During the mid-70s, two groups of astrophysicists have proposed independently that a grazing collision between a Mars-sized body and a proto-earth led to the formation of the moon. This giant impact hypothesis, which explained the large size of the moon relative to the earth and the rotation rates of the earth and the moon, has eventually became the leading theory for the lunar origin.
In 2001, scientists have learned that the isotopic compositions of a variety of elements in terrestrial and lunar rocks are almost similar. Based on the samples brought by the Apollo missions in the 70s, the lunar soil also contains low levels of organic matter in the form of amino acids which are actually the building block of proteins. It has also been known that the moon and the earth have the same abundance of the three isotopes of oxygen. However, numerical simulations of the collision have revealed that most of the material, around 60-80%, that cleaved into the moon came from the impactor rather than from earth.
Differences in isotopic compositions have served as fingerprints for planets and meteorites. The probability that the earth and the impactor have the same isotopic characteristics is very small. In line with this, the giant impact hypothesis has been affected by the isotopic crisis. Although accurate measurements of oxygen isotopes have been made, the isotonic compositions remain indistinguishable.
In 2015, geochemist Kun Wang of Washington University in St. Louis and geochemistry professor Stein Jacobsen of Harvard University have developed a technique for analyzing isotopes that can is ten times better than the best previous methods. In the low-energy impact model, it has been unveiled that the proto-earth and the moon have been covered in a silicate atmosphere. In the violent collision model, the impact has vaporizes the impactor and most of the proto-earth which led to the formation of the superfluid disk out of which the moon eventually crystallizes. This high-energy result has been published in the advance online edition of Nature in 2016.
Earth and Mars have isotopic compositions like those of chondritic meteorites which are thought to represent the original composition of the cloud gas and dust from which the solar system formed. The most likely large-scale event can be wholesale melting during the formation of the moon.
