Posted on Jul 19, 2019 in Astronomy, Science
The origin of Earth’s Moon has long been a puzzle, an enigma that has deep implications for the nature of planetary formation, Earth’s geophysical and geochemical history, and for habitability for small rocky worlds in the Cosmos.
Scientists propose in new research that a planet-scale object some 4.5 billion years hit proto-Earth while it was a vast magma ocean, explaining how the rocky parts of the Moon and Earth got to be so similar in elemental composition.
New calculations, reported by Nature, show that because of the large difference in shock heating between silicate melts and rock, a substantial fraction of the ejected, Moon-forming material is derived from ancient, proto-Earth’s magma ocean, even with a highly oblique collision. This model reconciles the compositional similarities and differences between the Moon and Earth within the physics of angular momentum.
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This new conceptual framework for the origin of the Moon explains both the chemical and the mechanical characteristics of the Earth–Moon system. The classic concept of an oblique giant impact explains the large angular momentum and the lack of a large iron-rich core to the Moon, but in this conjecture it is difficult to explain the similarity in the isotopic compositions of the Earth and Moon without violating the physics of angular momentum.
In March of 2018, The Galaxy posted a explanation for the Moon’s origin that had 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.
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Early models of lunar formation suggested 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 that 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.
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Synestias likely don’t last long – perhaps only hundreds of years. They shrink rapidly as they radiate heat, causing rock vapor to condense into liquid, finally collapsing into a molten planet.
The model starts with a collision that forms a synestia –the Moon forms inside the vaporized Earth at temperatures of four to six thousand degrees Fahrenheit and pressures of tens of atmospheres. The Moon would have emerged from the clouds of the synestia trailing its own atmosphere of rock vapor. The Moon inherited its composition from the Earth, but because it formed at high temperatures it lost the easily vaporized elements, explaining the Moon’s distinct composition.
The Daily Galaxy, Jake Burba, via Nature, University of California Davis and Scientific American
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