February 18th, 2013 by James Ayre
Small quantities of water have been discovered in the crystalline structure of lunar rock samples obtained during the Apollo missions by researchers at the University of Michigan. The new discovery directly contradicts the dominant theory of lunar formation.
The samples were taken from the lunar highlands, which is thought to represent what was the original crust of the Moon. The solidified surface of the original magma ocean that formed during the Moon’s formation. What the new discovery means is that the Moon was wet early in its life, and that it did not lose most of its water during formation as had been previously thought. It had been assumed that the water ice that is currently on the Moon originates from asteroid or comet impacts.
“The results seem to contradict the predominant lunar formation theory — that the moon was formed from debris generated during a giant impact between Earth and another planetary body, approximately the size of Mars.”
“Because these are some of the oldest rocks from the moon, the water is inferred to have been in the moon when it formed,” Youxue Zhang, a UM researcher said. “That is somewhat difficult to explain with the current popular moon-formation model, in which the moon formed by collecting the hot ejecta as the result of a super-giant impact of a martian-size body with the proto-Earth.”
“Under that model, the hot ejecta should have been degassed almost completely, eliminating all water.”
Does that mean that the Moon didn’t form from the Earth’s collision with another planetary sized body at all? Is it perhaps a gravitationally captured object that formed elsewhere?
During recent years, new observations taken by modern spacecraft and new lab measurements have taken apart the old theory that the Moon is completely dry.
“In 2008, laboratory measurement of Apollo lunar samples by ion microprobe detected indigenous hydrogen, inferred to be the water-related chemical species hydroxyl, in lunar volcanic glasses. In 2009, NASA’s Lunar Crater Observation and Sensing satellite, known as LCROSS, slammed into a permanently shadowed lunar crater and ejected a plume of material that was surprisingly rich in water ice.”
“Hydroxyls have also been detected in other volcanic rocks and in the lunar regolith, the layer of fine powder and rock fragments that coats the lunar surface. Hydroxyls, which consist of one atom of hydrogen and one of oxygen.”
For the new research, “Fourier-transform infrared spectroscopy was used to analyze the water content in grains of plagioclase feldspar from lunar anorthosites, highland rocks composed of more than 90 percent plagioclase. The bright-colored highlands rocks are thought to have formed early in the moon’s history when plagioclase crystallized from a magma ocean and floated to the surface. The infrared spectroscopy detected about 6 parts per million of water in the lunar anorthosites.”
“The surprise discovery of this work is that in lunar rocks, even in nominally water-free minerals such as plagioclase feldspar, the water content can be detected,” said Zhang, the James R. O’Neil Collegiate Professor of Geological Sciences.
“It’s not ‘liquid’ water that was measured during these studies but hydroxyl groups distributed within the mineral grain,” said Notre Dame’s Hui. “We are able to detect those hydroxyl groups in the crystalline structure of the Apollo samples.”
These hydroxyls represent strong evidence that the Moon’s interiors possessed large quantities of water when it was in its early molten state, before its crust solidified. This water could have had a significant role in the formatio of lunar basalts.
“The presence of water,” said Hui, “could imply a more prolonged solidification of the lunar magma ocean than the once-popular anhydrous moon scenario suggests.”
“The researchers analyzed grains from ferroan anorthosites 15415 and 60015, as well as troctolite 76535. Ferroan anorthosite 15415 is one the best known rocks of the Apollo collection and is popularly called the Genesis Rock because the astronauts thought they had a piece of the moon’s primordial crust. It was collected on the rim of Apur Crater during the Apollo 15 mission.”
“Rock 60015 is highly shocked ferroan anorthosite collected near the lunar module during the Apollo 16 mission. Troctolite 76535 is a coarse-grained plutonic rock collected during the Apollo 17 mission.”
This discovery may have implications for the planned eventual ‘colonization’ of the Moon, and the extraction of drinking water there.
The researchers just published their paper, titled “Water in lunar anorthosites and evidence for a wet early moon” February 17th in the journal Nature Geoscience.
Source: University of Michigan
Image Credits: NASA/Johnson Space Center; Moon Set via Wikimedia Commons
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