Did the Moon Come From a Giant Space Donut?

Artist concept based on a NASA rendering of a synestia: the vaporized planetary body bulging into a donut shape due to its rapid rotation.  (Sarah Stewart/UC Davis)

A new explanation for the moon's origin is making the rounds: the moon may have formed inside the hot maelstrom of a young, freshly vaporized Earth following a cataclysmic walloping by another planet.

The collision would have taken place about 4.5 billion years ago and resulted in the formation of an object called a "synestia," a swirling, hot, donut-shaped cloud of molten and vaporized rock bulging outward from its rapid spinning.

Fledgling Luna Pops Out

The new theory, proposed by researchers from the University of California, Davis and Harvard University, has the infant Moon forming within the hot cloud of the synestia, with blobs of molten rock clumping together by gravity and snowballing to ever greater proportions. There may even have been two smaller moons that formed in this way, which eventually merged into one.

Diagram comparing a normal planet, a planet with a debris disk or ring, and a synestia.
Diagram comparing a normal planet, a planet with a debris disk or ring, and a synestia. (Simon Lock/Harvard University and Sarah Stewart/UC Davis)

Conditions within this protoplanetary womb would have been intense, with temperatures as high as 6,000 degrees F (a steel mill's blast furnace operates at about 2,000 degrees F) and gas pressures of 10 atmospheres (equivalent to water pressure in the ocean at a depth of 300 feet).

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At the same time, the greater bulk of the synestia — which would later become the Earth — cooled and shrank, its vaporized rock and metal condensing into liquid lava and pooling toward the center. As the synestia's gases pulled back, the fledgling liquid moon emerged from the mists.

The image of a baby bird poised at the edge of its nest before flying away comes to mind -- sort of.

A Challenge to the Big Whack

This new idea challenges — or at least greatly modifies — the current leading contender in moon-origin theories, the "Giant Impact Hypothesis" — often referred to as "the Big Whack."

Artist concept of a collision between two planets--in this case a hypothetical Moon-sized world impacting a Mercury-sized planet. Such a collision is believed to have occurred between the young Earth and another planet named Theia, resulting in the formation of the Moon.
Artist concept of a collision between two planets, in this case a hypothetical Moon-sized world impacting a Mercury-sized planet. Such a collision is believed to have occurred between the young Earth and another planet named Theia, giving birth to the Moon. (NASA/JPL-Caltech)

As the Big Whack tale goes, the moon formed after the young Earth was struck by another planet, a Mars-sized object named Theia (from Greek mythology, the titan mother of Selene, the moon goddess).

The "splash" from the giant impact is thought to have sent a pall of material into space, which formed a disk of debris that later coalesced to become the young moon.

Rock samples from several Apollo landing missions tell us that some of the moon's composition matches the chemistry of Earth rock, a fact that supports both competing theories. The large size of Earth's iron core may also be explained by the collision of two planets, with each object's core merging into one.

Splash Versus Smash

As with any effort to piece together the puzzle of the past from scant clues, peering back almost five billion years to the solar system's chaotic beginnings and imagining how the young Earth and Moon came into existence is not easy detective work.

Though both the Giant Impact (Big Whack) and Earth-Synestia smash-up theories rely on a collision of two planets, the newer Earth-synestia theory has a couple of things going for it that the Big Whack does not.

First, for a Big Whack to splash out debris into a moon-forming disk, the collision would need to be a special-case glancing blow, while a synestia can result from a wide range of impact styles, from glancing strikes to a head-on collision.

So, just like rolling a seven on a pair of dice is the most probable outcome in craps -- because there are more possible combinations of die faces that yield 7 -- a synestia is more likely to result from any given planet-on-planet smash-up.

James B. Irwin, Apollo 15 astronaut, collecting a sample of lunar soil. Apollo rock and soil samples revealed that the Moon is composed largely of material from Earth.
James Irwin, Apollo 15 astronaut, collecting a sample of lunar soil. Apollo rock and soil samples revealed that the Moon is composed largely of material from Earth. (NASA)

Another strength of the new theory is that it better explains the Moon's chemical makeup. Forming within the high-temperature crucible of the synestia would have supplied Earth-based rock and metal, but more volatile elements would have been baked away, yielding the moon's distinct composition identified from the Apollo samples.

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For now, the synestia premise may be the best model to explain the moon we know today, but who knows? Someday another theory may crop up that puts both current contenders on notice, opening our imaginations to a scenario even more compelling than the splash of a planet-sized whack or a lunar birth from a giant donut of rock vapor.

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