Lunar Crystals from Apollo Missions Shed Light on the Moon’s Accurate Age

In 1972, Apollo 17 astronaut Harrison Schmitt found himself enveloped in lunar dust. Scientists have recently dated crystals collected during that mission and have determined that the Moon is approximately 4.46 billion years old, which is 40 million years older than previous estimations. Image Credit: NASA

Following the cooling of a magma ocean, the crystals on the Moon formed a minimum of 4.46 billion years ago.

Over 4 billion years in the past, when both the Earth and the Solar System were in their infancy, a celestial object roughly the size of Mars collided with Earth. The largest fragment from this collision eventually became the Moon. However, the specific timing of this occurrence has been a subject of continued uncertainty. A recent scholarly article in the journal Geochemical Perspectives Letters discloses that crystals retrieved from the Moon by the 1972 Apollo mission have helped clarify the Moon’s formation timing. The study posits that the Moon is at least 4.46 billion years old, effectively revising its age by an additional 40 million years.

The Importance of Moon Crystals

Philipp Heck, the Senior Director of the Negaunee Interactive Research Center and Robert A. Pritzker Curator for Meteoritics and Polar Studies at the Field Museum, as well as a professor at the University of Chicago, states that these crystals are the oldest known solids to have formed following the monumental collision. Heck, who is the senior author of the study, asserts that these ancient crystals provide a foundational benchmark for lunar chronology.

Microscopic observation of a lunar zircon crystal. Image Credit: Jennika Greer

This groundbreaking discovery stems from collaborative research led by Heck and the study’s lead author, Jennika Greer, a former doctoral candidate at the Field Museum and the University of Chicago. Greer, now a research associate at the University of Glasgow, recounts how their team was joined by co-authors Bidong Zhang and Audrey Bouvier to acquire a nanoscale view of these samples for a more complete understanding.

Insights into Moon Formation and Zircon Crystals

The lunar dust examined in the study was brought back by the Apollo 17 astronauts during the final crewed mission to the Moon in 1972. This lunar dust includes minuscule crystals that are billions of years old, serving as key indicators of the Moon’s age. Philipp Heck explains that when the Mars-sized object collided with Earth, the energy of the impact rendered the nascent Moon’s surface into molten rock. Crystals such as zircon could only have been formed once this molten surface had solidified, otherwise their chemical footprints would be obliterated.

A microscopically sharp point of a lunar crystal. Image Credit: Jennika Greer

Since these crystals could only have crystallized subsequent to the cooling of the magma ocean, analyzing the age of the zircon crystals would disclose the Moon’s minimum possible age. A preceding study by co-author Bidong Zhang had suggested an age for the Moon, but this latest research employs atom probe tomography for the first time to conclusively determine the age of the oldest known lunar crystal.

Atom Probe Tomography: A Method of Precise Analysis

Atom probe tomography involves initially sharpening a lunar sample to a pointed tip using a focused ion beam microscope. “We subsequently utilize ultraviolet lasers to evaporate atoms from the surface of this sharpened tip,” explains Greer. These evaporated atoms are then analyzed in a mass spectrometer, providing insights into their composition based on their velocity.

Lead author Jennika Greer operating the atom probe. Image Credit: Dieter Isheim, Northwestern University

By employing this technique of atom-by-atom evaluation at Northwestern University, the researchers could determine how many atoms within the zircon crystals had undergone radioactive decay, transforming from one element to another. For instance, uranium decays into lead. Using established decay rates, and by observing the ratio of uranium to lead isotopes in the sample, the scientists could ascertain the age of the sample, which was found to be approximately 4.46 billion years old. Therefore, the Moon must be at least as old.

Broader Implications

Greer articulates the significance of their findings: “Holding a rock that is confirmed to be the oldest known piece of the Moon provides a cornerstone for answering numerous questions about Earth’s history.” Heck underscores the Moon’s role in Earth’s ecological system, stating that a deeper understanding of its formation is crucial, as the Moon plays a significant role in stabilizing Earth’s rotational axis and influencing tides.

The study was co-authored by Jennika Greer at the University of Glasgow, Bidong Zhang at the University of California, Los Angeles, Dieter Isheim and D.N. Seidman at Northwestern University, Audrey Bouvier at Bayreuth University, and Philipp Heck at the Field Museum.

Reference: “4.46 Ga zircons anchor chronology of lunar magma ocean,” authored by J. Greer, B. Zhang, D. Isheim, D.N. Seidman, A. Bouvier, and P.R. Heck, was published in Geochemical Perspectives Letters on 23 October 2023. DOI: 10.7185/geochemlet.2334.

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More about Moon’s age

• Geochemical Perspectives Letters Journal
• Field Museum’s Meteoritics and Polar Studies
• University of Glasgow Research Departments
• University of California, Los Angeles Research Studies
• Northwestern University’s Analytical Instruments
• Bayreuth University Research Publications
• NASA’s Apollo Missions Archive
• Radioactive Decay and Radiometric Dating
• The Role of the Moon in Earth’s Ecological System