Artist's conception of two celestial bodies smacking into each other. Such a collision is believed to have formed Earth's moon. Credit: NASA/JPL-Caltech
Billions of years ago, so the theory goes, a Mars-sized body (sometimes called “Theia”) smashed into our young planet and caused a near-catastrophe. Earth fortunately survived the risk of blowing apart, and the fragments from the crash gradually coalesced into the Moon that we see today.
Even though this happened a heck of a long time ago, scientists believe they have found traces of Theia in lunar rocks pulled from the Apollo missions.
The isotopes or types of oxygen revealed in the new research appear to be different between the Earth and the Moon. And that’s important, because it implies that a body of different composition caused the changes. “If the Moon formed predominantly from the fragments of Theia, as predicted by most numerical models, the Earth and Moon should differ,” the study states.
An airplane at about 2,400 meters above the ground passes in front of the Moon on its way to landing at the Charles de Gaulle Airport in Paris, France. Taken from about 70 km from Paris. Credit and copyright: Sebastien Lebrigand.
Scientists scanned samples from the Apollo 11, 12 and 16 missions with scanning electron microscopes that are more powerful than what was available in the 1960s and 1970s, when scientists first looked at these samples from the manned moon missions.
Before, the “resolution” of these microscopes couldn’t find any significant differences, but the new data reveals the moon rocks have 12 parts per million more oxygen-17 than the Earth rocks.
“The differences are small and difficult to detect, but they are there,” stated lead researcher Daniel Herwartz, who was formerly with the University of Gottingen and is now with the University of Cologne. “This means two things; firstly we can now be reasonably sure that the giant collision took place. Secondly, it gives us an idea of the geochemistry of Theia.”
The work was published in Science and will also be presented at the Goldschmidt geochemistry conference in California on June 11.
Artist’s impression of an impact of two planet-sized worlds (NASA/JPL-Caltech)
Scientists have uncovered a history of violence hidden within lunar rocks, further evidence that our large, lovely Moon was born of a cataclysmic collision between worlds billions of years ago.
Using samples gathered during several Apollo missions as well as a lunar meteorite that had fallen to Earth (and using Martian meteorites as comparisons) researchers have observed a marked depletion in lunar rocks of lighter isotopes, including those of zinc — a telltale element that can be “a powerful tracer of the volatile histories of planets.”
The research utilized an advanced mass spectroscopy instrument to measure the ratios of specific isotopes present in the lunar samples. The spectrometer’s high level of precision allows for data not possible even five years ago.
Scientists have been looking for this kind of sorting by mass, called isotopic fractionation, since the Apollo missions first brought Moon rocks to Earth in the 1970s, and Frédéric Moynier, PhD, assistant professor of Earth and Planetary Sciences at Washington University in St. Louis — together with PhD student, Randal Paniello, and colleague James Day of the Scripps Institution of Oceanography — are the first to find it.
The team’s findings support a now-widely-accepted hypothesis — called the Giant Impact Theory, first suggested by PSI scientists William K. Hartmann and Donald Davis in 1975 — that the Moon was created from a collision between early Earth and a Mars-sized protoplanet about 4.5 billion years ago. The effects of the impact eventually formed the Moon and changed the evolution of our planet forever — possibly even proving crucial to the development of life on Earth.
(What would a catastrophic event like that have looked like? Probably something like this:)
“This is compelling evidence of extreme volatile depletion of the moon,” said Scripps researcher James Day, a member of the team. “How do you remove all of the volatiles from a planet, or in this case a planetary body? You require some kind of wholesale melting event of the moon to provide the heat necessary to evaporate the zinc.”
In the team’s paper, published in the October 18 issue of Nature, the researchers suggest that the only way for such lunar volatiles to be absent on such a large scale would be evaporation resulting from a massive impact event.
“When a rock is melted and then evaporated, the light isotopes enter the vapor phase faster than the heavy isotopes, so you end up with a vapor enriched in the light isotopes and a solid residue enriched in the heavier isotopes. If you lose the vapor, the residue will be enriched in the heavy isotopes compared to the starting material,” explains Moynier.
The fact that similar isotopic fractionation has been found in lunar samples gathered from many different locations indicates a widespread global event, and not something limited to any specific regional effect.
The next step is finding out why Earth’s crust doesn’t show an absence of similar volatiles, an investigation that may lead to clues to where Earth’s surface water came from.
“Where did all the water on Earth come from?” asked Day. “This is a very important question because if we are looking for life on other planets we have to recognize that similar conditions are probably required. So understanding how planets obtain such conditions is critical for understanding how life ultimately occurs on a planet.”
“The work also has implications for the origin of the Earth,” adds Moynier, “because the origin of the Moon was a big part of the origin of the Earth.”
An impact between a Mars-sized protoplanet and early Earth is the most widely-accepted origin of the Moon. Did smaller impacts seed the formation of continents? (NASA/JPL-Caltech)
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Recent research on lunar samples has shown that the Moon may be made of more Earth than green cheese — if by “green cheese” you mean the protoplanet impactor that was instrumental in its creation.
It’s an accepted hypothesis that Earth’s moon was created during an ancient, violet collision between our infant planet and a Mars-sized world called Theia, an event that destroyed Theia and sent part of Earth’s crust and upper mantle into orbit as a brief-lived ring of molten material. This material eventually coalesced to form the Moon, and over the next 4.5 billion years it cooled, became tidally locked with Earth, accumulated countless craters and gradually drifted out to the respectable distance at which we see it today.
Theia’s remains were once assumed to have been a major contributor to the material that eventually formed the Moon. Lunar samples, however, showed that the ratio of oxygen isotopes on the Moon compared to Earth were too similar to account for such a formation. Now, further research by a team led by scientists from The University of Chicago shows that titanium isotopes — an element much more refractive than oxygen — are surprisingly similar between the Moon and Earth, further indicating a common origin.
“After correcting for secondary effects associated with cosmic-ray exposure at the lunar surface using samarium and gadolinium isotope systematics, we find that the 50Ti/47Ti ratio of the Moon is identical to that of the Earth within about four parts per million, which is only 1/150 of the isotopic range documented in meteorites,” wrote University of Chicago geophysicist Junjun Zhang, lead author of the paper published in the journal Nature Geoscience on March 25.
If the Moon is more Earth than Theia, then what happened to the original impacting body? Perhaps it was made of heavier stuff that sunk deeper into the Moon, or was assimilated into Earth’s mantle, or got lost to space… only more research will tell.
But for now, you can be fairly sure that when you’re looking up at the Moon you’re seeing a piece of Earth, the cratered remnants of a collision that took place billions of years ago.
