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Swift
2018-Feb-28, 09:40 PM
The change isn't with the impact of proto-Earth and a Mars-sized object, but what happened after.

UC Davis (https://www.ucdavis.edu/news/how-moon-formed-inside-vaporized-earth-synestia)


A new explanation for the moon’s origin has it forming inside the Earth when our planet was a seething, spinning cloud of vaporized rock, called a synestia. The new model led by researchers at the University of California, Davis, and Harvard University resolves several problems in lunar formation and is published Feb. 28 in the Journal of Geophysical Research – Planets.

“The new work explains features of the moon that are hard to resolve with current ideas,” said Sarah Stewart, professor of earth and planetary sciences at UC Davis.

“The moon is chemically almost the same as the Earth, but with some differences,” she said. “This is the first model that can match the pattern of the moon’s composition.”

Current models of lunar formation suggest that the moon formed as a result of a glancing blow between the early Earth and a Mars-size body, commonly called Theia. According to the model, the collision between Earth and Theia threw molten rock and metal into orbit that collided together to make the moon.

The new theory relies instead on a synestia, a new type of planetary object proposed by Stewart and Simon Lock, graduate student at Harvard and visiting student at UC Davis, in 2017. A synestia forms when a collision between planet-sized objects results in a rapidly spinning mass of molten and vaporized rock with part of the body in orbit around itself. The whole object puffs out into a giant donut of vaporized rock.

Journal - abstract (http://onlinelibrary.wiley.com/doi/10.1002/2017JE005333/abstract)


The giant impact hypothesis remains the leading theory for lunar origin. However, current models struggle to explain the Moon's composition and isotopic similarity with Earth. Here we present a new lunar origin model. High-energy, high-angular momentum giant impacts can create a post-impact structure that exceeds the corotation limit (CoRoL), which defines the hottest thermal state and angular momentum possible for a corotating body. In a typical super-CoRoL body, traditional definitions of mantle, atmosphere and disk are not appropriate, and the body forms a new type of planetary structure, named a synestia. Using simulations of cooling synestias combined with dynamic, thermodynamic and geochemical calculations, we show that satellite formation from a synestia can produce the main features of our Moon. We find that cooling drives mixing of the structure, and condensation generates moonlets that orbit within the synestia, surrounded by tens of bars of bulk silicate Earth (BSE) vapor. The moonlets and growing moon are heated by the vapor until the first major element (Si) begins to vaporize and buffer the temperature. Moonlets equilibrate with BSE vapor at the temperature of silicate vaporization and the pressure of the structure, establishing the lunar isotopic composition and pattern of moderately volatile elements. Eventually, the cooling synestia recedes within the lunar orbit, terminating the main stage of lunar accretion. Our model shifts the paradigm for lunar origin from specifying a certain impact scenario to achieving a Moon-forming synestia. Giant impacts that produce potential Moon-forming synestias were common at the end of terrestrial planet formation.

grant hutchison
2018-Feb-28, 10:25 PM
Synestia. Terrible word, for which we can blame the first two authors, Lock and Stewart.
They coined it last year, in a paper describing these objects (https://arxiv.org/abs/1705.07858), which is an interesting read as background to the above.
They say that synestia is formed from Greek syn- (together, alike) and Hestia, the Greek goddess of architecture. I presume the idea is "being constructed together" - as in, a planet and its moon taking their origin from a single object.

Grant Hutchison

DaveC426913
2018-Feb-28, 10:45 PM
Yeah. Read this. Interesting. Gonna keep an eye on it.

John Mendenhall
2018-Feb-28, 11:47 PM
Maybe. The release from the 'just right' strike is nice.

grapes
2018-Mar-01, 10:01 AM
Synestia. Terrible word, for which we can blame the first two authors, Lock and Stewart.
They coined it last year, in a paper describing these objects (https://arxiv.org/abs/1705.07858), which is an interesting read as background to the above.
They say that synestia is formed from Greek syn- (together, alike) and Hestia, the Greek goddess of architecture. I presume the idea is "being constructed together" - as in, a planet and its moon taking their origin from a single object.

Thanks for finding that. Page 10, middle of second column.

I think it's more of a reference to the radical structure of the object itself. A co-rotating part, with a connected (through a transition region) non-corotating disk.

Ken G
2018-Mar-01, 12:06 PM
Interesting idea, though frankly it doesn't sound like a "new model," it just sounds like a specific model that works better. The basic idea that you have a glancing collision and the Moon gets thrown off somehow is still the basic idea, and since that's pretty much the only "model" anyone outside that subfield knows about, it's hard to call this one a "new model" because it sounds a lot like the old model to me! But no doubt, in this subfield, having a specific model for how it could happen is of great importance, as they now have some clear predictions that can get to work on confronting with observations.

grapes
2018-Mar-01, 03:38 PM
Interesting idea, though frankly it doesn't sound like a "new model," it just sounds like a specific model that works better. The basic idea that you have a glancing collision and the Moon gets thrown off somehow is still the basic idea, and since that's pretty much the only "model" anyone outside that subfield knows about, it's hard to call this one a "new model" because it sounds a lot like the old model to me! But no doubt, in this subfield, having a specific model for how it could happen is of great importance, as they now have some clear predictions that can get to work on confronting with observations.
Of course, it's a refinement. The claim is that it better accounts for the makeup of the resultant planet/moon system, that is, what we know about the earth and moon. In that sense, it's a new model.