Origin of the Moon: New evidence in old rocks for Lunar impact theory

Guest “wishing I could have been Jack Schmitt” by David Middleton

Earth Rocks and Moon Rocks Are More Different Than We Thought
New analyses of oxygen isotopes reveal terrestrial and lunar rocks aren’t as similar as previously thought, potentially changing the way we think the Moon formed.

By Javier Barbuzano

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.

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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.

Erik 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.

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Eos

This phrase doesn’t make any sense… “rock types that formed deeper in the lunar mantle, such as volcanic glass and basalts”… Volcanic glass and basalt form at the surface from rapidly cooling lava. The author has an MS in science journalism, so he may not have understood what he read or was told.

However, this article makes more sense…

We May Have Finally Found a Chunk of Theia Buried Deep Inside The Moon
MICHELLE STARR 9 MARCH 2020

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Planetary scientist Erick Cano and colleagues went a different route: a careful reanalysis of the lunar samples.

They acquired a range of samples from different rock types gathered on the Moon – both high- and low-titanium basalts from the lunar maria; anorthosites from the highlands, and norites from the depths, brought upwards during a process called lunar mantle overturn; and volcanic glass.

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For the new analysis, the research team modified a standard isotope analysis technique to produce high-precision oxygen isotope measurements. And they found something new indeed: that oxygen isotopic composition varied depending on the type of rock tested.

“We show,” they wrote in their paper, “that the method of averaging together lunar isotope data while ignoring lithological differences does not give an accurate picture of the differences between the Earth and Moon.”

In fact, the deeper the rock sample’s origins, the researchers found, the heavier the oxygen isotopes, compared to Earth’s.

This difference could be explained if only the outer surface of the Moon was pulverised and mixed during the impact, resulting in the similarity with Earth. But deep inside the Moon, the Theia chunk remained relatively intact, and its oxygen isotopes were left closer to their original state.

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Science Alert

Norites are intrusive igneous rocks that would have formed deeper in the lunar crust and/or mantle. The oxygen isotope differential could be indicative of Theia having formed farther away from the Sun than the Earth. Unfortunately, the paper is pay-walled. Here’s the abstract:

Article
Published: 09 March 2020
Distinct oxygen isotope compositions of the Earth and Moon
Erick J. Cano, Zachary D. Sharp & Charles K. Shearer

Nature Geoscience (2020)

The virtually identical oxygen isotope compositions of the Earth and Moon revealed by Apollo return samples have been a challenging constraint for lunar formation models. For a giant impact scenario to explain this observation, either the precursors to the Earth and Moon had identical oxygen isotope values or extensive homogenization of the two bodies occurred following the impact event. Here we present high-precision oxygen isotope analyses of a range of lunar lithologies and show that the Earth and Moon in fact have distinctly different oxygen isotope compositions. Oxygen isotope values of lunar samples correlate with lithology, and we propose that the differences can be explained by mixing between isotopically light vapour, generated by the impact, and the outermost portion of the early lunar magma ocean. Our data suggest that samples derived from the deep lunar mantle, which are isotopically heavy compared to Earth, have isotopic compositions that are most representative of the proto-lunar impactor ‘Theia’. Our findings imply that the distinct oxygen isotope compositions of Theia and Earth were not completely homogenized by the Moon-forming impact, thus providing quantitative evidence that Theia could have formed farther from the Sun than did Earth.

Nature Geoscience

While Earth and Moon rocks are extremely similar, there are distinct diagnostic differences. The Lunar and Planetary Institute has a fantastic compendium of lunar sample data from the Apollo and Luna missions. Each report has pictures, photomicrographs of thin sections, x-ray diffraction, x-ray fluoresce and other geochemical, petrological and mineralogical data. One of my long-term projects has been to look at the differences in basalts from the Earth and the Moon.

Figure 1. Basalt ferrous oxide vs alumina

Terrestrial basalt plots below the lunar trend line in a fairly tight cluster, irrespective of where (mid-ocean ridges, island volcanoes or continental flood basalts) or when (recently, Miocene Epoch or Triassic Era). Whereas the mare basalt samples collected by the Apollo 11 and 12 missions plot in a cluster around the top left of the lunar trend line. The mare basalts most likely formed at the surface, billions of years ago, from impact-related melts.

Hopefully, in a few years, the Artemis missions will reinvigorate lunar science.