Scientists at Washington University in St. Louis say they have discovered evidence of the moon’s violent origins. According to these scientists, the moon was created when a body the size of Mars smashed into the early Earth. The group’s evidence, while less-than-impressive to the layperson, gives scientists every reason to be excited. Scientists say that traces of heavier zinc atoms, found in moon rocks, condensed out of the cloud of vaporized rock faster than lighter zinc atoms. Scientists have been pouring over moon rocks for evidence of the moon’s violent origins since the Apollo missions first carried them back to Earth in the 1970s.
Scientists examining the moon rocks found that they lacked volatiles. A giant smashup justified the lack of volatiles, while other theories for the moon’s origin failed to do so. Scientists say that the lack of volatiles pointed to the violent origins theory but a massive collision should have also created isotopic fractionation, for which geochemists could not find evidence until recently.
“The magnitude of the fractionation we measured in lunar rocks is 10 times larger than what we see in terrestrial and Martian rocks,” says planetary scientist Frédéric Moynier, “so it’s an important difference.”
Scientists say that the Giant Impact Theory was first proposed at a conference in 1975. Supporters claimed that the moon came to be during a violent collision between a planetary body and the early Earth. They also said that the collision was extremely violent because the planetary body was roughly the size of the planet Mars. During the collision, the planetary body was vaporized as well as a lot of the early Earth’s mantle. The moon, the theory goes, then condensed out of the cloud of rock vapor. Scientists say that computer simulations supported this idea.
Studying moon rocks in the lab, geochemists noticed that they were very poor in sodium, potassium, zinc and lead.
“But if the rocks were depleted in volatiles because they had been vaporized during a giant impact, we should also have seen isotopic fractionation,” says Mr. Moynier. “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.”
The scientists, who are the first to find evidence of isotopic fractionation in the moon rocks, were cautious after realizing the implications of their discovery.
“When you find something that is new and that has important ramifications, you want to be sure you haven’t gotten anything wrong,” posits Mr. Moynier. “I half expected results like those previously obtained for moderately volatile elements, so when we got something so different, we reproduced everything from scratch to make sure there were no mistakes because some of the procedures in the lab could conceivably fractionate the isotopes.”
Mr. Moynier and his team set out to make sure that the fractionation was not a result of localized processes on the moon, such as fire fountaining. They looked at several lunar rock samples from different areas on the moon and found evidence of isotopic fractionation. To study the lunar rock samples, Mr. Moynier and his team made a plea to the Johnson Space Center to allow access to the space agency’s collection of moon rocks.
“What we wanted were the basalts,” says Mr. Moynier, “because they’re the ones that came from inside the moon and would be more representative of the moon’s composition.”
Scientists found that the low- and high-titanium basalts had the same zinc isotopic ratios.
“The work also has implications for the origin of the Earth,” notes Mr. Moynier, “because the origin of the moon was a big part of the origin of the Earth.”
Without the moon, scientists believe that the Earth would spin faster, days would be much shorter, weather crazier, and climate more unpredictable. Without a moon, scientists theorize, we may never have existed.
The findings were published recently in the journal Nature.