4 Theories of How Moon was Formed

Moon Via NASA

Birth of Moon:

Have you ever wondered how moon was formed or it came to be as natural satellite of earth. In the history of astronomy, a few handful of theories were posited by various scientist and communities. Some of them were debunked right away and few were pondered for decades. This  theories had a impact so much so that one of the main aims of, NASA's infamous, Apollo missions was to identify the origins of moon. There are many different theories, but these theories can be coalesced to four categories.

1. Giant-Impact  Hypothesis

Nasa.gov

This is the most popular and widely accepted theory in the community of science. The theory goes like this, Apparently a mars-sized proto planet had collision with proto-earth and the impact expunged part of debris formed from collision to space. But some portion of debris condensed and formed together as moon. This theory explains most of the aspects related to formation of moon such as, angular momentum between earth and sun, similar ispotopic properties between moon and sun etc, the core of moon being smaller compared to that of earth etc. The mars sized proto planet was given a name called Theia, a name derived from mother of greek moon godess Selene.
Note: Below four theories were prevalent and widely debated before the sample moon rocks were brought from Apollo mission.

2. Capture Theory

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As the name suggests, this theory states that the moon may have formed elsewhere in the solar system or milky way and got captured by earth's gravity when passing nearby. But the isotopical similarity between earth and moon contradicts the theory of moon forming elsewhere. The sample's of moon from apollo space mission showed astounding chemical and isotopic similarities, which suggest's a possible link between earth and moon. It was also suggested that moon may have been captured by earth from Venus.

3. Fission Theory

ESO Supernova-European Southern Observatory

This theory was conceptualized  by George Darwin, son of  Charles Darwin in 19th century. He suggested that at some point in early history, earth may have rotated so fast that a portion of it may have been expelled to outer space. some theorists also suggest part of current pacific ocean was expelled. But this was contradicted when the moon samples showed that the mineral content of moon is slightly different from pacific ocean or earth.

4. Co-Accretion Theory

Formation of moon-jara

Also known as Co-Formation Theory  or Condensation theory, it posits that earth and moon both may have formed at the same time from coleascion of dust and gas particle from pro-planetory disk or even rotting around black holes. But it does not explain angular momentum of earth and moon nor why radious of moon's iron core is less than that of earth's (moon's core is 25 percent of it's body where as earth's core is 50 percent of it's body).