The lunar highlands are thought to represent the original crust, crystallized from a magma ocean on a mostly molten early moon. The new findings indicate that the early moon was wet and that water there was not substantially lost during the moon’s formation.
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, according to U-M’s Youxue Zhang and his colleagues.
“Because these are some of the oldest rocks from the moon, the water is inferred to have been in the moon when it formed,” Zhang 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,” Zhang said.
Called the “Genesis Rock,” this lunar sample of unbrecciated anorthosite collected during the Apollo 15 mission was thought to be a piece of the moon’s primordial crust. In a paper published online Feb. 17 in Nature Geoscience, a University of Michigan researcher and his colleagues report that traces of water were found in the rock. Photo courtesy of NASA/Johnson Space Center
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, were also detected in the lunar anorthosite study reported in Nature Geoscience.
In the latest work, 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 work, which was conducted at Zhang’s U-M lab and co-author Anne H. Peslier’s lab, 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, 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.”
The hydroxyl groups the team detected are evidence that the lunar interior contained significant water during the moon’s early molten state, before the crust solidified, and may have played a key role in the development 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.”
SOURE – University of Michigan
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