The Origin of the Moon: discussion on the gradual perfection of proposed hypotheses
Introduction
The Moon is the easiest and most familiar object that human can catch in the clear night. The Moon as known as our Earth’s only natural satellite is indicated by Rick Carlson in recent research to have an age of between 4.40 and 4.45 billion years, which is not long after the origin of the Earth. However, the origin of the Moon is still not defined and further research on this topic is continuous. The exploration of formation of the Moon is gradually per fected by the various proposed theories started from 1878 to nowadays. This research essay will discuss both the past and present hypotheses of origin of the Moon. The past hypotheses also known as “The Big Three” referring to Fission, Capture and Accretion hypotheses are popular in different times and centuries before the Apollo mission flew in 1972 (Tyson, 2000). However, the study on the moon rock brought by Apollo’s program gives unavailable clues for “The Big Three”(Tyson, 2000). Nowadays, the Giant Impact Hypothesis is regarded as the most leading theory that is most accepted in present. Nevertheless, it is widely accepted, some studies supported this theory; others may create new theory to challenge this Giant Impact Hypothesis, which make this hypothesis still arguable. However, whatever these hypotheses failed or will achieve, they bring a better understanding on the formation of planet’s satellites in our Solar System (Zhong, 2013), which is instrumental for further study on planetary science and even galaxy.
The Big Three------The past hypotheses
• Fission Hypothesis
The Fission Hypothesis was proposed by George Howard Darwin, the son of the Charles Darwin (the famous evolutionist) in 1878 and became popular accepted untilApollo’s trip (Tyson, 2000). It is interesting that this hypothesis described the Moon as the daughter of the Earth (Tyson, 2000). His work described the Earth as a rapidly spinning viscous bodies and calculated that the early Earth spun very rapidly at about a 2.5 hour day that gave a centrifugal force that pushed the
Moon out from the Earth, another force is from the Sun’s strong gravity attraction (Anon, 2007). The diagram 1 presents how the Moon is pulled out from the Earth. Furthermore, couple years later,
Osmond Fisher, a geologist, believed that the Pacific Ocean Basin was a scar due to this event (Tyson, 2000). Therefore, during that time, it is taught in school that the Pacific Ocean was formed by the fission of the moon from the Earth’s crust (Anon, 2007). Even though this hypothesis seems to be inconceivable at present, as the first scientific and comprehensive model to explain the formation of the Moon at that time, it had its meaningful values at that time.
Diagram 1. Fission Hypothesis

• Capture Hypothesis
This hypothesis can be easily understood by its literal word-“capture”, which is developed by Thomas Jefferson Jackson in 1909 and was popular until the 1980s (Tyson, 2000). Jackson believed that there might have a large atmosphere around early Earth, which can slow down the movement of the Moon and capture it (David &Nader, 2007). In other words, as the diagram 2 show that he believed that the Moon was like a fly wandering in the space, however, our Earth was like a spider web that attracts this “Fly” using gravity (Tyson, 2000). Furthermore, the unusual large size, non-equatorial orbit and tidally locked orientation of the Moon also give possibility to a capture origin (Anon., 2007).
Diagram 2. Capture Hypothesis
• Accretion Hypothesis
The astronomer Edouard Roche proposed the third theory---Accretion hypothesis, it is believed that both Earth and the Moon were formed from the same material as all other planets in the solar system and they are formed side-by-side independently as it used be (Tyson, 2000). Moreover, it is suggested that the Earth and the Moon were formed together in the solar system, based on the primordial accretion disk double system (Burro, n.d.)
Diagram 3. Accretion Hypothesis

The Failure of “The Big Three”
• Apollo Program
The Apollo program is carried out by the National Aeronautics and Space Administration (NASA) as the third United States human spaceflight program, which achieved human’s first landing on the Moon from 1969 to 1972 (Wikipedia, 2015).Otherwise, the program brought back about 800 pounds of moon rocks and soil samples, which explored a lot facts of the Moon that enhanced people’s understanding of the Moon. As the well known statement proclaimed by Neil Armstrong, "That’s one small step for a man, one giant leap for mankind."( NASA, 2014) The Apollo’s trip gives human beings a better understanding of the Moon, including the clues that can explain the failure of “The Big Three”.
As the examination of moon rocks’ compositions, it is found that there is no water elements and less volatile elements comparing to Earth rocks, which means much of moon have experience molten activity with a very high temperature that our Earth hasn’t been experienced (Tyson, 2000). Furthermore, the lunar samples showed that moon has much less iron, that is Moon core only account for 3% of its total mass, while by contrast, Earth’s core represents 30% (Tyson, 2000). Therefore, the Fission Hypothesis failed to explain why the Moon lacks of iron if moon is the daughter of the Earth; they should have similar proportional composition of cores (Herres &
Hartmann, 1997). Another reason that Fission Hypothesis failed is much brilliant as itis known that oceanic crust is much younger, as the Pacific basin formed about 70million years ago, while the Moon has an age of about 4-4.5 billion-year-old (Tyson,2000).However, the Earth and the Moon have similar characteristic. The results of radiogenic elements and nonradioactive, stable isotopes of oxygen, shows that they have roughly same age and suggesting that they were formed at same distance from the Sun (Tyson, 2000). All these facts challenged the Capture Hypothesis as it failed to explain why the oxygen isotopes of the Moon rocks match with Earth’s rocks so much (Herres & Hartmann, 1997).Finally, the failure of Accretion Hypothesis is that it failed to explain the total angular momentum and energy in present Earth- Moon system. It is explored that the Earth-Moon has a high angular momentum that is unusual comparing to other planet-satellites in the solar system, since the Moon is more massive relative to Earth(Tyson, 2000). In addition, the second failing point is the same problem as Fission Hypothesis regarding of different proportion of core(Tyson, 2000).

