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How the first nuclear bomb test is helping scientists learn about the Moon’s formation

Glass from the Trinity test site is helping us figure out the birth of our lunar companion

The Trinity Site explosion.
Government photo

Radioactive glass left over from the first ever test of a nuclear bomb is providing scientists with clues about the formation of Earth’s Moon. This glassy bomb byproduct is revealing how certain materials may have evaporated from the Moon when it first took shape more than 4 billion years ago.

For decades now, the leading theory has been that our Moon formed when Earth was hit by a Mars-sized object dubbed Theia. This collision is thought to have been so strong and hot, that it caused certain types of chemicals known as volatiles — elements with really low melting points — to evaporate from the newly forming Moon. Researchers have been studying volatiles in Moon rocks for decades, in order to better understand how our lunar neighbor formed. They have speculated about how this evaporation process occurred, but it’s been difficult to back these theories up with empirical evidence. You’d need to re-create the same conditions of the Moon’s birth on an enormous scale.

That’s why researchers at the University of San Diego decided to use a nuclear test as a proxy for what the Moon experienced billions of years ago. “It dawned on me that if we're going to use a big experiment, it needed to be something of sufficient size to see [this effect],” James Day, the lead author of a study published today in Science Advances and a geochemist at Scripps Institution of Oceanography at the University of California at San Diego, tells The Verge. “And I thought about it, and the Trinity test site would be the best place for this.”

The Trinity test is the first full-scale detonation of a nuclear bomb. In July 1945, the bomb exploded with a force of 84 terajoules in the New Mexico desert — a relatively small explosion compared to the nuclear weapons of today. Still, the heat of the Trinity blast was so intense that it melted the sands at the test site, creating a radioactive, green-colored glass that’s called trinitite. This strange glass extended from 1,000 to 1,600 feet in every direction from the center of the bomb’s blast.

Trinitite samples used in the study.
Photo: Day et al. Sci. Adv. 2017;3:e1602668

Day and his team collected samples of this trinitite and measured the amount of volatiles the materials had lost. Specifically, the researchers analyzed the loss of the volatile zinc, as it’s one of the easiest elements to measure, according to Day. Plus, experts believe the Moon lost a lot of its zinc when it first formed; if you compare lunar rocks to similar rocks found on Earth, they’re about 100 times more depleted in the element. Rocks spread across the entire surface of the Moon all show a similar pattern of zinc and other volatile loss, which is why researchers think it must have been a large-scale, high-temperature event that caused this evaporation.

After studying the trinitite, the researchers confirmed that that theory had been right. The glass at the Trinity blast site had lost a significant fraction of its zinc, just like the Moon. Not only that, but the trinitite found closest to the blast center had suffered the largest fraction of zinc loss compared to the glasses near the edge of the blast. That means this evaporation was much more pronounced where the temperatures were the highest. “It’s telling us our theories of how the Moon formed and how the volatile elements are lost from the Moon are supported by this empirical test,” says Day. “In science that’s important, because you need to know that your hypotheses are either correct or incorrect.”

Video: UC San Diego

The trinitite samples are also helping the researchers refine their models of how zinc evaporated when the Moon formed. Zinc has five stable isotopes — versions of the element that all have different masses. The lighter isotopes tend to evaporate more readily than the heavy ones, a process known as fractionation. It turns out that researchers were overestimating the fraction of zinc that gets lost during these intense events — the Trinity test showed that fractionation is more subdued than previously thought.

Knowing exactly how volatiles evaporate will help experts better understand how the process works when other planets are formed. For instance, that could help us figure out how volatiles were lost when our own planet formed. It’s crucial knowledge that researchers may never have gotten from lab tests. “Laboratory experiments can barely reach such high-pressure-temperature conditions,” Kun Wang, a planetary scientist at Washington University in St. Louis who was not involved with the study, wrote in an email to The Verge. “Using the by-products of the nuclear test, they saved the trouble to do such...experiments in the lab, which is not easy or even impossible to do.”

Above all, Day says he is happy to have used the Trinity test, an event centered around destruction, to further our understanding of our planetary system. “We’ve taken what was a historic life-changing event for all of us on this planet — the nuclear age has had a major impact on everyone’s lives — and we’ve used it for scientific benefit,” says Day. “There was something gratifying in doing this.”