The leading hypothesis for the origin of the moon is the giant-impact hypothesis. The moon is believed to be the result of a giant impact between the Earth and Mars-sized astronomical body that occurred 4.5 billion years ago. This massive impact would have resulted in the loss of all gases from the debris of the collision that are believed to have formed the early moon. New research has found evidence of water in rock samples from the moon that are dated to the time of its speculated origins, which casts some doubt on the giant-impact hypothesis for the origin of the moon.
It is hypothesized that the Earth and a planetary-mass object the size of Mars collided and the debris from this collision would come together to form the moon. Scientists believe that the result of the impact would have caused a de-gassing of the moon, where gases like oxygen and hydrogen, which are need to create water, would have disappeared. This hypothesis is favored among scientists because there is some strong evidence for this collision. Evidence from moon samples indicates that the surface of the moon was covered in molten rock, which you would expect from a high energy collision between two large astronomical bodies. Oxygen isotope composition found in lunar rock samples were found to be virtually identical to those found in terrestrial rocks. Additionally, the authors of the oxygen isotope study found that it was unlikely that the three oxygen isotopes found in lunar samples were the result of lunar impacts.
New research using lunar samples from the Apollo missions has found evidence of more water on the moon than what was originally thought. In the event of a de-gassing event due to the collision, there shouldn’t be much evidence of water on the surface of the moon. Previous studies of lunar rock samples found evidence of the existence of hydrogen and hydroxyls, a chemical functional group consisting of one atom each of hydrogen and oxygen. Scientists hypothesized that these gas may have been brought to the moon on comets and asteroids that have impacted the moon since the giant-impact. This idea has come into question when researchers found six parts per million of water in a lunar rock sample, suggesting that these elements were from extraterrestrial origin due to the large amount.
The discovery of water on the moon doesn’t necessarily disprove the giant-impact hypothesis, but it does present an interesting conflict. It could very well be that comets or asteroids brought hydrogen and oxygen to the moon or not all elements of water disappeared due to the collision. Other hypotheses on the origins of the moon face more difficult challenges. The matching isotope composition of lunar and terrestrial rocks make it less likely that the moon came from a different part of the solar system and was brought into the orbit of the Earth by gravitational pull. The binary accretion theory hypothesizes that the moon was formed at the same time and place as the Earth, which may explain the matching isotope compositions. Binary accretion faces the same problem with the gravitational pull hypothesis in that the high-altitude orbit of the moon does not support binary accretion.
It has been long known that the elements of water are present on the moon. This could be explained by the idea that these elements could have been brought to the moon by comets or asteroids and we are aware of these impacts, evident by the large visible craters on the moon. The presence of water in relatively large quantities in rocks that have been dated back to the 4.5 billion years ago, which is when scientists speculate is the time that the large-impact occurred, brings up some doubt regarding the large-impact hypothesis. It very well could be that some hydrogen and oxygen survived the collision and scientists do believe that the presence of water does not refute the large-impact hypothesis.
