Old Moon's young crust mystery explained

Old Moon's young crust mystery explained
by Robert Schreiber
Berlin, Germany (SPX) Dec 20, 2024

Scientists have long debated the Moon's true age, with estimates varying by hundreds of millions of years. While some studies suggest the Moon formed 4.35 billion years ago, others place its origin at 4.51 billion years. Adding to the complexity, most lunar rock samples point to a younger age, but a handful of ancient zircon crystals appear much older. Researchers now propose that the Moon's crust was largely remelted after its formation, preserving only a few zircons unaffected by these extreme conditions.

The Impact that Created the Moon

The Moon's history begins with a massive collision in the early Solar System. A Mars-sized body struck the young Earth, generating enough heat to liquefy the planet and eject material into space. This debris eventually coalesced into the Moon, initially covered in a molten ocean of rock. Over time, the Moon cooled and drifted to its current orbit, approximately 384,400 kilometers from Earth.

"We are particularly interested in the phase when the distance between the Earth and the Moon was about one-third of today's distance," said Prof. Dr. Francis Nimmo of the University of California, Santa Cruz, the study's lead author. During this period, changes in the Moon's orbit caused intense tidal forces that generated significant internal heating. A similar phenomenon is observed on Jupiter's moon Io, the Solar System's most volcanically active body. Researchers believe Earth's early Moon underwent similar extreme volcanic activity.

A Molten Interior and Remelted Crust

According to the study, heat from the Moon's interior during this phase was sufficient to churn through its mantle, melting much of the crust over millions of years. While a global magma ocean did not reform, localized lava flows surfaced in some areas, while magma intrusions beneath the surface heated surrounding rocks.

This volcanic activity reset the geological clock of most lunar rocks. As Prof. Dr. Thorsten Kleine of the Max Planck Institute for Solar System Research explained, "Lunar rock samples don't reveal their original age but only when they were last strongly heated." However, heat-resistant zircon crystals avoided this reset in regions untouched by magma, preserving evidence of the Moon's earliest history.

Unlocking the Moon's Timeline

The research team concluded that the Moon is between 4.43 and 4.51 billion years old, with its crust undergoing remelting around 4.35 billion years ago. This new timeline resolves inconsistencies in prior studies and sheds light on the Moon's formation and its subsequent volcanic history.

The findings also address the apparent scarcity of lunar craters. "Lava from the Moon's interior could have filled the early impact basins, making them unrecognizable," said co-author Prof. Dr. Alessandro Morbidelli of the College de France. Additionally, differences between the compositions of the Moon's mantle and Earth's mantle may stem from a second melting event, which allowed some elements to escape into the Moon's iron core.

"The new results mean that all the pieces of the puzzle that previously didn't fit together now form a coherent overall picture of the Moon's formation," Kleine added.

Research Report:Tidally Driven Remelting Around 4.35 Billion Years Ago Indicates the Moon is Old