The moon is incredibly important to the Earth. It provides the basis for many calendars and affects the oceans' tides. But how did it get there?
It's an answer we thought we knew, more or less. But now a group of scientists have used complex modelling to propose a new theory — and it involves a violent collision that vapourized much of our planet.
"Despite smart people working on this problem for 50 years, we're still discovering surprisingly basic things about the earliest history of our world," lead researcher Matija Cuk said in a news release.
The findings, published this week in the journal Nature, were co-authored by scientists at the University of California, Davis; Harvard University; the University of Maryland and the SETI Institute — an organization perhaps better known for its efforts to find alien life — where Cuk is a researcher.
"It's quite humbling," he said.
Old theory: boom
For decades, one of the leading explanations about the moon's origin has been the "impact theory."
The story goes that billions of years ago, Earth and a Mars-sized protoplanet called Theia collided.
The debris formed a ring around the Earth's equator and was eventually drawn together by gravity to form the moon.
The moon would have formed very close to Earth, eventually drifting farther away.
It sounds nice and tidy, but there are some facts that throw a monkey wrench into this theory — chiefly, the moon's chemistry and weird angle.
"This idea came around before computers could really do detailed simulations of planet formation," senior author Sarah Stewart, professor of earth and planetary sciences at the University of California, Davis, told CBC News.
Computer simulations now show that the impact would have created the moon mostly out of Theia's debris, but scientists have since learned that the Earth and moon have strikingly similar chemical makeups.
"So that really put a brake on the giant impact hypotheses," Stewart said.
What's more, had the moon formed from debris around the equator, we would expect it to orbit along the equator today. In fact, its orbit is tilted five degrees.
"This large tilt is very unusual. Until now, there hasn't been a good explanation," astronomer and co-author Douglas Hamilton, of the University of Maryland, said in a statement.
"But we can understand it if the Earth had a more dramatic early history than we previously suspected."
New theory: much bigger boom
Under the new, tweaked theory, the collision would have occurred at a much sharper angle and been much more intense.
"It takes a lot of energy to change an orbit," Stewart said.
The new model proposes a high-energy impact that would have been so violent that it vapourized not only Theia, but much of the Earth as well.
Vapour and molten material would have formed a massive cloud, 500 times the size of today's Earth. Some of that material would have cooled and fallen back to Earth, while the rest would have formed the moon.
What's more, the bigger collision would have been so powerful, and at such an angle, that it knocked Earth's rotational axis off by between 60 and 80 degrees and caused the planet to spin so rapidly that days only lasted two hours.
Slowly, the moon moved away from the Earth, which slowed down and turned somewhat upright again, the new model proposes.
"Over billions of years, the moon's [orbital] tilt slowly decayed down to the five degrees we see today," Cuk said. "So today's five-degree tilt is a relic and a signature of a much steeper tilt in the past."
Still, there are questions left unanswered.
The next step is to better understand the chemical makeup of the moon — exactly why and how Theia's and Earth's debris combined.
"We have work in progress on that part, but until that comes out, I don't think people will buy in completely," Stewart said.
The benefit of the new model, she said, is that it accounts for the moon's current orbit without relying on some as-of-yet unknown intervening steps.
"You don't have to have something else come by and doing anything to the Earth. It really is just a giant impact, and then stand back and watch."