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Head-on Collision With Another Planet Formed Earth and Moon

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Our galactic neighborhood is pretty peaceful these days. So peaceful, in fact, that it’s easy to forget its violent and spectacular beginnings. Fortunately, we have scientists to remind us. A paper published last week in the journal Science reports that Earth once smashed into, then mixed with, another planet. They say the Moon formed out of the resulting wreckage too.

That planet’s name was Theia. At the time of the collision, Theia was so young that researchers refer to it as a “planetary embryo.” Though it's important to note that Earth was only 100 million years old itself—also young in cosmic time.

While knowledge of the planets’ collision isn't new, the paper's co-authors uncovered new details of the event. Most scientists believed the impact had occurred at a 45-degree angle, like one car sideswiping another. But this new research shows it was probably more like a brutal head-on collision. 

How can you be sure about something that happened to our planet nearly 4.5 billion years ago? Take a good look at the Moon. Researchers tested lunar rock samples collected by astronauts on the Apollo 12, 15, and 17 missions. They also looked at six volcanic rocks from Hawaii and Arizona. The scientists focused on the rocks’ chemical makeup—specifically on their oxygen atoms.

Researchers Paul Warren, Edward Young and Issaku Kohl with a moon rock sample. Image Credit: Christelle Snow/UCLA

There are several isotopes of oxygen, each named for its number of protons and neutrons. O-16, which makes up nearly all of the oxygen on Earth (including in rocks), has eight protons and eight neutrons. We also have small quantities of O-17 and O-18. The ratio of these three isotopes acts as a chemical signature, so if two entities have the same ratio, it’s likely they’re made of the same stuff.

The theory was that Theia’s hit-and-run impact flung shards of both planets outward into space, where gravitational forces compressed them into a single ball: the Moon. If that’s so, then the Moon’s ratio of isotopes should look different from Earth’s. And some scientists believe that’s the case.

But the researchers behind the new paper argue otherwise. "We don't see any difference between the Earth's and the moon's oxygen isotopes; they're indistinguishable," lead author Edward Young said in a press release

What does this mean? It means, the researchers say, that Theia hit Earth hard and fast, and likely straight-on. Some of the debris from Theia and Earth combined to form the planet we call home today, and some of it combined to form the Moon. 

"Theia was thoroughly mixed into both the Earth and the Moon, and evenly dispersed between them," Young said. "This explains why we don't see a different signature of Theia in the Moon versus the Earth."

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Animals
These Strange Sea Spiders Breathe Through Their Legs
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Sylke Rohrlach, Wikimedia Commons // CC BY-SA 2.0

We know that humans breathe through their lungs and fish breathe through their gills—but where exactly does that leave sea spiders?

Though they might appear to share much in common with land spiders, sea spiders are not actually arachnids. And, by extension, they don't circulate blood and oxygen the way you'd expect them to, either.

A new study from Current Biology found that these leggy sea dwellers (marine arthropods of the class Pycnogonida) use their external skeleton to take in oxygen. Or, more specifically: They use their legs. The sea spider contracts its legs—which contain its guts—to pump oxygen through its body.

Somehow, these sea spiders hardly take the cake for Strangest Spider Alive (especially because they're not actually spiders); check out, for instance, our round-up of the 10 strangest spiders, and watch the video from National Geographic below:

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Food
How to Make Perfect Fried Chicken, According to Chemistry
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Cooking amazing fried chicken isn’t just art—it’s also chemistry. Learn the science behind the sizzle by watching the American Chemical Society’s latest "Reactions" video below.

Host Kyle Nackers explains the three important chemical processes that occur as your bird browns in the skillet—hydrolysis, oxidation, and polymerization—and he also provides expert-backed cooking hacks to help you whip up the perfect picnic snack.

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