Chemical study confirms theory of Moon formation

In a new study published in the journal Nature, researchers have used state-of-the-art techniques to support the theory that the moon was formed by violent, high-energy impact rather than a mild, low-energy impact.

In the 1970s, two sets of astrophysicists independently came to the conclusion that the Moon was created by a glancing collision between a Mars-sized object and the still-forming Earth. The massive impact theory explained many things, like the large size of the Moon in relation to the Earth and the rotation rates of the Earth and Moon, and it gradually became the primary theory for the Moon’s formation.

In 2001, however, researchers reported the isotopic makeup of various elements in terrestrial and lunar rocks are almost identical. Studies of samples acquired by the Apollo missions in the 1970s indicated Moon rocks have the same amounts of the three stable isotopes of oxygen as Earth rocks.

With the odds that an impactor would have the same isotopic signature as the Earth being quite small, this finding is a major stumbling block for the glancing, low-energy impact theory. One prevalent high-energy impact model describes how the impact was so violent, the impactor and Earth’s mantle vaporized and blended to form a thick melt/vapor mantle atmosphere that grew to more than 500 times bigger modern-day Earth. The Moon formed as this cloud of material cooled.

Earth and lunar landscape

This study shows that the Moon has a violent origin (Credit: Thinkstock)

Studying Moon Rocks

In the study, researchers evaluated seven lunar specimens from several missions and examined their potassium isotope ratios. They learned that the lunar rocks were enriched by around .4 parts per thousand in the heavier isotope of potassium, potassium-41.

The only high-temperature process that could split the potassium isotopes in this way, according to the study team, is unfinished condensation of the potassium from the vapor phase during the Moon’s formation. As opposed to the lighter isotope, the heavier isotope would preferentially drop out of the vapor and condense.

Research has shown, however, that if this sequence occurred in an absolute vacuum, it would lead to an enrichment of heavy potassium isotopes in lunar specimens of approximately 100 parts per thousand, much greater than the value the team discovered. High atmospheric pressure would subdue fractionation, and for this reason, the study team said, the Moon likely condensed in a pressure of greater than 10 bar, or about 10 times the atmospheric pressure at sea level on Earth.

Hence, the team’s finding the lunar rocks are filled with the heavier potassium isotope props up a violent, high-energy mantle atmosphere simulation, with lunar rocks containing more of the heavier isotope than terrestrial rocks.


Image credit: Thinkstock