POSTED 8 NOV 2001
About 4.5 billion years ago, when Earth was just an adolescent, she met the planet of her dreams -- Theia. He was just her type: as big as Mars and plenty hard-core. But he was from another orbit and that made it difficult for the two to meet. For weeks, the star-crossed lovers whispered sweet-nothings and blew kisses through the solar system.
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.
Small bang theory
Are you my daddy?
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
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
Oxygen Isotopes and the Moon-Forming Giant Impact. U. Wiechert, A. N. Halliday, D.-C. Lee, G. A. Snyder, L. A. Taylor, and D. Rumble. Science 2001 October 12; 294: 345-348. (in Reports)
Age and Origin of the Moon, Der-Chuen Lee, Science, 7 November 1997, p. 1098-1103.
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