New Evidence That Two Planets Collided To Form The Moon

©NASA

Moon formation

"There's a huge difference between the modern elemental makeup of the Earth and Moon and we wanted to know why," said NASA planetary scientist Justin Simon. "Now, we know that the Moon was very different from the start, and it's probably because of the 'Giant Impact' theory."

Simon and graduate fellow Tony Gargano, both from the agency's Astromaterials Research and Exploration Science division at Johnson Space Center in Houston, led the research and recently published the results in the journal Proceedings of the National Academy of Sciences.

The Giant Impact

Scientists have proposed many ideas for how the Moon formed. A leading contender, the Giant Impact theory, speculates that when Earth was a young planet and just beginning to form, it was hit by another emerging planet named Theia, located nearby. The collision caused both planets to temporarily splatter apart into globs of gas, magma, and chemical elements before reforming into the bodies we know today to be the Earth and Moon. The research by Simon and Gargano is adding support that further confirms this theory.

A new look at something old

Simon, Gargano, and their research team found evidence for the collision theory when they were conducting a study to understand the significant differences in chemical composition between Earth and Moon rocks. The lunar samples came from rocks gathered 50 years ago by the Apollo missions and saved for research in the future, when new techniques and tools would be available.

The researchers zeroed in on looking at the amount and types of chlorine found in the rocks. They chose chlorine because it's a volatile element, meaning it vaporizes at relatively low temperatures, and tracking it is helpful for understanding planetary formation. Chlorine exists in two abundant and stable forms: light and heavy. The terms heavy and light are used to describe chemicals which have variations in their atomic structure, also called isotopes.

What they found is that the Moons rocks contain a higher concentration of heavy chlorine, whereas Earth rocks are richer in light chlorine.

Heavy chlorine has a tendency to resist change and stay put, but light chlorine is more reactive and responsive to forces. In the Giant Impact model, both the Earth and Moon blobs initially contained a mix of heavy and light chlorines. But, as the planets came back together, the larger Earth dominated the unfolding processes and drew the lighter, easily vaporized chlorine to itself, leaving the Moon depleted of light chlorine and other more easily evaporated elements. According to the measurements the scientists took, this is exactly what appears to have happened.

As a sort of cross-check, Simon and Gargano analyzed the rocks samples for differences in other elements that are part of the same family of chemicals as chlorine, called the halogens. They saw this family of more easily evaporated elements were lost from the Moon. However, they didn't see a pattern of differences among the halogen chemicals that might be caused by something that happened a later time between the Earth and Moon. This means that the Moon's lighter chlorine composition and relative halogen abundances must have been set at the very beginning.

"The chlorine loss from the Moon likely happened during a high-energy and heat event, which points to the Giant Impact theory," said Gargano.

Opportunity meets experience

Gargano is a Ph.D. student at the University of New Mexico and led the study as a member of NASA's graduate fellowship program. The program offers Gargano unique access to funding, materials, labs, and most importantly, mentoring expertise only available through NASA.

"It's been incredibly beneficial to me because I get to see the inner workings of NASA and learn how world-class researchers determine how to best tackle scientific work and related issues," said Gargano.

Gargano has been working closely with Simon, who is an expert on planetary chemistry and using devices called mass spectrometers to determine the composition of cosmic substances, such as asteroids and lunar rocks. Simon and his team developed the complex technical approaches the study needed for getting the most accurate halogen measurements from the rock samples.

"Many previous lunar studies have looked at chlorine inside a specific mineral, called apatite, but we developed a way to measure chlorine throughout the rock, which gives us a more complete story," said Simon.

Gargano is advised by Professor Zachary Sharp, who is a co-author of the study and a pioneer in chlorine cosmic chemistry. The research team also included NASA's Wayne Buckley, Apollo Next Generation Sample Analysis Co-Lead Chip Shearer, and world-renowned scientist Sir Alex Halliday.

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