A Terrestrial Magma Ocean Was Key to the Moon’s Formation
The origin of the Moon is one of the fundamental questions in planetary science. Its presence and size relative to us are unique in the inner solar system, and its influence on the Earth’s rotation and tides has been crucial to the development of life. Samples brought back from the Apollo missions show that the Moon is composed largely of material derived from Earth’s mantle, and that it formed later than the Earth itself.
This discovery became the core of the Giant Impact Hypothesis. The hypothesis states that a large planetary body, about the size of Mars, impacted Earth early in its history. Material dislodged from that collision was ejected into orbit, eventually coalescing to form the basis of the Moon. However, when this idea was tested in computer simulations, it turned out that the material that formed the Moon would be made primarily from the impacting object — not Earth’s mantle. Simulation conditions in which the Moon had its proper complement of Earth-derived materials required a small subset of impact angles and angular velocities, making the formation of the Moon by this method seem less likely.
A new study published April 29 in Nature Geoscience, co-authored by Yale geophysicist Shun-ichiro Karato, offers an explanation. The key, Karato says, is that the early, proto-Earth — about 50 million years after the formation of the Sun — was covered by a sea of hot magma, while the impacting object was likely made of solid material. Karato and his collaborators set out to test a new model, based on the collision of a proto-Earth covered with an ocean of magma and a solid impacting object.
The model showed that after the collision, the magma is heated much more than solids from the impacting object. The magma then expands in volume and goes into orbit to form the Moon, the researchers say. This explains why there is much more Earth material in the Moon’s makeup. Previous models did not account for the different degree of heating between the proto-Earth silicate and the impactor.
“In our model, about 80% of the Moon is made of proto-Earth materials,” said Karato, who has conducted extensive research on the chemical properties of proto-Earth magma. “In most of the previous models, about 80% of the Moon is made of the impactor. This is a big difference.”
Karato said the new model confirms previous theories about how the Moon formed, without the need to propose unconventional collision conditions. The requirement that Earth’s surface be a magma ocean at the time is more plausible than it sounds. At the likely time of the Moon-forming impact, the solar system was still a chaotic place, filled with small planetary bodies that had yet to coalesce into planets. An impact by a body large enough to melt the Earth’s surface, but not large enough to disrupt it (as suggested by the Giant Impact Hypothesis) was a much more common event.
For the study, Karato led the research into the compression of molten silicate. A group from the Tokyo Institute of Technology and the RIKEN Center for Computational Science developed a computational model to predict how material from the collision became the Moon.
Portions of this article were provided by Yale News.