June 6, 2014
Analysis Of Apollo Rocks Supports Collision Theory Of Moon Formation
redOrbit Staff & Wire Reports - Your Universe Online
The Moon was most likely formed by a collision between the Earth and a planet-sized object that took place roughly 4.5 billion years ago, a team of German scientists report in research published Friday by the journal Science.
That object was a planetesimal named Theia, Dr. Daniel Herwartz of the University of Goettingen and his colleagues explained in their study. Most numerical models of the collision imply that there is a higher amount of this planetesimal in the Moon than in the Earth, though previous attempts to identify the isotopic component of Theia in lunar rocks had been unsuccessful.
Due to the lack of definitive evidence supporting the Theia theory, “it was getting to the stage where some people were suggesting that the collision had not taken place,” Dr. Herwartz told BBC News science correspondent Pallab Ghosh on Thursday. “But we have now discovered small differences between the Earth and the Moon. This confirms the giant impact hypothesis.”
The research team used a new technique guaranteeing perfect separation of oxygen isotopes from other trace gases, Dr. Herwartz told Irene Klotz of Reuters. Based on their research, they concluded that the moon’s composition is roughly 50 percent Theia and 50 percent Earth, though he noted that additional research is needed to confirm those estimates.
“This work is the first to claim to see such a difference in the isotopes of oxygen,” Robin Canup, a Southwest Research Institute planetary scientist who was not involved in the research, told Klotz. “The reported difference between the Earth and moon is extremely small, small enough that I think there will be debate as to whether the difference is real or an artifact of how one interprets the data.”
Specifically, Dr. Herwarz and his associates compared the ratios of 17O/16O in the Apollo samples with rocks originating from Earth. Originally, they used lunar samples that had reached our planet in meteorites, but since those samples had exchanged isotopes with Earth-based water, fresher samples had to be obtained. When compared to Earth samples, the Apollo rocks were found to have significantly higher 17O/16O levels.
“The differences are small and difficult to detect, but they are there,” he said in a statement. “This means two things; firstly we can now be reasonably sure that the Giant collision took place. Secondly, it gives us an idea of the geochemistry of Theia. Theia seems to have been similar to what we call E-type chondrites.”
“If this is true, we can now predict the geochemical and isotopic composition of the Moon, because the present Moon is a mixture of Theia and the early Earth. The next goal is to find out how much material of Theia is in the Moon,” he added. The results of the study will also be presented at the Goldschmidt international geochemistry conference in Sacramento, California on June 11.