A lunar rock sample collected by Apollo 14 astronauts in 1971 contains traces of minerals with a chemical composition common to Earth and very unusual for the Moon, according to research led by Dr. Jeremy Bellucci from the Swedish Museum of Natural History and Curtin University’s Professor Alexander Nemchin. The rock may be originally from Earth, thrown into space when an asteroid struck our planet about 4 billion years ago (Hadean eon). It was subsequently mixed with other lunar surface materials into one sample.
The two Moon-exploring crew men of the Apollo 14 lunar landing mission photographed and collected the large rock pictured just above the exact center of this picture. The rock, casting a shadow off to the left, is the lunar breccia 14321. It lies between the wheel tracks made by the modular equipment transporter (MET) or rickshaw-type portable workbench. A few prints of the lunar overshoes of the crew members are at the left. This photo was made near the boulder field near the rim of Cone Crater. Image credit: NASA.
Dr. Bellucci, Professor Nemchin and their colleagues analyzed a small felsite (granite) fragment of the lunar breccia 14321 (also known as ‘Big Bertha’), the largest sample returned during Apollo 14 mission.
The team’s analysis shows the fragment crystallized in a terrestrial-like oxidized system, at terrestrial temperatures, rather than in the reducing and higher temperature conditions characteristic of the Moon.
“The 1.8-gram fragment showed mineralogy similar to that of a granite, which is extremely rare on the Moon but common on Earth. It also contains quartz, which is an even more unusual find on the Moon,” Professor Nemchin said.
“By determining the age of zircon found in the Apollo 14 sample, we were able to pinpoint the age of the host rock at about 4 billion years old, making it similar to the oldest rocks on Earth.”
“In addition, the chemistry of the zircon in this sample is very different from that of every other zircon grain ever analyzed in lunar samples, and remarkably similar to that of zircons found on Earth.”
“It is an extraordinary find that helps paint a better picture of early Earth and the bombardment that modified our planet during the dawn of life,” added team member Dr. David Kring, a Universities Space Research Association (USRA) scientist at the Lunar and Planetary Institute.
The lunar breccia 14321. Image credit: NASA.
It is possible that the Apollo 14 sample is not of terrestrial origin, but instead crystallized on the Moon, however, that would require conditions never before inferred from lunar samples. It would require the sample to have formed at tremendous depths, in the lunar mantle, where very different rock compositions are anticipated. Therefore, the simplest interpretation is that the sample came from Earth.
The team’s analyses are providing additional details about the sample’s history.
The rock crystallized about 12 miles (19 km) beneath Earth’s surface 4-4.1 billion years ago. It was then excavated by one or more large impact events and launched into cis-lunar space. Once the sample reached the lunar surface, it was affected by several other impact events, one of which partially melted it 3.9 billion years ago, and which probably buried it beneath the surface.
The sample is therefore a relic of an intense period of bombardment that shaped the Solar System during the first billion years. After that period, the Moon was affected by smaller and less frequent impact events.
The final impact event to affect this sample occurred about 26 million years ago, when an impacting asteroid hit the Moon, producing the 1,115-foot (340 m) wide Cone Crater, and excavating the sample back onto the lunar surface where astronauts collected it almost exactly 48 years ago.
“Impacts on the Moon would have mixed the Earth rocks with lunar rocks, including at the future Apollo 14 landing site, where it was collected by astronauts and brought back home to the Earth,” Professor Nemchin said.
The study was published in the journal Earth and Planetary Science Letters.
_____
J.J. Bellucci et al. 2019. Terrestrial-like zircon in a clast from an Apollo 14 breccia. Earth and Planetary Science Letters 510: 173-185; doi: 10.1016/j.epsl.2019.01.010
