The Moon is the only natural satellite on Earth. With an equatorial diameter of 3476 km, it is the fifth largest satellite in the solar system. It has a diameter of approximately a quarter of Earth's diameter and a mass of 7,349×1022 kg, which represents 1.23% of the mass of our planet (which is 5,9736×1024 kg). It is located at a distance of 384,400 km.
The Moon is in synchronous relationship (or in tidal coupling) with Earth, that is, always showing the same face towards the planet. The visible hemisphere is marked with dark lunar seas of volcanic origin, between the bright ancient mountains and the prominent impact craters, also called astroblems.
The Moon is the only celestial object, outside of our planet obviously, where man has been able to step on, through manned descents. Although the Moon program of the Soviet Union was the first to reach the Moon with an unmanned spacecraft, the United States Apollo program conducted the only manned missions to Earth´s satellite to date, beginning with the first manned lunar orbit by the Apollo 8 in 1968, and six manned moon landings between 1969 and 1972, the first being Apollo 11 in 1969, and the last one Apollo 17 in 1972. To date, since the American astronaut Neil Armstrong did so on July 20, 1969 , only eleven other men have put their feet on lunar surface, the last of them being the American Eugene Cernan in December 1972.
However, today it is still not entirely clear what the origin of our satellite is. The scientific community still remains hesitant, and in numorous debates around the question: where does it come from and how did it get there? This is not an easy question to answer, and various theories have been put forward with different arguments.
We know that Moon formed approximately 4527 million years ago. This age has been calculated according to the dating of the isotope of the lunar rocks that have been collected in manned missions, between 30 and 50 million years after the origin of the solar system. But regarding the way it was formed, there are basically four main possibilities:
- It was an independent object that, when passing near Earth, was captured in its orbit.
- Earth and Moon were born from the same mass of matter that orbited the Sun.
- Moon emerged from a kind of "swelling" of Earth that broke off by centrifugal force.
- It originated as a result of a collision between the young Earth and a protoplanet the size of Mars, which is called Tea (or Theia) or occasionally Orpheus.
The first hypothesis, called “capture”, assumes that Moon was an independent planetesimal object, formed at a different time from ours and in a remote place. Moon initially had an elliptical orbit with an aphelion (furthest point from the Sun) located at the distance that now separates it from the Sun, and with a perihelion (point closest to the Sun) near the planet Mercury. This orbit would have been modified by the gravitational effects of giant planets, which altered the entire planetary system by expelling several bodies from their orbits, including our satellite. The Moon traveled for a long time through space until it approached Earth and was captured by terrestrial gravitation.
However, it is difficult to explain with this hypothesis how the significant deceleration of the Moon happened, necessary so that it does not escape from the Earth's gravitational field.
The second hypothesis, or "fission" assumes that originally Earth and Moon were a single body and then part of the mass was expelled, due to the instability caused by the strong rotational acceleration that our planet was experiencing at that time. The detached part retained part of the angular momentum of the initial system and, therefore, continued in rotation that, over time, synchronized with its translation period.
It is believed that the area that fell off corresponds to the Pacific Ocean, which has about 180 million square kilometers and an average depth of 4,049 meters. However, in order to be able to separate such an important portion of our planet, it should have rotated at such high speed as a complete rotation every three hours. Such speed seems impossible, since, when rotating too fast, Earth would not have formed because it would have an excess of angular momentum.
The third hypothesis, also called "binary accretion" involves the formation of both the Earth and the Moon at the same time, from the same material and in the same area of the solar system. In favor of this theory is the radioactive dating of the lunar rocks brought to our planet by the space missions, which date between 4,500 and 4,600 million years the lunar age, approximately the age of Earth.
But the main fail of this hypothesis is that, if the two were created in the same place and with the same matter: how is it possible that both have a chemical composition and density so different? On the Moon, titanium and exotic compounds abound, elements by no means abundant on our planet, or at least not in the most superficial layers.
The “big impact” hypothesis stands as the preferred or perhaps the most successful for decades, partly because it explains the large size of our satellite and the absence of water in it. It assumes that our satellite formed after the collision wof a body approximately one seventh the size of our planet with the Earth. The impact caused gigantic blocks of matter to jump into space for later, and, through an accretion process similar to the one that formed the rocky planets near the Sun, generate the Moon.
The most doubtful of this theory is that they should have had too many coincidences together. Although the probability of impacting with a wandering object was very high at the beginning of the Solar System, it is more difficult that the collision did not completely disintegrate the planet and that the fragments were large enough to generate a satellite.
However, it has recently been proposed that our satellite, Moon, was formed not as a result of a single giant collision (Theia) but by a series of large concatenated impacts. This would explain the reason why Moon seems to be composed of material very similar to Earth¨s and not by a combination of land remnants and remains of another planet.
Lately, another explanation has also appeared under the name of “Precipitation Hypothesis” according to which, the energy released during the formation of our planet heated part of the material, forming a hot and dense atmosphere, especially composed of metal vapors and oxides. These spread around the planet and, when cooled, precipitated the grains of dust that, once condensed, gave rise to the only satellite on Earth.
But as can be seen, each one of the theories presented raises a different origin and partly answers some questions about the Moon, but they also have “gaps” or elements that they cannot explain at all or that they simply cannot explain. So, perhaps the origin of our satellite is another mystery that will remain unsolved about space, even if it is a mystery only 384,000km away.
Josher
December 21, 2019
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