The Apollo missions may have been launched as part of a Cold War-era race with the Soviet Union, but the geological knowledge we gleaned from them was considerable. Until Apollo, we didn’t know that the Earth and the Moon appear to have similar ratios of atomic isotopes locked within their respective rocks, suggesting they formed from the same base material.

But the Apollo 14 astronauts may have brought back something even more extraordinary than the lunar rocks they thought they were collecting. A new geological analysis, published in Earth and Planetary Science Letters, suggests that one of the lunar rock samples the astronauts returned contained a rock sample that originated on Earth.

This is Lunar Sample 14321, also known as Big Bertha. The Lunar and Planetary Institute, which has an extensive set of slides and cross-sections of LS14321 on-file and available, writes:

[L]argest sample returned during Apollo 14 mission; also known as “Big Bertha”. This sample is the third largest sample returned by any Apollo mission. This breccia was collected during the second EVA at Station C1, near the rim of Cone Crater… This large sample is typical of the apparently dominant rock type in the Cone Crater ejecta blanket. It is a moderately well-indurated breccia, in which predominately dark clasts are set in a lighter matrix.

This rock contains a 2-gram felsite clast (felsite is a very fine-grained volcanic rock) with quartz, feldspar, and zircon embedded within it. These are all uncommon elements in lunar rocks, but common in terrestrial samples. The chemical analysis of the fragment also appears to demonstrate that it formed under conditions analogous to those found on Earth rather than within the Moon.

“What we did was use the composition of minerals in the fragment to show it formed under conditions that only occur on Earth,” Robinson told Gizmodo. “For example, the composition of certain minerals are sensitive to temperature and pressure; they contain more or less of various elements if they crystallize in hot or cool, and/or deep or shallow environments. Other minerals can indicate if the rock formed in the presence of lots of oxygen, or in a very oxygen-poor environment. Our data shows that this fragment formed in a higher-pressure, more oxygen-rich, and lower temperature environment than occurs on the Moon. Essentially, it had to come from an Earth-like environment.”

The paper lays this out in somewhat more detail, but the basic conclusion is the same. Either this fragment of rock formed under highly unusual conditions within the Moon, or it represents a bit of Earth flung heavenward as the result of a meteorite impact. Later lunar impacts and geological events resulted in a bit of Earth rock being embedded in a larger sample of a more typical moon rock.

This is less fanciful than one might think. The rock sample is an estimated 4.0-4.1B years old, from a time when the Moon was much closer to our own planet. It’s entirely possible that massive meteor strikes from early in our own history delivered significant amounts of material to the Moon in earlier epochs. The complete lack of erosion on the Moon’s surface may also have helped to preserve samples of this time period. The oldest definitely dated rocks on Earth have been dated to approximately this epoch, though there is some uncertainty in the exact age of such ancient samples.

The theory isn’t proven, and there have been some skeptics who have argued that lunar impacts could have created the rocks in question. But it’s also possible that the Apollo 14 astronauts brought back a tiny fragment of something even more remarkable than they knew.

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