How did the Moon form 4.5 billion years ago, and why is it so different from the Earth? According to a team led by John Day at Scripps Institution of Oceanography at the University of California San Diego, part of the answer may be found in a mineral created by the first nuclear bomb test in 1945. By comparing samples of the atomic-age glass called "trinitite" with samples returned from the Moon by the Apollo astronauts, the Scripps team are gaining a better understanding of the chemistry of our satellite in its earliest days.
On July 16, 1945, a group of scientists, soldiers, and engineers gathered in the desert outside of Alamogordo, New Mexico, in the early dawn to witness the culmination of years of top secret work. On top of a steel tower sat a device called the "Gadget," which was the first atomic bomb ever to be scheduled for detonation. Despite the tiny amount of nuclear material available to arm the bomb, the desert test was deemed necessary because the design of the plutonium fission bomb was extremely complicated, and there were fears that it might not work – or that it would work too well.
At 5:29 am MWT, the Gadget was detonated. In a millionth of a second, the bomb and tower vanished as if suddenly dropped on the surface of the Sun. The 22-kiloton explosion was small by modern standards, but it was over twice what was anticipated. The countryside was lit up by a light many times more powerful than that of the midday Sun.
When a lead-lined Sherman tank was sent in later to take chemical samples, the driver saw that the ground was covered to a distance from blast site of 300 to 350 m (985 to 1,200 ft) with a green, glassy material. This was the first trinitite – a man made mineral that was formed when the sandy desert floor was subjected to a temperature of 8,430⁰ K (14,714⁰ F, 8157⁰ C). The heat was so intense that some samples of trinitite have a reddish tint due to absorbing the iron, lead, and copper that were part of the bomb tower before it vaporized in the nuclear fireball.
Day and his team hypothesized that the conditions that created trinitite were very similar to those that occurred when a Mars-sized object may have struck the primordial Earth and formed the Moon. Their reasoning was that by studying the volatile elements, like zinc, in samples of trinitite and comparing them to lunar samples, the Moon's formation would be less of a mystery.
For their study, the Scripps scientists collected test samples from the original Trinity site (now a national monument) at distances of 10 to 250 m (33 to 820 ft) from ground zero. They then returned to the lab and the trinitite was run through a mass spectrometer to measure the quantity and type of zinc isotopes in each sample. They found that the zinc had dried out of the trinitite, with less zinc in samples that were closer to the blast.
"The results show that evaporation at high temperatures, similar to those at the beginning of planet formation, leads to the loss of volatile elements and to enrichment in heavy isotopes in the left over materials from the event," says Day. "This has been conventional wisdom, but now we have experimental evidence to show it."
The next step was to compare the Alamogordo findings with isotope measurements of lunar rock samples brought back by the Apollo missions. In both cases, there was a lack of volatile elements and little or no water present. In other words, the same chemical processes at high temperatures that created the trinitite were present at the creation of the Moon, which supports the idea of it being formed by a giant impact billions of years ago.
"We used what was a history-changing event to scientific benefit, obtaining new and important scientific information from an event over 70 years ago that changed human history forever," says Day.
The findings are published in Science Advances.