In 1971, Apollo 14 brought back the 20-pound rock Big Bertha; in 2019, scientists found a fragment that likely came from Earth, excavated about 4 billion years ago

When Apollo 14 astronauts Alan Shepard and Edgar Mitchell picked up a hefty, football-sized rock near the rim of Cone Crater in February 1971, they had no idea it might be carrying a secret piece of home. This rock, later nicknamed "Big Bertha," weighed almost 20 pounds, making it the third-largest sample returned during the Apollo program, smaller only than Big Muley from Apollo 16 and Great Scott from Apollo 15. After almost five decades, a study titled ‘Terrestrial-like zircon in a clast from an Apollo 14 breccia’ published in Earth and Planetary Science Letters found that a tiny piece within this rock was most likely not formed on the Moon at all.

A football-sized rock with a hidden passenger

According to NASA's lunar sample records, Big Bertha is officially known as lunar sample 14321. It is a breccia rock, meaning that it is a collection of rock fragments joined together through ancient impacts. The astronauts collected Big Bertha from near Cone Crater during the mission’s second moonwalk. For years, scientists studied small pieces of the sample until a team led by geochemist Jeremy Bellucci and planetary scientist Alexander Nemchin decided to study one specific tiny fragment buried inside it.

A two-gram clue that did not belong

It was a rather odd discovery. This piece, which weighed 1.8 grams, approximately the same as a small coin, consisted of quartz, feldspar, and zircon. While these minerals are common on Earth, they are pretty uncommon on the Moon. As Professor Nemchin explained in a Curtin University statement, the mineralogy of the sample is similar to that of granite, which is widely distributed on Earth but extremely rare on the Moon. He added that the sample also contained quartz, which he called "an even more unusual find on the Moon."

What the chemistry actually revealed

The team did not focus only on the unusual minerals. They examined the zircons closely since the mineral has the capability of retaining evidence of its formation, such as the temperature and pressure in which it was formed. The study suggests that the zircon may have crystallized at much cooler temperatures in an oxygen-rich environment resembling Earth's crust rather than the Moon's mantle. Analysis of the mineral dated the rock to be about 4 billion years old, similar in age to some of the most ancient rocks known on Earth.

A journey that started with a giant impact

So how is it possible for a chunk of Earth to be present on the Moon? According to a press release by the Lunar and Planetary Institute, part of the Universities Space Research Association, the rock crystallized approximately 20 kilometers beneath Earth's surface about 4 to 4.1 billion years ago. Following an enormous asteroid or comet impact, the rock may have been ejected from Earth's surface and out of its atmosphere. Back then, the Moon sat about three times closer to the Earth than its current position, and therefore it had less distance to travel. The fragment may later have been deposited on the lunar surface, where other impacts folded it into lunar rocks and may have partially melted it about 3.9 billion years ago. A much more recent impact, roughly 26 million years ago, formed Cone Crater and exposed the Big Bertha boulder itself at the surface, where Apollo 14 astronauts found it in 1971; the tiny terrestrial fragment inside it would only be identified decades later, once scientists cut into the sample in the lab.

Why scientists are not fully settled on this yet

It is a great hypothesis, but not a proven fact. The study's authors acknowledged the possibility that the fragment could have been created on the Moon under circumstances never encountered in any previous lunar samples. According to Nemchin, the process should involve unusual local conditions, resulting in the creation of a rock indistinguishable from one created on Earth. The researchers left the possibility open, but said an Earth origin was simpler because it does not require a new kind of lunar chemistry not seen elsewhere on the Moon. Dr. David Kring, whom the Lunar and Planetary Institute credits with challenging the team to search for a piece of Earth on the Moon, said in the same statement that he expects the terrestrial origin hypothesis to be debated within the geology community but added that other pieces of ancient Earth were likely scattered across the lunar surface and would be identified as more samples get studied.

Why the fragment matters

If confirmed, this tiny fragment could end up becoming the first known sample of Earth ever recovered from another world. As Kring said in the same statement, this finding can help in better understanding Earth’s early formation and the massive impacts that occurred in the early days of our planet’s creation. Earth's own plate tectonics and erosion have wiped out almost all rocks from its earliest history, so scientists have very little material to study from that period. A rock like this, preserved on the airless, geologically quiet Moon, could offer a rare window into what young Earth actually looked like.