Solar Wind likely Source of Lunar Water

A team of researchers from three U.S. universities has suggested that the most likely source of the water locked inside lunar soils is the constant stream of charged particles from the Sun.

In 2009, NASA’s Lunar Crater Observation and Sensing satellite slammed into a permanently shadowed lunar crater and ejected a plume of material that was surprisingly rich in water ice. Water and related compounds have also been detected in the lunar regolith, the layer of fine powder and rock fragments that coats the lunar surface.

But the origin of this water has remained unclear. Theoretical models of lunar water stability dating to the late 1970s suggest that hydrogen ions from the solar wind can combine with oxygen on the Moon’s surface to form water and related compounds called hydroxyls, which consist of one atom of hydrogen and one of oxygen and are known as OH.

A new study, published in the journal Nature Geoscience, confirms solar wind as a source for water embedded in the lunar surface.

The team used infrared spectroscopy and mass spectrometry analyses of Apollo samples to reveal the presence of significant amounts of hydroxyl inside glasses formed in the lunar regolith by micrometeorite impacts. When combined, the techniques of Fourier transform infrared spectroscopy and secondary ion mass spectrometry can be used to determine the chemical form of the hydrogen in a substance, as well as its abundance and its isotopic composition.

“We found that the ‘water’ component, the hydroxyl, in the lunar regolith is mostly from solar wind implantation of protons, which locally combined with oxygen to form hydroxyls that moved into the interior of glasses by impact melting,” said Prof Youxue Zhang of the University of Michigan, who co-authored the study.

“Lunar regolith is everywhere on the lunar surface, and glasses make up about half of lunar regolith. So our work shows that the ‘water’ component, the hydroxyl, is widespread in lunar materials, although not in the form of ice or liquid water that can easily be used in a future manned lunar base.”

“This means water likely exists on Mercury and on the asteroids such as Vesta or Eros further within our Solar system,” added lead author Dr Yang Liu of the University of Tennessee’s Planetary Geosciences Institute. “These planetary bodies have very different environments, but all have potential to produce water. The finding also implies solar-wind contributes to water ice in lunar poles.”

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Bibliographic information: Yang Liu et al. Direct measurement of hydroxyl in the lunar regolith and the origin of lunar surface water. Nature Geoscience, published online 14 October 2012; doi: 10.1038/ngeo1601