A new analysis of a lunar rock indicates that the Moon’s interior is dry

Close-up of the rust on the surface of Rusty Rock (Photo NASA)
Close-up of the rust on the surface of Rusty Rock (Photo NASA)

An article published in the journal “Proceedings of the National Academy of Sciences” describes a research on the presence of water on the Moon. A team led by the geochemist James M. D. Day of the University of California at San Diego examined amounts and compositions of zinc, chlorine and lead isotopes in a lunar rock concluding that water and other volatile compounds evaporated very early in the Moon’s history. This contradicts a research published only a few weeks ago in which it’s argued that ancient Moon volcanic deposits contain large amounts of water.

This new research is based on a new analysis carried out using new techniques on fragments of the lunar rock sample cataloged as 66095 and nicknamed “Rusty Rock”, collected by Charlie Duke and John Young during the Apollo 16 mission and brought to Earth. It’s one of the lunar rocks richest in volatile compounds and the only one brought to Earth to show traces of rust, a characteristic that has always intrigued the scientists who wondered where the water that formed that rust came from.

The conclusion of this new chemical analysis is paradoxical because the composition of Rusty Rock and especially its rust was consistent with the formation of a very dry Moon interior. The rust is full of light zinc isotopes and this means that it’s probably the product of the zinc that condensed on the lunar surface after it evaporated during the period of its formation where it was extremely hot.

Zinc is a volatile element so its behavior has some similarities with that of the water in the conditions in which the Moon formed. James M. D. Day compared it to the clouds that form from the ocean: they’re rich in lighter oxygen isotopes and the ocean is rich in heavy isotopes of the same element. Likewise, the Moon’s interior must be rich in heavy isotopes and lost the light ones and volatile compounds. This means among other things that the Moon’s interior is dry.

A month ago about an article published in the journal “Nature Geoscience” provided a very different perspective in which the researchers brought what they considered to be evidence of the presence of large amounts of water in the Moon’s ancient volcanic deposits. James M. D. Day stated that he’s skeptical of those results because the researchers were unable to explain the mechanism of formation of volcanic glass containing water. Actually, it’s certain that at least some beads brought on Earth during the Apollo 15 and 17 missions contained water, the uncertainty is whether they represent the rule or the exception.

Carrie McIntosh, one of James M. D. Day’s PhD students, is conducting another research on lunar volcanic glass beads. This will allow to obtain more data that could allow to understand that apparent contradiction. A number of researches analyzed a number of lunar rocks obtaining different results and trying to put together the pieces of the lunar jigsaw puzzle some pieces don’t seem to match. This shows that our knowledge is still fragmentary.

James M. D. Day explained that every time the lunar rocks collected during the Apollo missions get analyzed with a new technique, the researchers can get new insights. Unfortunately, there are few rocks so the work is slow and complex, which means that it may take a long time to understand the Moon’s secrets. That’s not just about the presence of water but also concern the details of its formation, connected to that of Earth.

The sampe 66095 aka Rusty Rock getting collected (Photo NASA)
The sampe 66095 aka Rusty Rock getting collected (Photo NASA)

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