lunar volcanism new evidence moon age

Old Moon, Young Crust: New Evidence Points to Lunar Volcanism and Greater Age

Introduction: The Mystery of the Moon's Age

After its formation, the moon underwent periods of extreme volcanism, during which its crust repeatedly melted and was thoroughly mixed. At the time, the moon was in a closer orbit to Earth, and the tidal forces generated by this proximity caused internal heating that drove the volcanic activity. Such conditions are reminiscent of lo, Jupiter's moon, which remains the most volcanically active object in the solar system.

New Study Resolves Contradictions

Resolving earlier contradictions, a study published today in Nature by researchers from the University of California Santa Cruz, the Max Planck Institute for Solar System Research (MPS), and the Collège de France concludes that the moon formed between 4.43 and 4.51 billion years ago. The crust, however, is estimated to be 80–160 million years younger.

The moon has proven to be surprisingly enigmatic when it comes to revealing its age. Scientific estimates vary widely, spanning several hundred million years: some researchers propose its formation occurred 4.35 billion years ago, while others place its origin at 4.51 billion years ago.

A Clash of Ages: The Zircon Conundrum

One of the most notable discrepancies lies in the lunar rock sample: while the majority suggest a younger age, a few rare zirconium silicate crystals, or zircons, indicate a significantly older timeline. How can this be reconciled? In their recent study, researchers resolve this contradiction by calculating that the moon's crust underwent widespread melting after its formation, leaving only a handful of zircons intact under these extreme conditions.

A Collision and Its Ripple Effects

The Moon's Formation and Early History

The moon's history began with a cataclysmic impact. In the nascent solar system, a Mars-sized body collided with the young Earth, generating immense heat that melted the planet and ejected vast quantities of material into space. Over time, this debris coalesced to form the moon, initially enveloped in an ocean of molten rock. As millions of years passed, the moon cooled and gradually drifted away, eventually settling into its current orbit approximately 384,400 kilometers from Earth.

The Early Moon's Volcanic Activity

"We are especially intrigued by the period when the moon orbited Earth at a distance about one-third of today's," says Francis Nimmo, first author of the study and research at the University of California Santa Cruz.

At this stage, the moon's orbit experienced notable shifts in both shape and position. The increasingly elliptical trajectory led to variations in its speed and proximity to Earth, creating forces that extensively churned and heated the moon's interior.

A comparable phenomenon can be observed today on Jupiter's moon lo, which follows a slightly elliptical orbit around the gas giant. The immense tidal forces exerted by Jupiter render lo the most volcanically active body in the solar system. In its early history, Earth's moon likely rivaled lo in volcanic activity.

Intense Heat Flow and Crustal Melting

According to the researchers' calculations, the heat flow from the moon's interior was intense enough to melt and churn the entire mantle. Although a global magma ocean never formed during this phase, the interior heat gradually reached the entire surface over several million years, liquefying much of the crust-possibly on multiple occasions. In some regions, hot lava erupted onto the surface, while in others, magma intrusions beneath the surface heated the surrounding rocks.

Recalibrating the Geological Timeline

Volcanism and Lunar Rock Dating

Understanding the volcanic history is essential for accurately dating crustal rocks on the moon. Similar to Earth's rocks, lunar samples contain radioactive isotopes, which are atoms distinguished by varying neutron numbers.

Given the known decay rates of isotopes, their current concentrations allow scientists to determine the rock's age. The key detail lies in thermal dynamics: while the rock remains hot, isotopes can migrate between it and its environment. Once cooled, the composition becomes fixed, and the radioactive isotopes begin to decay—initiating the geological clock.

The Resetting of the Geological Clock

Thorsten Kleine, Director at the MPS and co-author of the study, explains that "vigorous volcanism on the moon effectively reset its geological clock. Consequently, lunar rocks reveal the age of their last major thermal exposure, not their primordial age."

The researchers' calculations indicate that only a few heat-resistant zircons retain evidence of the moon's more distant past. In regions where lava failed to reach the surface, these zircon grains stayed cool, preserving their internal clock.

"The lunar rock samples narrate the moon's tumultuous journey, form its formation to its intense volcanic activity," explains Kleine. "We simple misinterpreted these clues until now." According to the study, the moon formed 4.43 to 4.51 billion years ago, with violent volcanism shaping its crust around 4.35 billion years ago.

Revealing the Answer to the Riddle

The Moon's Crater Mystery

The recent discoveries also clarify several other long-standing contradictions that had baffled researchers. For instance, the relatively small number of craters on the moon seemed to contradict its ancient age. Given the vast time span, the moon should have experienced more impacts. Volcanic activity now provides a plausible explanation. According to co-author Alessandro Morbidelli from the Collège de France, "Lava from the moon's interior may have filled early imèact basins, rendering them unrecognizable."

Understanding the Lunar Mantle

The composition of the lunar mantle presented another challenge for scientist. This layer of rock, located just beneath the moon's crust, has a distinctly different composition compared to Earth's mantle. However, if the moon's interior underwent a second melting phase, certain substances could have migrated from the mantle into the iron core beneath.

"The latest findings suggest that the previously disconnected pieces of the puzzle now come together, forming a unified understanding of the moon's formation," explains Kleine.

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