Illustration of a huge planetoid impacting Earth
This artist’s rendition shows a giant impact similar to the one that scientists think created the Earth-Moon system 4.5 billion years ago. Credit: NASA/JPL-Caltech

One of the most important scientific outcomes of the Apollo program was giving scientists the opportunity to explain our Moon’s origins.

Geochemical analysis of the Apollo lunar samples suggested that our Moon was formed 4.5 billion years ago, when a Mars-sized body known as Theia hit Earth when our planet had almost completely formed. Computer models indicate that in this “big splat,” most of the material that ended up forming the Moon—between 70% and 90% of the satellite’s composition—came from Theia.

Although most planetary scientists think the giant impact actually happened, evidence of Theia has been hard to find. Lab measurements of the isotopic ratios of multiple elements such as oxygen have found that Earth and the Moon are virtually indistinguishable. They couldn’t find a trace of Theia’s chemical signature.

This conundrum left researchers with just two likely explanations. On the one hand, Theia might have had the same exact isotopic composition as Earth. This idea does not bode well for current theories about the formation of the solar system and is largely ruled out. On the other hand, the impact could have been so powerful that it caused a thorough mixing of Earth’s and Theia’s material or was at least able to hide its results from our current array of instruments.

Researchers have tried to model impact scenarios that fulfill those requirements as well as other particularities of the Earth-Moon system, such its large mass ratio and its high angular momentum, but the perfect model has remained elusive.

A New Way of Looking at Lunar Rocks

Now a group of researchers has finally detected oxygen isotope differences between terrestrial and lunar rocks, something that could ease constraints when creating lunar formation models and rule out some of the most extreme scenarios.

Erick Cano, a graduate student at the University of New Mexico in Albuquerque, and his colleagues conducted new high-precision measurements of the oxygen isotope compositions of a variety of lunar samples from the Apollo missions. Although rock that formed near the lunar surface appeared to have an oxygen isotope composition identical to that of Earth rocks, rock types that formed deeper in the lunar mantle, such as volcanic glass and basalts, had a distinct isotopic composition. The new work, which originated as part of Cano’s master’s thesis, was published online on 9 March in Nature Geoscience.

Cano doesn’t have a clear answer about why the difference among lunar samples wasn’t found in previous analysis. “It could be just the selection of samples that we got from NASA…that covered a wide variety of the different rock types.”

“It’s not that Theia is being preserved in the core necessarily, but we believe that the lunar interior is preserving this kind of original composition of the Moon postimpact.”

Cano and his colleagues propose that the difference in isotopic composition among Moon rocks could have been generated after the big splat, when vaporized rocks excavated from Earth’s mantle formed a silicate-rich atmosphere around the young Moon. This atmosphere “contaminated” the upper layers of the lunar mantle, still molten and exposed. Later on, this material was incorporated into the modern lunar surface, masking Theia’s isotopic signature.

“It’s not that Theia is being preserved in the core necessarily, but we believe that the lunar interior is preserving this kind of original composition of the Moon postimpact,” Cano said.

The isotopic difference is not very large. It’s much smaller than the isotopic difference between Mars and Earth, for instance, which, in turn, is much smaller than that between Earth and most types of meteorites. However, it’s enough to “put a slightly different constraint on models of the impact that made the Moon,” said Jay Melosh, a planetary scientist at Purdue University in West Lafayette, Ind., who was not involved in the new research. “I think it’s an important contribution.”

According to Melosh, the finding “is not something that completely turns the models on their heads, but it might make more difficult to accept the extreme mixing models.”

“Models will always have to include some degree of mixing between the proto-Earth and Theia because that’s kind of inevitable in a collision that is as violent as the giant impact hypothesis predicts,” Cano said. “This does eliminate the necessity for these models to include a mechanism that will completely homogenize the oxygen isotopes between the Earth and the Moon, so it essentially kind of opens the door for new parameters that can be used for an entirely new range of impact scenarios.”

The oxygen isotope system is the most precisely known of all isotope systems between Earth and the Moon. If lunar lithology keeps yielding varying rates of isotopic compositions across different elements, science could be a step closer to finding what’s left of Theia. “Hopefully, we will start seeing this in other elements,” Cano said.

—Javier Barbuzano (@javibarbuzano), Science Writer


Barbuzano, J. (2020), Earth rocks and Moon rocks are more different than we thought, Eos, 101, Published on 09 March 2020.

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