Origin of the Moon
Telescope observations and space missions have probably diminished the "poetry" surrounding this celestial body, yet despite being so studied, it still retains mysterious sides, especially regarding its origin.
Let us first summarize the main data available to us:
- the Moon has the same chemical composition as the Earth or rather the Earth's mantle and is rich in refractory elements, which have a higher melting temperature such as calcium and titanium;
- the average density is 3.36 g/cm3 versus 5.52 g/cm3 on Earth
- the age is the same as Earth's
- the lunar diameter is 3.476 km, about 1/4 that of Earth
- the volume is 22 BILLION km3, 1/49 of Earth's, while the mass is 1/81 of Earth's; this is due to the disproportion existing with Earth's density
Of the origin of the Moon there is no irrefutable evidence, but on the basis of the data in our hands, the following 4 hypotheses have been developed:
- Capture: Formed in another region of the solar system it would then have been diverted from its original orbit and captured by the Earth's gravitational field. This would only explain the different chemical composition from Earth; while "capture" would seem to be an unlikely dynamic process; in fact, a body of lunar mass passing in the vicinity of Earth would only experience a deviation of its trajectory, otherwise for this to be possible it would have to have a very low orbital velocity.
- Accretion: The Moon would be a companion of the Earth, formed together with it in the same part of the protoplanetary nebula, but born separately and formed from the fragments that were in orbit around the Earth. It is unclear why the lighter planetesimals remained in orbit around the protoEarth instead of aggregating with the others to form the Earth. Another data point against this hypothesis is that based on the current rate at which the Moon is receding, due to the tides, the two celestial bodies should have been very close to each other a little over 1 BILLION years ago, while the common origin lies 4.6 billion years ago. So this hypothesis would only explain the different chemical composition of the two bodies.
- Fission: The Moon would be "daughter" of the Earth; more than 4.5 BILLION years ago within the protoplanetary nebula surrounding our young Sun the still-forming Earth in a semi-fluid state was subjected to tidal forces induced by a large passing celestial body that pertubed its dynamic equilibrium and caused the fission of a part of it, perhaps also accomplice excessive rotational velocity. In this hypothesis, the difference in chemical composition would be explained by the fact that the Moon would consist of the material from the Mantle and Earth's surface regions.
- Collision: It would have happened in the first few million years of the solar system's formation when large asteroids and planetesimals that escaped the initial moments of planet aggregation and formation were still roaming in interplanetary space. A frightening collision with a planetoid of about 1/10th the mass of Earth (roughly the size of the planet Mars) would have caused part of Earth to break apart. The catastrophic collision would have scattered material from Earth into orbit; later the fragments would have regrouped to form the protomoon.
But which of the listed hypotheses is the most likely? Let us start with the fission hypothesis, in vogue in the late 1800s and early 1900s proposed by astronomer George H. Darwin, son of Charles the theorist of biological evolutionism.
It has been reconsidered in light of the careful study of data and computer simulations by an Australian researcher, A.E.Ringwood, published in the influential scientific journal "Nature" in 1986.
His study starts from an unequivocal geological fact, namely that the average density of the Moon is very similar to that of Earth's surface layers. Now, based on the fact that the average density of the Earth is 5.52 times that of water, the representative geological model of the planet tells us that the iron and nickel core is 32 percent of the total mass;
while based on the average density of the Moon, which is 3.36 g/cm3, we get a core equal to 2 percent of the total mass; thus our satellite turns out to be a sphere composed, almost entirely, of the same material of which the Earth's crust is made.
Ringwood to verify the data of the models compared the concentration of chemical compounds in lunar and terrestrial rocks; the percentages of the elements were found to be practically the same.
It now remains to explain how splitting of the forming Earth is possible.
Ringwood explains his hypothesis:
More than 4 BILLION years ago, the still-forming Earth was being bombarded by numerous massive planetesimals whose orbits intersected Earth's; the Earth at that time was rotating on itself within about 5 hours;
it had an extensive primordial atmosphere composed essentially of hydrogen and had around it an accretion disk that reached the distance of about 3 Earth diameters to the "Roche limit", within which tidal forces do not permit the aggregation of a satellite (as we can presently verify with Saturn, whose rings are a clear demonstration of this).
Due to collisions with planetesimals, much material from the Earth's mantle would have been flung into the atmosphere and would have fed, for tens of millions of years, the accretion disk that, growing beyond the Roche limit, would finally generate the Moon.
This is one of the hypotheses most widely accepted by the majority of the scientific world, in fact in this hypothesis would explain both the different chemical composition of the two celestial bodies and the discrete lunar mass, although it must be acknowledged that so far no theory explains all the questions about the origin of the Moon with due precision.
