Two new collision-simulation studies may solve an intractable problem… (NASA )
Scientists may never know exactly how the moon and Earth were formed some 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 that 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 before long 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.
"What's happening now is an attempt to salvage the theory," said Erik Asphaug, a planetary scientist at UC Santa Cruz who was not involved in the new research.
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 modelers 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.
"Everything is molten," Cuk said.
Most of the heavy iron from both planetary cores would combine and coalesce to form Earth's core. The blended lighter rock from both bodies would form the outer layers of the Earth as well as the 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.
By figuring out how Earth's spin might have slowed, Canup said, scientists have "greatly broadened the class of impacts that might be viable."
Caltech planetary scientist David Stevenson, who was not involved with the research, said that the new models "are a stepping stone toward a more satisfying story" but that "we're only part of the way."
David Paige, a moon expert at UCLA who was also not part of either modeling study, said it might not be possible to know exactly what happened.
"So much of what existed prior to the impact has been obliterated," he said. "It's a whodunit mystery with very few clues lying around."
He said, however, that isotopic research might offer part of the solution.
In the report published in Nature, Moynier and his colleagues used sophisticated mass spectrometry to show that the blend of different zinc isotopes on the moon is not the same as the blend on Earth. Lighter versions of the metal were slightly depleted on the moon, suggesting that the lighter zinc must have evaporated during some kind of impact, the team reported.
That doesn't do much to determine whether either collision scenario is correct. It may point a way forward for the planetary scientists who'll try to figure it out, however, Paige noted.
"It's through more measurements like this zinc one that we're able to better sort it out," he said.
For his part, Moynier said he planned to examine rubidium isotopes in lunar rocks next.