Mysterious Origin of the Moon Finally Explained by Strange New Synestia Theory

Wednesday, 28 February 2018 - 12:11PM
Astrophysics
Space
Moon
Wednesday, 28 February 2018 - 12:11PM
Mysterious Origin of the Moon Finally Explained by Strange New Synestia Theory
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Image credit: Sarah Stewart/UC Davis based on NASA rendering

Plenty of planets have moons, but our Moon is special for a couple of major reasons—including the fact that it's proportionally huge compared to our own planet. It also may be unique in the way that it stabilizes' the tilt of the Earth, which may keep us from straying into new ice ages.

 

Despite dozens of missions and six manned landings, we still don't know one major thing about it: where it came from. However, a breakthrough new theory, proposed by researchers at the University of California-Davis and Harvard, claims to solve all of these issues by posing a totally new model for how the Moon (and Earth) formed: a synestia.

 

Some astronomers think the Earth's gravity snagged the wandering Moon and trapped it in orbit, while others think the two formed together. A more violent hypothesis claims that the Moon was originally part of the Earth, but was spat out as a molten chunk in a kind of planetary fission.



Each of these theories has their own advantages, but all of them have to deal with two major facts: the Moon has a low density, which probably means it doesn't have an iron core like Earth, and there's a distinct lack of "volatiles" (elements that easily evaporate into gases), which suggest it underwent a long period of heating that burned all of those elements off.


On top of that, the Moon and Earth seems to have very similar compositions, suggesting their origins are intimately linked.


So, just what is a synestia?


Here's how the researchers at UC Davis describe it: "A synestia...[is] a huge, spinning, donut-shaped mass of hot, vaporized rock, formed as planet-sized objects smash into each other. And at one point early in its history, the Earth itself was likely a synestia."

 

Researchers investigating synestia have been examining how collisions of spinning objects interact with one another. 

 

"A rotating object has angular momentum, which must be conserved in a collision. Think of a skater spinning on ice: If she extends her arms, she slows her rate of spin, and to spin faster she holds her arms close. Her angular momentum is the same," UC Davis explains.

 

"Now consider two ice skaters turning on ice: if they catch hold of each other, the angular momentum of each adds together, so their total angular momentum must be the same. [Researchers] Lock and Stewart modeled what happens when the 'ice skaters' are Earth-sized rocky planets colliding with other large objects with both high energy and high angular momentum. The researchers found that over a range of high temperatures and high angular momentum, planet-sized bodies could form a new, much larger structure, an indented disk rather like a red blood cell or a donut with the center filled in. The object is mostly vaporized rock, with no solid or liquid surface. They have dubbed the new object a 'synestia,' from 'syn-,' 'together' and 'Hestia,' Greek goddess of architecture and structures.


"The synestia structure also suggests new ways to think about lunar formation," says UC Davis. "Earth's moon is remarkably similar to Earth in composition, and most current theories about how the moon formed involve a giant impact that threw material into orbit. But such an impact could have instead formed a synestia from which the Earth and moon both condensed."



The idea is that the Moon emerged from this synestia along with the Earth, which explains why they seem to be made of similar material.


Though no one has ever seen a synestia, the simulations created by the research teams show that this new model could account for most of the Moon's characteristics.


Of course, a major coup would be to actually spot one of these space-donuts, but their short lifespans probably make them hard to spot.

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