Theia, not good with long-distance relationships, grew impatient. So, one day he came crashing into Earth thus consummating their love. In that instant, they gave birth to something radiant -- a molten rock they called Moon.

But like any sentimental tale, woe followed: Theia, the rake he was, busted into a thousand pieces and left Earth alone with child. Because her planetary pregnancy was unplanned, she decided to let Moon drift away. But the gravitational attraction between them was too strong.

Though Earth nurtured Moon as it circled around her, she vowed never to confess that she was really Moon's Mother.

Alphonsus crater on the Moon from 440 km
Courtesy NASA

Small bang theory
Scientists today believe that the scandalous affair between Earth and Theia created the second brightest object in the sky -- the Moon. When Theia crashed into Earth, they claim, it knocked off a chunk of Earth's surface. This chunk, along with pieces of Theia, swirled around the Earth in clouds of dust. Eventually, those clouds came together, forming the moon. Scientists call this conception the Giant Impact. We call it the Small Bang Theory.

Are you my daddy?
Scientists from the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland, decided to do what any good soap opera character would do to determine the daddy -- they ran a paternity test. Instead of using DNA to match offspring to parent, the scientists used oxygen isotopes to find variations in certain chemical compositions between Moon and its alleged mother.

This view of the V-shaped valley on the moon, called Rima Hadley (Hadley Rille), clearly shows blocks along the walls and at the bottom The sides slope at about 20 degrees, and the rille is 1 to 1.5 km from rim to rim.
Courtesy NASA

If such variation existed in lunar rock samples, they'd tell us something about Moon's father. To do this, the scientists tested oxygen isotopes in lunar samples from NASA using a very precise laser fluorination technique.

"We were looking for material from Theia in lunar rocks," says Uwe Wiechert, one of the researchers. He and his team expected to find it for several reasons. "Oxygen isotopes are heterogeneously distributed in the solar system," he says. As a result, you'd expect to find a mix of isotopes in a sample, especially if that sample was born from different bodies. Also, computer models show that the Moon inherited 70 to 90 percent of its material from Theia.

Despite predictions, Wiechert's paternity test revealed something unexpected -- it found little variation between the Earth and Moon. "We were surprised that there is not even a tiny difference," he explains.

Chip off the old rock
You'd think that Moon, like you, would show traits from both parents -- a little rock from Mom and a little rock from Dad. But if the Moon's oxygen isotopes resemble those of Earth, what does that suggest about Theia and the Giant Impact theory? Were Theia's isotopes, like recessive genes, dominated by those of Earth? Or, was Moon not the love child of two celestial beings but rather an asexual offspring of Earth? (Scientists call this the Fission Theory of origin.)

Apollo 17 astronaut Harrison Schmitt standing next to boulder at Taurus-Littrow during third EVA. Courtesy NASA

Wiechert explains that the two bodies that created the moon must have been very similar: "an identical oxygen isotopic composition for the Earth and a Mars-sized impactor requires Theia formed from the same mix of components as the Earth." And similar composition, he adds, depends on how far the planets are from the Sun. Wiechert says, "It is astonishing that two so different bodies like the bone-dry Moon and the blue-planet Earth formed out of the same material."

-- Emily Carlson

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