Moon’s Age Significantly Older Than Previously Estimated, Say Scientists

According to the prevalent theory, during a time when Earth was in its nascent stage, a celestial body roughly the size of Mars collided with our planet. This dramatic impact occurred while Earth was still in a molten and relatively malleable state. The collision resulted in the ejection of an extensive cloud of debris, which subsequently gathered and congealed to form the Moon in orbit around Earth.

The Moon’s apparent youthful appearance might be misleading. Recent research analyzing minuscule zircon grains within lunar samples retrieved from the Apollo missions indicates that the Moon’s age surpasses previous estimates, stretching back an additional 40 million years.

This finding indicates that the Moon’s age, as determined by a team led by geologist Jennika Greer from the University of Glasgow, is now established at a minimum of 4.46 billion years. This age places it just slightly younger than Earth, which is estimated to be around 4.54 billion years old.

These crystals are the oldest known solids that formed after the giant impact and because we know how old these crystals are, they serve as an anchor for the lunar chronology.

Philipp Heck
Cosmochemist of the Field Museum and University of Chicago

The Moon’s exact origin and formation timeline remain uncertain, but the presence of certain elements strongly hints at a connection to Earth. Currently, the prevailing hypothesis is the “Giant impact hypothesis,” which suggests that the Moon was formed through a colossal impact event during the early stages of the Solar System. This period was marked by a higher frequency of large celestial bodies and protoplanets colliding with one another.

Different estimations have been proposed regarding the timing of the colossal impact, yet a mounting body of evidence drawn from the dating of lunar samples indicates that it occurred significantly earlier than previously thought, possibly as early as 4.51 billion years ago, deviating from the initial approximations of approximately 4.4 billion years.

Zircon crystals provide a precise method for determining the age of a sample due to their unique formation process. During their formation, zircon crystals take in uranium while strongly repelling lead. As time passes, the radioactive uranium within the zircon gradually transforms into lead at a well-established rate. By examining the uranium-to-lead ratios within a zircon crystal, scientists can accurately calculate the time when the zircon crystal originated.

Tiny zircon crystals are present in lunar soil samples collected during the Apollo missions. Researchers led by Jennika Greer examined zircon crystals from samples gathered during the final Apollo mission, Apollo 17, in 1972. The team concluded that these crystals must have developed after the Moon’s surface had solidified, likely from the molten global ocean that initially enveloped the Moon after its creation.

When the surface was molten like that, zircon crystals couldn’t form and survive. So any crystals on the Moon’s surface must have formed after this lunar magma ocean cooled. Otherwise, they would have been melted and their chemical signatures would be erased.

Philipp Heck
Cosmochemist of the Field Museum and University of Chicago

The scientists employed atom probe tomography to analyze their samples. This technique involved refining the crystals into sharp points and using lasers to vaporize atoms from these points. A mass spectrometer was then used to assess the weight of the vaporized material, enabling the researchers to calculate the uranium-to-lead ratios.

As a result, the analysis revealed that these particular crystals had an age of 4.46 billion years. Consequently, this implies that the Moon, as the host of these crystals, must be at least as old. Such data provides valuable insights for scientists seeking to unravel other facets of the Moon’s history, including the duration required for its formation and solidification, as well as a more precise estimation of the timing of the colossal impact event.

It’s amazing being able to have proof that the rock you’re holding is the oldest bit of the Moon we’ve found so far. It’s an anchor point for so many questions about the Earth. When you know how old something is, you can better understand what has happened to it in its history.

Jennika Greer
Doctoral candidate at the Field Museum and the University of Chicago

Resources

1. ^WEBSITE Starr, M. (2023, October 23). The moon is millions of years older than we thought, scientists say. ScienceAlert. [ScienceAlert]
2. ^JOURNAL Will, P., Busemann, H., Riebe, M. E. I., & Maden, C. (2022). Indigenous noble gases in the Moon’s interior. Science Advances, 8(32). [Science Advances]
3. ^{WIKIPEDIA ENTRY} Age of Earth. (2023). In Wikipedia. [Wikipedia English]
4. ^WEBSITE EurekAlert!. (2023, October 23). Crystals brought back by astronauts show that the Moon is 40 million years older than scientists thought. EurekAlert! [EurekAlert!]