February 21, 2013
There is water inside the moon – so much, in fact, that in some places it rivals the amount of water found within the Earth. Over the last five years, spacecraft observations and new lab measurements of Apollo lunar samples have overturned the long-held belief that the moon is bone-dry. Traces of water have been detected within the crystalline structure of mineral samples from the lunar highland upper crust obtained during the Apollo missions, according to a University of Michigan researcher and his colleagues. “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 Youxue Zhang, James R. O’Neil Collegiate Professor of Geological Sciences.
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.
“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.”
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.
Reference: Hejiu Hui et al., Water in lunar anorthosites and evidence for a wet early Moon, Nature Geoscience, 2013, DOI: 10.1038/ngeo1735
In 2011, Brown University scientists found super-tiny melt inclusions in lunar soil samples –image top of page– that opened the door for measurements that revealed the magnitude of water inside the moon.
The Daily Galaxy via University of Michigan
Image credit: Credit: Saal lab/Brown University