Samples from Tranquility Base suggest solar wind brings water to lunar soil. Photo Credit: NASA
The solar wind is a likely contributor to widespread surface level water deposits in lunar soil churned by micrometeoroid impacts over billions of years, according to an analysis of tiny glass-like features called agglutinates extracted from samples of the moon gathered by the Apollo 11, 16 and 17 astronauts.
A similar process could be at work on other airless solar system bodies, including Mercury and the large main belt asteroid Vesta, according to the study "Direct Measurement of Hydroxyl in the Lunar Regolith and the Origin of Lunar Surface Water" led by geologist Yang Liu, of the University of Tennessee, Knoxville, and published Oct. 14 in the journal Nature Geoscience.
"Because agglutinates make up a major proportion of lunar soils, often reaching 50% volume of a mature soil, the detected OH in agglutinates represents an unanticipated, abundant reservoir of OH/H2O in lunar regolith," according to the research team "The exposure age of a soil at the lunar surface is proportional to its agglutinate content. Therefore, our results indicate that older lunar soils might contain higher OH than younger lunar soils."
The sample analysis, conducted using Fournier transform infrared spectroscopy and secondary ion mass spectroscopy, provides a traceable transfer mechanism: hydrogen ions carried by the solar wind reach and are implanted in the lunar soil and perhaps other planetary bodies where there is no atmosphere or magnetic field to act as shields. On the moon, the solar wind hydrogen ions are subsequently transferred to the agglutinitic features through steady energetic pelting of the upper lunar surface by micrometeorites.
A "large fraction" of the examined agglutinates registered 200 to 300 parts per million of hydroxyl, OH, and water, according to the study.
A half-dozen researchers from Cal Tech, the University of Michigan as well as the University of Tennessee claim they are first to provide evidence for a solar system water distribution process previously attributed largely to comet and large meteorite impacts. On the moon, those impacts are believed responsible for ice deposits detected in permanently shadowed regions of craters at the lunar South Pole.
The study pieces together findings from Apollo as well as subsequent NASA missions, including Genesis, which launched in 2001 and returned three years later with a harvest of solar wind particles for constituent analysis. The Lunar Reconnaissance Orbiter and Lunar Crater Observation and Sensing Satellite, both launched in 2009, mapped the moon's surface in new detail, while permitting LCROSS to make direct observations of lunar South Pole ice. Lunar Prospector, a 1998 orbital mission, looked for evidence of polar ice, with neutron and gamma ray mass spectrometers.