down into the core and lower layers of magma. Allegedly, this is due to the difference in the lunar soil.

It turned out that the earth's satellite was not primary, as, for example, that of Jupiter, but secondary - in addition, the question of why the mass of the Moon is so large in comparison with the mass of the Earth itself was removed. Also, the hypothesis of the Americans explained why there is absolutely no water on the moon: when the planets collide, the debris should have flared up to thousands of degrees - the water simply evaporated and flew into space. Another thing is that after the Apollo flights, the idea of a waterless moon was stubborn ignorance of the facts.

The hypothesis looked just fine for three whole years. But already in 1978, Charon, a satellite of Pluto, was discovered. If the Moon is 80 times less massive than Earth, then Charon is only nine times lighter than Pluto. It turned out that there is nothing unique about the moon. Doubts arose: large planets, most likely, collide too rarely for so many large satellites to appear.

New inconveniences were brought by the analysis of lunar rocks in laboratories and the first data on meteorites of alien origin. It turned out that the Moon is isotopically indistinguishable only from the Earth, and all other planets of the solar system are clearly different. How did this happen if the Moon supposedly contains the substance of another planet - the hypothetical ancient Theia? To explain the contradiction, the hypothesis of the mega-shock was finalized: the place of birth of Theia was considered … the earth's orbit - that's why the isotopic composition of both planets is the same. In one place, two planets were formed at once, which then collided.

But it was not clear why two planets appeared on the earth's orbit, and one at a time on the orbits of other planets of the system. Added problems and geologists. Another question arose: if the mega-collision of two planets heated the Earth and its debris, where did the water come from on the planet? By all accounts, it should have evaporated.

The mega-impact theory had already become extremely popular, they did not want to abandon it, so the idea was put forward that water appeared on Earth later - it was brought by comets that fell on the planet for billions of years. But it was soon discovered that the ratio of isotopes of hydrogen and oxygen in cometary water is very different from that on Earth. It is more similar to Earth's water from asteroids, but there is very little of it on them, that is, they cannot be the source of our oceans.

Finally, in the 21st century, traces of water began to be found on the moon. And when the proponents of the mega-impact hypothesis suggested that comets brought this water, the Dutch geologists showed that lunar rocks could not have formed in their present form without the presence of water from the very beginning of the formation of the satellite. The situation was aggravated by Russian astronomers: according to them, a typical collision of a comet with the Moon leads to the departure of more than 95% of the water back into space.

Best of all, the situation was reflected in the article of 2013 with the telling heading "Impact theory is exhausted".