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Moon’s Interior May Contain Vast Amounts Of Water

June 15, 2010

A new study by NASA-funded scientists at the Carnegie Institution’s Geophysical Laboratory finds that the interior of the moon may hold 100 times more water than previously believed.

The scientists analyzed samples of moon rocks collected by Apollo astronauts nearly four decades ago, and found that the volume of water molecules inside minerals in the moon’s interior could exceed that of the entire U.S. Great Lakes. 

The researchers concluded that the water was likely preserved from magma that was present when the moon began to form some 4.5 billion years ago ““ meaning that water is native to the moon.

“For over 40 years we thought the moon was dry,” said Francis McCubbin of Carnegie, lead author of a report about the study.

“In our study we looked at hydroxyl, a compound with an oxygen atom bound with hydrogen, and apatite, a water-bearing mineral in the assemblage of minerals we examined in two Apollo samples and a lunar meteorite,” he said.

The scientists conducted tests that detect elements in the parts per billion range.

By combining their measurements with models that characterize how the material crystallized as the moon cooled during formation, they determined that the minimum water content ranged from 64 parts per billion to 5 parts per million.

Those figures are more than two orders of magnitude greater than previous results from lunar samples, which estimated the moon’s water content to be less than 1 part per billion.

“For over 40 years we thought the Moon was dry,” said McCubbin.

“Recently, scientists detected water from Apollo samples on the order of 46 parts per million.”

However, “we studied two other Apollo samples and a lunar meteorite using secondary ion mass spectrometry (SIMS), which can detect elements in the parts per million range.”

“We found that the minimum water content ranged from 64 parts per billion to 5 parts per million””at least two orders of magnitude greater than previous results.”

Jim Green, director of the Planetary Science Division at NASA Headquarters in Washington, explained the nature of the water discovered within the Moon’s interior.

“In this case, when we talk about water on the moon, we mean water in the structural form hydroxyl,” he said.

“This is a very minor component of the rocks that make up the lunar interior.”

Scientists believe the origin of the moon was the result of a planet-sized object that slammed into the Earth some 4.5 billion years ago.  The impact deposited a large amount of material into the Earth’s orbit, which ultimately condensed to form the moon.

A lunar magma ocean is believed to have formed at some point during the compacting process, which later began to cool.  It was during this cooling phase that water either escaped or became preserved as hydroxyl molecules in the crystallizing minerals.

While previous studies had found evidence of water on the lunar surface and inside the moon, the Carnegie researchers looked within crystalline rocks known as KREEP, which contain more potassium (K), rare Earth elements (REE), phosphorus (P), and other heat-producing elements such as uranium and thorium.  All of these elements are all components of lunar impact melt and basaltic rocks.

“Since water is insoluble in the main silicates that crystallized, we believed that it should have concentrated in those rocks,” said Andrew Steele of Carnegie, who co-authored the report.

“That’s why we selected KREEP to analyze.”

The researchers specifically studied hydroxyl, a compound with an oxygen atom bound with hydrogen, in the mineral apatite””the only water-bearing mineral in the assemblage.
“It is gratifying to see this proof of the OH contents in lunar apatite,” said Bradley Jolliff, a lunar scientist at Washington University in St. Louis.

“The concentrations are very low and, accordingly, they have been until recently nearly impossible to detect. We can now finally begin to consider the implications””and the origin””of water in the interior of the Moon.”

The identification of water from multiple types of lunar rocks that display a variety of incompatible trace element signatures suggests that water may be ubiquitous at low concentrations within the moon’s interior, potentially as early as the time of lunar formation and magma ocean crystallization.

McCubbin’s report was published in Monday’s Online Early Edition of the Proceedings of the National Academy of Sciences. 

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