The giant impact theory or hypothesis claims that the Moon was formed from debris left from collision between a proto-planetary Earth and a Mars-sized proto-planet called Theia. The debris left from the collision later cooled and coalesced to form the Moon.
A prediction that follows from the giant impact theory is that more than 60 percent of the Moon should be composed of chemical material derived from Theia. Scientists reasoned that since most planetary bodies in our solar system have significantly different compositions, the odds of a collision between Earth and a proto-planet with similar composition was very low, about 1 percent. Thus, the Moon’s chemical composition should be different from the Earth’s if indeed it was formed from a collision between the Earth and another planetary body.
But analysis of rock samples collected from the Moon during the Apollo missions revealed that the chemical composition of the Moon is nearly identical with the Earth. Chemical analysis of Moon rocks, which showed that the Moon has a nearly identical isotopic composition with the Earth, appeared to contradict scientist’s prediction from the giant impact theory that the chemical composition of the Moon should have a different from that of the Earth.
[Note: An isotope of an element, such as oxygen, has the same number of protons but different number of neutrons.]
Alessandra Mastrobuono-Battisti, astrophysicist at the Israel Institute of Technology, Haifa, and lead author of the new study, titled “A primordial origin for the compositional similarity between the Earth and the Moon,” published in the journal Nature, told Space.com: “In terms of composition, the Earth and moon are almost twins, their compositions differing by at most few parts in a million. This contradiction has cast a long shadow on the giant-impact model.”
But the new study by Mastrobuono-Battisti and Hagai Perets from Israel Institute of Technology, Haifa, collaborating with French scientist Sean Raymond from University of Bordeaux in France, might have resolved the problems posed to the giant impact theory by the comparative chemistry of the Earth and the Moon.
The study demonstrates for the first time that the Moon could have been formed from a catastrophic collision between a proto-planetary Earth and a Mars-sized proto-planet similar to the Earth in chemical composition.
The scientists created a computer simulation that showed the likely patterns of collisions between planetary bodies at the early stages of evolution of the solar system. The simulation showed the interaction between 85 to 90 protoplanets about 10 percent of the Earth’s mass, and 1,000 to 2,000 smaller bodies or planetesimals with about 0.25 percent of the Earth’s mass.
The researchers found a consistent pattern between different variations of the simulation models. They found that in the first 100 million to 200 million years, three or four rocky planets comparable to the inner planets, Mercury, Venus, Earth and Mars, emerged. They also found that the largest of the planets was comparable to the Earth in size and mass.
Although, many of the proto-planets had distinct material and chemical compositions, they found that in 20 to 40 percent of the cases, planets had very similar chemical composition to the last proto-planet that collided with them.
The similarity in chemical composition between planets and the last proto-planet that collided with them in the early stages of formation of our solar system was due to the fact that as the solar system evolved, planetary bodies with similar composition, density and mass tended to share orbits making them more likely to collide.
Explaining further why proto-planetary bodies with similar compositions tended to share similar orbits, the researchers pointed out that the proximity of a planet to the sun influences it chemical composition. For instance, planets farther away from the sun were colder and thus had a tendency to retain relatively heavy isotopes of oxygen, compared with those closer to the sun.
The simulation thus showed that although several proto-planets and planetismals collided with the Earth during the early stages of evolution of our solar system, one of the last of the bodies to collide with the Earth was one similar to the Earth in composition.
The debris thrown up by the collision finally cooled and coalesced to form the Moon, with a chemical composition similar with the Earth’s.
In an interview with Space.com, astrophysicist Hagai Perets of the Israel Institute of Technology in Haifa, who co-authored the study, said, “The most exciting and surprising thing was to find out that we can shed new light on a 30-year-old mystery. Compositionally similar planet-impactor pairs are not rare at all.”
In a separate but related study by geologists at University of Maryland, also published in the journal Nature, scientists found evidence confirming the so-called “late-veneer hypothesis,” which tries to explain why most of the Earth’s tungsten is found in its crust and mantle rather than in its iron-rich core.
Tungsten is siderophilic (iron-loving), that is, it tends to bind to iron. Thus, if all of the tungsten composition of the Earth was derived from impacts prior to the giant impact formation of the Moon, we should see more of the tungsten in the iron-rich core rather than in the Earth’s crust or mantle.
The “late-veneer hypothesis” suggests that the excess of tungsten and other siderophilic elements in the Earth’s crust was due to material that accumulated from asteroid impacts over millions of years after the Moon had been formed by the giant impact.
To test the hypothesis, the scientists reasoned that if the “light veneer hypothesis” was true then although the Earth should have more tungsten than the Moon, it should have a lower proportion of the light isotope tungsten-182. This is because while the Earth has collected more tungsten-rich material from asteroid impacts than the Moon, asteroidal debris has a relatively low level of the light isotope tungsten-182.
Analysis of Moon and Earth rocks for relative levels of tungsten-182 isotope confirmed the hypothesis that the Earth should have a lower composition of tungsten-182 than the Moon.
The researchers also found that the Moon and Earth have identical tungsten compositions after correction is made for the estimated amount of tungsten accumulated from asteroid impacts.
This observation incidentally gives evidence in support of the revised giant-impact theory which states that the Moon was formed from debris thrown up by the impact of a Mars-sized planet with composition identical with the the Earth.
