When you think of the moon, the adjective “wet” doesn’t generally come to mind.
Earth’s natural moon is known to be very dry; A gray ball of dust scented with gunpowder and ancient volcanic rock. It’s been a long time since anyone thought there might be an ocean of liquid water out there.
However, the Moon is not as water-free as appearances suggest… and early in its history, it may have been still wetter.
According to a new analysis of a meteorite determined to have originated on the Moon, the lunar crust was once richer in water than we previously thought. A team led by lunar geologist Tara Hayden of the University of Western Ontario found a mineral in it that indicates the lunar crust was rich in volatile elements 4 billion years ago, including water.
This mineral is a common phosphate known as apatite, and this is the first time we have seen apatite in material coming from the Moon.
“Detecting apatite in the Moon’s early crust for the first time is very exciting — that we can finally begin to piece together this little-known phase of the Moon’s history,” says Hayden.
“We found that the Moon’s early crust was richer in water than we expected, and its volatile stable isotopes reveal a more complex history than we previously knew.”
When scientists got involved in lunar work with the Apollo missions, they noticed something strange. The Moon appeared seriously depleted in volatiles such as carbon, chlorine, hydrogen and sulfur, compared to Earth. The presence of these other volatiles strongly suggested the presence of water, so the conclusion was that the Moon must be, and always must be, “completely dry.”
But there have been persistent hints that the Moon is not as dry and bare as we thought. Water has been found trapped in lunar volcanic glass. They are also thought to be trapped as ice in deep, shadowy pits.
We have had difficulty studying the history of volatiles on the Moon because there are no volatile carrier minerals to study – apatite in particular. But when Hayden was asked to verify a collector’s rock sample, she found that not only was the rock originally from the moon, but it contained a significant amount of apatite.
The rock itself, named Arabian Peninsula 007, is a piece of lunar breccia made up of a bunch of different minerals bound together in a rocky matrix, a bit like a mineral fruitcake. The researchers said that apatite was found in most of the different types of minerals identified in the sample.
This suggests that the Apollo samples cannot be considered related to other parts of the Moon, as local lunar rocks tell different parts of the Moon’s story.
“We know a lot about the history of water on the Moon from Apollo samples, but these samples are thought to only represent about five percent of the entire surface of the Moon,” Hayden says.
“I was very fortunate that the meteorite not only came from the Moon, but it remarkably showed chemistry that was very vital to our understanding of water-bearing minerals on the Moon.”
Currently, our ability to explore the Moon’s chemistry is very limited, but a new lunar era is blossoming. Many space agencies are planning missions to the Moon, and NASA’s Artemis program aims to send humans to the Moon in 2026. Although we don’t have many answers now, it may not be long until we do.
Until then, lunar meteorites provide a window into the Moon’s history, and where its water may be hiding.
“It has long been thought that the Moon’s surface dried out for thousands and even millions of years, but there may be more water available on the Moon than we thought, and we just need to find a way to extract it,” Hayden says.
The research was published in Nature astronomy.
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