The present hypothesis
The Giant Impact Hypothesis
This Giant Impact Hypothesis is regarded as the leading modern hypothesis for nowadays and accepted widely. It is first proposed by Hartmann and Davis in 1975(Hartmann & Davis, 1975). The general idea is that 4.5 billion years ago, during the time when solar system experienced early accretion evolution, other smaller planetary objects were growing and colliding the Earth while the Earth also experienced formation process (Hartmann & Davis, 1975). However, a fraction of collision debris, which is a Mars-sized, went into orbit that around the Earth and then gradually aggregated into present the Moon (Hartmann & Davis, 1975). This hypothesis explained why the proportion of composition of the Earth and the Moon are different. That is the Earth has a 30 % iron core, but the Moon has only 3%. The further study by Hartmann (2014) explained that the Earth’s core is the result of drainage Earth’s iron into the core before collision, but debris’ iron came from the breakage of mantle of the Earth (Hartmann, 2014). Therefore, the Moon has much less iron comparing to that of the Earth. Furthermore, Analysis of composition of Moon rocks’ samples collected by Apollo Program showed evidences that supports the giant impact hypothesis that is the Moon was created through impact of a large astronomical body with the Earth (Burrows, 1999). Finally, this hypothesis also explains why oxygen isotope composition between the Moon and Earth is the same since the Moon formed in Earth’s neighbourhood.
Additional theories and challenges
New hypothesis is raised by Robin M. Can up, funded by the NASA Lunar Science Institute, which challenges the Giant Impact Hypothesis as the new theory hypothesizes “our early Earth and moon were both created together in a giant collision of two planetary bodies that were each five times the size of Mars.” However, there is also study on the depletion of zinc isotopes on the Moon supports the Giant Impact Hypothesis in late 2012 by Paniello et al.
Conclusion
The Moon, as the easiest object that people can share in the clear night, has its attractions for scientists exploring its origin. Even though, the formation of the Moonies still a mystery that requires further research on this topic. We have already seen how our ancients’ efforts on exploring the formation of the Moon, and it is known as the knowledge of the Moon is gradually perfected by the various proposed theories started from 1878 to nowadays. As this research essay discussed the past hypotheses as known as “ The Big Three” referring to Fission, Capture and Accretion hypotheses are popular in different times and centuries seems to failed after the great Apollo mission in 1972 (Tyson, 2000). However, they gave a better understanding for later works on this topic. Nowadays, the Giant Impact Hypothesis is regarded as the most leading theory that is widely accepted. Nevertheless, some studies supported this theory; others may create new theory to challenge this Giant Impact Hypothesis, which make this hypothesis still arguable. Therefore, our human being should put our hopes on the development of planetary science and technology that may come to day we can figure out the truth of origin of the Moon.
Reference
Anon., 2007. Lunar Capture Origins of the Earth’s Moon. Retrieved on 27 Sept 2015: http://lunarorigin.com/lunar-origin-models
Burrows, W.E. (1999). This New Ocean: The Story of the First Space Age. Modern Library.pp.431
Hartmann, W.K. (2014). The Giant impact hypothesis: past, present (and future?). The Royal Society, Vol.372.
Hartmann, W.K. & Davis, D.R. (1975). Icarus, Planetary Science Institute, Vol.24, No.505. Retrieved on 27 Sept 2015: http://www.psi.edu/epo/moon/moon.html
Jenvey, K. (2012). NASA Lunar Scientists Develop New Theory on Earth and Moon Formation. Retrieved on 27 Sept 2015: http://www.nasa.gov/topics/solarsystem/features/moon_formation.html
Paniello, R.C., Day, J.M.D., Moynier, F. (2012). Zinc isotopic evidence for the origin of the Moon. Nature, Vol. 490, pp.376-379
Tyson, P. (2000). Origins. Retrieved on 27 Sept 2015: http://www.pbs.org/wgbh/nova/tothemoon/ origins.html
Zhong, C. (2013). Origin and Evolution of the Moon. International Astronomical Union. 9(s298), pp. 457-457.