Water, water everywhere: Even on the Moon

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  • Published: Mar 1, 2013
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Water, water everywhere: Even on the Moon

Blue moon

An analysis of tiny mineral deposits from lunar anorthosites brought back to Earth by Apollo astronauts adds new evidence to suggestions that water was not lost to space during the formation of the Moon. The analysis was carried out using Fourier-transform infrared (FTIR) spectroscopy.

An analysis of tiny mineral deposits from lunar anorthosites brought back to Earth by Apollo astronauts adds new evidence to suggestions that water was not lost to space during the formation of the Moon. The analysis was carried out using Fourier-transform infrared (FTIR) spectroscopy.

Youxue Zhang of the University of Michigan, Ann Arbor and his colleagues have found trace amounts of water trapped within the crystalline structure of mineral samples from the lunar highland upper crust. Astronomers consider the lunar highlands to be the last vestiges of the Moon's original crust, which crystallized from a magma ocean billions of years ago. The presence of water included in samples of the crystals brought back to Earth suggest that the early Moon may have been wet and that the violent formation of our orbiting companion did not lead to substantial water loss. 

Wet, wet, wet

"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 explains. This observation contradicts the predominant lunar formation theory and is difficult to explain in terms of a primordial body assimilating hot ejecta produced when a Mars-sized body collided with a body that would ultimately form the Earth itself. "Under that model, the hot ejecta should have been degassed almost completely, eliminating all water," Zhang says.

Writing in the journal Nature Geoscience, Zhang and postdoctoral researchers Hejiu Hui now at the University of Notre Dame, together with Clive Neal also at Notre Dame and Anne Peslier of Jacobs Technology and NASA's Johnson Space Center, describe the analysis of the moon rocks. Rock 60015 is a 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. The researchers also investigated grains from ferroan anorthosite 15415 - the Genesis Rock, so-called because the Apollo astronauts suspected that they had found a piece of the Moon's primordial crust on the rim of Apur Crater during the Apollo 15 mission.

The team used FTIR spectroscopy to detect water in the brightly coloured rocks and measured water at about 6 parts per million 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," Zhang says.

Water world

Work during the last five years has detected not ice water, but hydroxyl groups, distributed within mineral grains, which are most likely there because of the presence of water in the lunar interior. 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.

The present work is further 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," adds Hui, "could imply a more prolonged solidification of the lunar magma ocean than the once-popular anhydrous moon scenario suggests."

Related Links

Nature Geosci 2013, online: "Water in lunar anorthosites and evidence for a wet early Moon"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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