By Matt Williams
For centuries, scientists have been attempting to explain how the Moon formed. Whereas some have argued that it formed from material lost by Earth due to centrifugal force, others asserted that a preformed Moon was captured by Earth’s gravity. In recent decades, the most widely-accepted theory has been the Giant-impact hypothesis, which states that the Moon formed after the Earth was struck by a Mars-sized object (named Theia) 4.5 billion years ago.
According to a new study by an international team of researchers, the key to proving which theory is correct may come from the first nuclear tests conducted here on Earth, some 70 years ago. After examining samples of radioactive glass obtained from the Trinity test site in New Mexico (where the first atomic bomb was detonated), they determined that samples of Moon rocks showed a similar depletion of volatile elements.
The study was led by James Day – a professor of geoscience at the Scripps Institution of Oceanography at the University of California, San Diego. Along with his colleagues – who hail from the Paris Institute of Earth Physics, the McDonnell Center for the Space Sciences, and NASA’s Johnson Space Center – they examined samples of glass retrieved from the Trinity test site to determine their chemical compositions.
This glass, known as trinite, was created when the plutonium bomb was detonated at the Trinity test site in 1945 as part of the Manhattan Project. To a distance of 350 meters (1,100 feet) from ground zero, arkosic sand (which is primarily composed of quartz grains and feldspar) was converted to green-colored glass by the extreme heat and pressure caused by the massive explosion.
For years, scientists have been studying these glass deposits, which they determined was the result of sand being sucked up into the explosion, and then rained down as molten liquid onto the surface. When Day and his colleagues examined it, they noted that samples of the glass were depleted of zinc and other volatile elements – which are known to evaporate under extreme heat and pressure – depending on how far they were from ground zero.
According to their study, which was published in Science Advances on February 8th, 2017, samples of trinite that were obtained between 10 and 250 meters (30 to 800 feet) from the blast site were depleted of these elements far more than samples that were taken from farther away. In addition, the isotopes of zinc that remained were heavier and less-reactive than in others.
They then compared these results to studies performed on lunar rocks, which showed a similar depletion of volatile elements. From this, they determined that similar heat and pressure conditions existed at one time on the Moon which caused these elements to evaporate. This is consistent with the theory that a massive impact took place in the past that turned the Moon’s surface into an ocean of magma.
As Day explained in a UC San Diego press release:
“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. This has been conventional wisdom, but now we have experimental evidence to show it.”
While the predominant theory since the 1980s has been the Giant impact hypothesis, the debate has been ongoing and subject to new findings. For example, back in January of 2017, a new study published in Nature Geoscience – which was led by by Raluca Rufu of the Weizmann Institute of Science in Rehovot, Israel – indicated that the Moon may have been the result of many smaller collisions.
Using computer simulations, the Weizmann team found that multiple small impacts could have formed many moonlets around Earth which would have then coalesced to create the Moon. But by showing that volatile elements undergo the same kinds of reactions to heat and pressure, regardless of where the reaction takes place, Day and his colleagues have offered some solid evidence that points towards a single impact event.
This study is just the latest in a series that is helping Earth scientists to put constraints on when and how the Moon formed, which are also helping us to get a better understanding of the history of the Solar System and its formation.