MOON MYSTERY

An extract from OMEGA Lifetime Magnetic Edition


By Clay Dillow 


The genesis of our Moon has always mesmerised science.Is it the celestial child of a cataclysmic collision with another planetary body, or is it a primordial object born in the dawn of our solar system? Magnetic lunar rock might hold the answer.

The Moon has always held a special attraction for humans, a magnetism which is both spiritual and secular, poetic and practical.

Anchored in the night sky, its phases marking the passage of days, months, and seasons. Among themany celestial bodies visible from Earth, the Moon has always been the closest to us, otherworldly yet familiar. Throughout most of human history it could be easily seen but not touched, its composition and origins a complete mystery. Half a century ago, Apollo astronautsreturning from the lunar surface punctured that mystique, bringing back the first physical artefacts from another planetary body: rocks, not so very different from the rocksfound on Earth. But by placing the once-untouchable Moon in the palm of scientists’ hands, those same rock samples also gave rise to new lunar mysteries. Prior to the Apollo landings, planetary scientists knew the Moon lacked the kind of global magnetic field characteristic of evolved, volcanic planets like Earth. But the rock samples brought back by astronauts told a very different story – that of a dynamic Moon, with a strong global magnetic field that had inexplicably vanished billions of years before. More than four decades later, researchers are still unravelling the mystery of the Moon’s lost magnetism, and they may finally be zeroing in on some answers.

“Is it a planet with its own geological history or is it really just a conglomeration of space dust?”

Dr Benjamin Weiss, Professor of planetary sciences

THE LUNAR DYNAMO

“The big question still driving lunar science today is: To what extent is the Moon a body like a planet – an evolved planet like Earth or Mars – and to what extent is it a kind of primordial object, a relic from the formation of the solar system?” asks Dr Benjamin Weiss, a professor of planetary sciences at the Massachusetts Institute of Technology (MIT). “Is it a planet with its own geological history or is it really just a conglomeration of space dust?”

For planetary scientists, the answer could shed light on the very origin of the Moon itself. The leading theory for the Moon’s beginnings positsthat the Moon and the Earth are made from the same celestial fabric,the Moon having been shorn from the Earth by a massive celestial collision during the solar system’s chaotic early existence. A competing theory says the Moon formed elsewhere in the solarsystem, an accretion of matter more akin to an asteroid which simply got caught in Earth’s gravity. The presence of a global magnetic fieldon the Moon – and the hot, molten history associated with that magnetism – would lend more weightto the collision theory, strengthening our understanding of our own planet’s history and the formation of the early solar system. But the magnetised Apollo samples initially puzzled scientists. The Earthgenerates its global magnetic field via the motion of molten rock sloshing around the outer core of theplanet, creating what’s known as a dynamo. But the Moon’s small sizerelative to the Earth dictates that any dynamo that existed in the Moon’s early history would havequickly cooled, slowed, and eventually ceased churning. Scientists were further stumped by the field’s relative strength. Measurements of the Apollo rocks indicate that the Moon at one point produced a magnetic field as strong as the Earth’s. But the Moon’s diminutive size – and far smaller dynamo – relative to its neighbour should have resulted in a far weaker magnetic field. In the decades following the Apollo era, scientists continued to mull over the problem. Some explored theories as to how a lunar dynamo might have powered itself for hundreds of millions of years beyond what conventional models considered possible. Others sought alternative explanations for the magnetism itself, theorising that asteroid impacts in the Moon’s surface could have created temporary magnetic fields strong enough to magnetise molten lunar material as it cooled into rock. Only in the last several years have scientists, armed with new tools, techniques, and data, found ways to move the investigation forward. And, in the hunt for the Moon’s lost magnetism, it turns out that timing is everything.

IMPACT THEORY

Starting in 2006, Weiss and some colleagues began re-examining the Apollo rocks with new, more sophisticated instruments in an effort to better understand their history – not only when they formed, but how long it took them to cool and solidify.

“Central to this whole thing is timing,” he says. “If we want to know the history of the magnetic field, we have to date the rock. That’s a reallyimportant part of our whole approach: combining the magnetisation of the rock with chronometry, which tells us when therock formed.” That chronometryallowed the team to build a more detailed history of the lunar samples,and to shake the foundation of the ‘impact theory’ for Moon’s magnetism. Using technologies unavailable to scientists during theApollo era, the team was able to identify rocks that had cooled over the course of days, months, and even thousands of years – far longer than any fleeting, impact-relatedmagnetic field could have persisted at the lunar surface. For these slow-cooling rocks to remain magnetised,the magnetic field must have been present for an extended period – forthousands of years, in some cases – suggesting the Moon indeed possessed a dynamo like the Earth’sbillions of years ago. But how did the dynamo produce such a strongmagnetic field? “The reason we study this is [because] we don’t really understand in detail how planets generate magnetic fields,” Weiss says. “It does seem like it had one, but the Moon is also so small that we can’t figure out how it could generate such a strong field. It’s a real mystery, and it’s also one of the reasons people doubted this in the beginning, because it’s so strange. It remains strange.” Nonetheless, the presence of a dynamo indicates that the Moon enjoyed a hot, dynamic geological history, a “critical test of the impact origin theory of the Moon’s origins,” he says. That’s not just significant for the study of the Moon, but for our understanding of the Earth as well, bringing into sharp focus how similar the Earth and the Moon likely are – and how they’ve diverged over time. “The Earth and the Moon appear to be the same, they are two pieces of the same primordial object,” Weiss says. “It’s fascinating to see how differently they evolved, based on what’s happened to them since. It underscores how special the Earth really is.”

“The Earth and the Moon appear to be the same, they are two pieces of the same primordial object.”

Dr Benjamin Weiss, Professor of planetary sciences

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