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 University of Michigan researcher 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. 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," said Zhang, a professor in the Department of Earth and Environmental Sciences and one of three co-authors of the study.
"Under that model, the hot ejecta should have been degassed almost completely, eliminating all water," Zhang added.
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.
"It's not 'liquid' water that was measured during these studies but hydroxyl groups distributed within the mineral grain. We are able to detect those hydroxyl groups in the crystalline structure of the Apollo samples," said first author Hejiu Hui, postdoctoral research associate of civil and environmental engineering and Earth sciences at the University of Notre Dame.
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."
The discovery was reported in a paper published online in the journal Nature Geoscience.