Full Moon as seen from Earth's northern hemisphere

The moon, or Luna, is Earth's only natural satellite. It is littered with crater impacts and has a thin atmosphere, much like Mercury. There are several competing theories about how the moon came into existence, such as it being an asteroid that got trapped by Earth's gravity, or that it was born from Earth's ejected material that resulted from a hypothetical asteroid or planetoid impact.[1] Some attribute the name Theia to the solar body that hypothetically impacted Earth to create the Moon. However, the most likely theory is that the Moon formed 4.54 billion years ago along with the Earth through a process called accretion during the birth of the Solar System from the solar nebula.


Proposing the giant-impact hypothesis is the "simplest explanation" for the Moon's origins because it "fits in well with computer simulations". The main drawback with the theory is that there is no evidence for the popularized hypothetical Theia in lunar rock samples.[2] In 2001, a team at the Carnegie Institution of Washington reported that the rocks from the Apollo program carried an isotopic signature that was identical with rocks from Earth, and were different from almost all other bodies in the Solar System.[3] In 2007, researchers from the California Institute of Technology showed that the likelihood of Theia having an identical isotopic signature as the Earth was very small (less than 1 percent).[4] In 2014, a team in Germany reported that the Apollo samples had a slightly different isotopic signature from Earth rocks.[5]

Because the isotopic signatures of Moon rock are mostly identical with rocks from Earth, and only from Earth, with only slight differences, seems to lean more toward a co-formation theory than the giant-impact hypothesis. The Moon may have been born from the Earth through a process similar to that of the fission hypothesis, first proposed by George Darwin in the 19th century. Most scientists discount the fission hypothesis, saying that Earth could not have been spinning fast enough to expel rock. However a 2010 study suggested that a natural nuclear explosion, set up by the superconcentration of radioactive elements, may have provided enough kick to dislodge a moon-size piece, from the early Earth, into its orbit.[6] The co-formation theory proposes that moons form at the same time as their parent planet. Under such an explanation, gravity would have caused material in the early solar system to draw together at the same time as gravity bound particles together to form Earth. Such a moon would have a very similar composition to the planet, and would explain the Moon's present location. Even though the Earth and the moon share much of the same material, it should be noted that the moon is much less dense than our planet, which lends some argument against the co-formation theory.[7]

Moon phasesEdit

The rhythm of the moon's phases has guided humanity for millennia — for instance, calendar months are roughly equal to the time it takes to go from one full moon to the next. The same hemisphere of the moon is always in view because it takes 27.3 days both to rotate on its axis and to orbit Earth. Each phase gives a full moon, half moon or no moon (new moon) as the moon reflects sunlight. How much of it we see depends on the moon's position in relation to Earth and the sun. The hemisphere of the Moon that always faces away from Earth, is called the far side of the Moon.

Relation to EarthEdit

The moon, with a diameter of about 2,159 miles (3,475 kilometers), is bigger than Pluto. The moon is a bit more than one-fourth (27 percent) the size of Earth, a much smaller ratio (1:4) than any other planets and their moons. This ratio allows the moon to have a great effect on the planet, which is considered to make life on Earth possible.

Inside the MoonEdit

The moon very likely has a very small core, just 1 to 2 percent of the moon's mass and roughly 420 miles (680 km) wide. It likely consists mostly of iron, but may also contain large amounts of sulfur and other elements. Its rocky mantle is about 825 miles (1,330 km) thick and made up of dense rocks rich in iron and magnesium. Magmas in the mantle made their way to the surface in the past and erupted volcanically for more than a billion years — from at least four billion years ago to fewer than three billion years past. The crust on top averages some 42 miles (70 km) deep. The outermost part of the crust is broken and jumbled due to all the large impacts it has received, a shattered zone that gives way to intact material below a depth of about 6 miles (9.6 km).

On the MoonEdit

The moon is marked by craters formed by asteroid impacts millions of years ago. Because there is no weather, the craters have not eroded. The average composition of the lunar surface by weight is roughly 43 percent oxygen, 20 percent silicon, 19 percent magnesium, 10 percent iron, 3 percent calcium, 3 percent aluminum, 0.42 percent chromium, 0.18 percent titanium and 0.12 percent manganese. Orbiters have found traces of water on the lunar surface that may have originated from deep underground. They have also located hundreds of pits that could house explorers who remain on the moon long-term.


  1. YouTube, Documentary: Solar System, Mysteries of the Moon
  2. BBC, Traces of another world found on the Moon (2014), by Pallab Ghosh
  3. Wiechert, U.; et al. (October 2001). "Oxygen Isotopes and the Moon-Forming Giant Impact". Science. 294 (12): 345–348. Bibcode:2001Sci...294..345W. PMID 11598294. doi:10.1126/science.1063037. Retrieved 2009-07-05.
  4. Pahlevan, Kaveh; Stevenson, David (October 2007). "Equilibration in the Aftermath of the Lunar-forming Giant Impact". Earth and Planetary Science Letters. 262 (3–4): 438–449. Bibcode:2007E&PSL.262..438P. arXiv:1012.5323 Freely accessible. doi:10.1016/j.epsl.2007.07.055.
  5. Herwartz, D.; Pack, A.; Friedrichs, B.; Bischoff, A. (2014). "Identification of the giant impactor Theia in lunar rocks". Science. 344 (6188): 1146. Bibcode:2014Sci...344.1146H. PMID 24904162. doi:10.1126/science.1251117.
  6., How the Moon Formed: 5 Wild Lunar Theories, Fission
  7., How Was the Moon Formed?, Co-formation theory (2013)



Warning: the scope of Wikipedia articles are presented through popular and mainstream consensus which heavily supports, the likely erroneous, Theia hypothesis.