Establishing the age of the Moon is critical to understanding solar system evolution and the formation of rocky planets, including Earth. However, despite its importance, the age of the Moon has never been accurately determined.
A new paper by Barboni et al. titled “Early formation of the Moon 4.51 billion years ago” published in Science Advances presents uranium-lead dating of Apollo 14 zircon fragments that yield highly precise, concordant ages, which indicate the formation of the Moon happened within the first ~60 million years after the birth of the solar system.
The surface of the Moon provides the most accessible record of planetary formation processes and the early evolution of our solar system. Geochemical analyses of Apollo samples and lunar meteorites have contributed to the present paradigm of lunar formation through a giant impact with the proto-Earth, followed by rapid accretion of the orbiting debris and nearly complete melting of the proto-Moon.
Chemical differentiation and crystallization produced dense material that sank to the base of the lunar magma ocean and a buoyant crust that formed the lunar highlands. The data unambiguously show that the Moon was differentiated and mostly solidified 4.51 billion years ago, so the young ages obtained on lunar highland samples cannot be directly dating the age of the Moon.
Although there is consensus for this general model of lunar formation and early evolution, the timing of the giant impact and subsequent events remains controversial, with some planetary scientists favoring the formation within ~100 million years after the formation of the solar system and others arguing for a relatively late giant impact ~200 million years after the beginning of the solar system.
The “young” ages for lunar formation are difficult to reconcile with the zircon records from the Hadean era of Earth’s history and from the Moon. In addition, the vast majority of dynamical models are inconsistent with a Moon-forming impact occurring 100 million years after the birth of the solar system.
Knowledge of the age of the Moon is important not only for developing a detailed understanding of the duration and processes occurring in the lunar magma ocean but also for constraining competing models of solar system evolution during the later stages of planetary accretion.
A more direct constraint on the age of the Moon can be obtained by dating the chemical differentiation events accompanying the crystallization of the lunar magma ocean. Lunar zircons are thought to form in a reservoir enriched with potassium, rare-Earth elements, and phosphorus, which formed only at the end of the lunar magma ocean crystallization. Therefore, uranium-lead isotopic data on lunar zircons can be used to determine the age of bulk solidification of the Moon.
This work has significantly improved the precision and accuracy with which a chondritic uniform reservoir is known and therefore allows the determination of model ages with high confidence.
The team’s results provide minimum ages for differentiation of the lunar magma ocean and a minimum age for the formation of the Moon that is ~120-150 million years older than previous estimates. As a minimum estimate for the age of the Moon, scientists now believe the formation of the Moon must have occurred later than ~50 million years after the beginning of the solar system.
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Posted by: Soderman/SSERVI Staff
Source: M. Barboni, P. Boehnke, B. Keller, I. E. Kohl, B. Schoene, E. D. Young, K. D. McKeegan, Early formation of the Moon 4.51 billion years ago. Sci. Adv. 3, e1602365 (2017).