Scientists may never know exactly how the moon and Earth were formed 4.5 billion years ago, but this week their understanding of the cataclysmic event made a significant leap forward.
In a slew of studies published Wednesday, planetary scientists provided new evidence supporting the long-standing - but imperfect - theory that the Earth and moon formed after the proto-Earth collided with another huge planetary body, sometimes referred to as Theia.
Some of that evidence comes from super-precise measurements of the zinc in lunar rock samples collected by Apollo astronauts. These findings, reported in the journal Nature, support the idea the moon's birth had to have resulted from "a big event with lots of energy," strong enough to vaporize rock, said study leader Frederic Moynier, a geochemist at Washington University.
Separately, two studies published in the journal Science detailed two scenarios of what such a powerful crash might plausibly have looked like.
Both collision-simulation papers may solve an intractable problem with the classic story scientists told about the moon's birth. That story goes something like this: Two planets, one Earth-sized and one Mars-sized, slammed together. The smaller body, Theia, was obliterated completely, its materials flung asunder to form a disk around the Earth that coalesced to form the moon.
The theory explains the distance between the two bodies, their relative sizes and other physical properties. But in the last decade or so, a problem arose: The chemistry didn't match up with the physics. According to computer simulations of the theorized collision, the moon should have been composed mainly of materials from Theia.
Instead, analysis showed that rock samples from the moon and Earth appeared to contain the same amounts of the same types of oxygen, titanium, silicon and other elements.
The similarity of these distinct chemical isotopes was taken as a sign that the Earth and moon were actually made of the same stuff - and meant that planetary scientists would need to rethink the details of how the giant impact happened, said Harvard University researcher Matija Cuk, a coauthor of one of the new simulations.
The main problem the computer modellers faced was that any collisions resulting in an Earth and a moon with shared geochemistry required the ancient Earth to be spinning too fast to allow for the 24-hour rotation that exists today.
Cuk and his Harvard colleague Sarah Stewart solved the conundrum by suggesting that a fast-spinning proto-Earth could have slowed during a period when the moon and the sun aligned in such a way that gravity warped Earth's orbit, putting the brakes on its rotation.
Plugging the appropriate conditions into their computer simulation, they found that a small body about half the size of Mars striking the early Earth nearly head-on would completely obliterate both bodies, with all the material mixing together. Most of the heavy iron would coalesce to form Earth's core. The lighter rock from both bodies would form the outer layers of the Earth and moon.
Robin Canup, a planetary scientist at the Southwest Research Institute in Boulder, Colo., used Cuk's and Stewart's idea about how the Earth's rotation might have slowed and developed another scenario for the moon's creation. Also writing in Science, she showed that two similarly sized bodies, each about half the mass of the modern Earth, could have collided at a relatively slow speed and merged, their contents creating a pool of material that later split apart into Earth and moon.
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