Titanium paternity test fingers Earth as moon's sole parent

March 29th, 2012
Nicolas Dauphas, UChicago associate professor in geophysical sciences, holds vials of material collected from the moon during the Apollo 14 mission. He and graduate student Junjun Zhang also worked with samples from the Apollo 15, 16 and 17 lunar missions in their new study on the origin of the moon. Credit: Lloyd DeGrane

In the giant-collision scenario, computer simulations suggest that the moon had two parents: Earth and a hypothetical planetary body that scientists call "Theia." But a comparative analysis of titanium from the moon, Earth and meteorites, published by Junjun Zhang, graduate student in geophysical sciences at the University of Chicago, and four co-authors indicates the moon's material came from Earth alone.

If two objects had given rise to the moon, "Just like in humans, the moon would have inherited some of the material from the Earth and some of the material from the impactor, approximately half and half," said Nicolas Dauphas, associate professor in geophysical sciences at UChicago, and co-author of the study, which appears in the March 25 edition of Nature Geoscience.

"What we found is that the child does not look any different compared to the Earth," Dauphas said. "It's a child with only one parent, as far as we can tell."


Titanium also contains different isotopic signatures forged in countless stellar explosions that occurred before the sun's birth. These explosions flung subtly different titanium isotopes into interstellar space. Different objects in the newly forming solar system gobbled up those isotopes in different ways through collisions, leaving clues that let scientists infer where the solar materials including the moon came from.

Planetary DNA

"When we look at different bodies, different asteroids, there are different isotopic signatures. It's like their different DNAs," Dauphas said. Meteorites, which are pieces of asteroids that have fallen to Earth, contain large variations in titanium isotopes. Measurements of terrestrial and lunar samples show that "the moon has a strictly identical isotopic composition to the Earth," he said.

"We thought that the moon had two parents, but when we look at the composition of the moon, it looks like it has only one parent," Zhang said.


The Earth and the moon are constantly bombarded by cosmic rays from the sun and from more distant sources in the galaxy. Earth's atmosphere and magnetic field prevents most of these rays from reaching its surface, but the moon has no such protection.

"We compared the titanium isotopic composition with samarium and gadolinium since those two systems are very sensitive to the cosmic-ray effect," Zhang said. The only compositional differences the scientists expected to see in samarium and gandolinium between Earth and moon would be the result of cosmic rays. "We found a very nice linear correlation between titanium and samarium or gadolinium," she said.


Lunar Conundrum

Solving the conundrum of the moon's origin probably will prove challenging because all of the alternative scenarios for the moon's formation have drawbacks.



According to a third scenario, Earth collided with an icy body lacking entirely in titanium. There are no bodies made purely of ice in the solar system, however. "They would always have a significant fraction of solid material, so you would still expect the object to deliver some titanium," Dauphas said.



More information:
"The proto-Earth as a significant source of lunar material," by Junjun Zhang, Nicolas Dauphas, Andrew M. Davis, Ingo Leya, and Alexei Fedkin, Nature Geoscience, Advance Online Publication, March 25, 2012.

Provided by University of Chicago


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