It may seem strange, but the formation of Earth's closest neighbor in the solar system has caused countless problems for astronomers and physicists for centuries. The moon, Earth's only natural satellite, and an unusual satellite compared to the other planet's moons, lends very few clues to its origins. In the past few decades, a compelling theory about the moon's origins has gained attention in the scientific community, but the jury is still out, in a sense.
Early Theories About The Moon's Formation
The predominant theory about the formation of the solar system explains, in simple terms, that there was a ball of matter which was disturbed in some way, causing a gravitational collapse that coalesced into the sun. The remaining material, caught in orbit, gradually accreted together to form the planets and their satellites.
This protoplanetary disc model does not seem to work for Earth's moon, however. No other planet in our solar system has a satellite even close to the size ratio between the Earth and the moon. In other words, Earth's moon is just too big to have formed this way without being absorbed into the early Earth or flung off into another orbit of its own.
Another early theory about the moon's formation was proposed by Charles Darwin's son, G.H. Darwin. He thought that there was a period of very rapid rotation in Earth's early history, while the planet was still molten, and the moon spun out of this rotation. This theory fell apart when calculations of the angular momentum necessary for an event of this magnitude to occur showed it to be impossible.
The Iron Catastrophe and Oxygen Isotope Ratios
The idea that the Earth and its moon formed simultaneously out of the same protoplanetary material was a popular explanation for a long time. However, after discovering the fact that the moon lacks an iron core like the Earth, this notion had to be scrapped as well.
In the early molten formation of the Earth, an enormous event occurred that has made life possible on this planet for billions of years. Called "the iron catastrophe," this event was the process by which heavy iron and nickel began to sink through the lighter materials of the proto-Earth into the core. Aside from slowing the process of cooling due to radioactive decay of heavy elements deep within the Earth, the iron catastrophe is the root of the Earth's geomagnetic field which protects it from solar winds and allows liquid water to exist on its surface.
The moon, on the other hand, does not contain an iron core and is mostly homogeneous in its composition. Therefore, no iron catastrophe for the moon, and no possibility for formation from the same cloud of matter.
It was thought after this that the moon formed somewhere else in the solar system and was captured by Earth's gravity. After the moon landings, however, it was discovered that the oxygen isotope ratios of the moon and the Earth were identical. Comparing this to isotope ratios in meteorites and Mars rocks, formed in other places in the solar system, proves that the capture theory was also incorrect. How did the moon form?
The Impact Theory of Moon Formation
In the 1970s and 1980s, a new theory began to gain prominence in the scientific community. The impact hypothesis suggests that early in the formation of the solar system, while the Earth was still largely molten, and alone in its neighborhood, a Mars size body entered its orbit and collided with it at tremendous force and speed. The heavy iron was retained by the larger body, which had already experienced the iron catastrophe, and the lighter materials were flung out into space to form the early moon, which stayed in orbit with its new parent.
This hypothesis explains nicely the matching oxygen isotope ratios, the lack of a large iron core in the moon, and the similarity of the moon's surface to the current composition of Earth's mantle. It also explains the abundance of lava flow like features on the moon's surface, despite its lack of tectonic activity. With the intense energy released in the impact, the moon would have been a molten ball for a long time after.
While it may never be possible to exactly determine the origins of the moon, much like the origins of anything else in the universe, the impact theory is the most widely regarded today. It has held up now for 25 years, the blink of an eye in geologic terms, but a seeming lifetime in the constantly changing world of astronomical research.
Source
Hartmann, W.K. and D.R. Davis, "Satellite Sized Planetesimals and Lunar Origin," Icarus, 1975, 24 (4), 504-505
Jarod Rhoades - Jarod is a musician and writer who resides in the beautiful Pacific Northwest. While his main focus is creative writing, he also churns ...
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