May 26, 2011
Scientists Find Evidence Of Water Within Lunar Samples
A team of NASA-funded researchers has measured water from the moon that shows some parts of the lunar mantle have as much water as the Earth's upper mantle.
The research, from Case Western Reserve University, Carnegie Institution for Science, and Brown University, shows 100 times more water than measured before. Scientists discovered water along with volatile elements in lunar magma trapped inside of crystals that are themselves trapped inside tiny volcanic glass beads.
The inclusions were found in lunar sample 74220, the famous high-titanium "orange glass soil" of volcanic origin collected during the Apollo 17 mission in 1972. The team used a state-of-the-art ion microprobe instrument to measure the water content of the inclusions, which were formed during explosive eruptions on the Moon about 3.7 billion years ago.
"These samples provide the best window we have to the amount of water in the interior of the Moon," said James Van Orman, professor of geological sciences at Case Western Reserve. "The interior seems to be pretty similar to the interior of the Earth, from what we know about water abundance."
The researchers also found the concentrations of water and volatile elements including fluorine, chlorine and sulfur in lunar magma are nearly identical to concentrations in solidified magma from primitive mid-ocean ridges on Earth.
The discovery strengthens the theory that the Moon and Earth have a common origin. But it also forces scientists to reconsider the current theory of the process: that a huge impact in Earth's early history ejected material into orbit that helped create the Moon.
Part of the origin theory says that water and other volatile elements and compounds were depleted due to the heat and violence of the impact. But this research indicates otherwise and provides new clues to the process of lunar formation.
"Water plays a critical role in determining the tectonic behavior of planetary surfaces, the melting point of planetary interiors, and the location and eruptive style of planetary volcanoes," said Hauri, a geochemist with Carnegie's Department of Terrestrial Magnetism (DTM), and lead author of the study.
"We can conceive of no sample type that would be more important to return to Earth than these volcanic glass samples ejected by explosive volcanism, which have been mapped not only on the Moon but throughout the inner solar system," he said.
The presence of this much water in lunar magma also forces scientists to consider volcanic activity as a possible source of ice found in the shadows of crates at the poles.
In a paper in Nature in 2008, the same team then led by Alberto Saal, associate professor of geological sciences at Brown, reported the first evidence for the presence of water on the Moon and used models to estimate how much was originally in the magmas before eruption.
"The bottom line," said Saal, "is that in 2008, we said the primitive water content in the lunar magmas should be similar to the water content in lavas coming from the Earth's depleted upper mantle. Now, we have proven that is indeed the case."
The new finding got a critical assist from a Brown undergraduate student, Thomas Weinreich, who found the melt inclusions that allowed the team to measure the pre-eruption concentration of water in the magma and to estimate the amount of water in the Moon's interior.
Weinreich searched through thousands of grains from the famous "orange soil" samples discovered by astronaut Harrison Schmidt during the Apollo 17 mission before finding ten that included melt inclusions.
"It just looks like a clear sample with some black specks in it," said Weinreich, a co-author on the paper.
Compared with meteorites, Earth and the other inner planets of our solar system contain fairly low amounts of water and volatile elements, which were not abundant in the inner solar system during planet formation. The even lower quantities of these volatile elements found on the Moon has long been claimed as evidence that it must have formed following a high-temperature, catastrophic impact. But the new findings show that aspects of this theory must be re-examined.
The paper -- "High Pre-Eruptive Water Contents Preserved in Lunar Melt Inclusions" -- was written by Hauri, Weinreich, Saal, Van Orman and Malcolm Rutherford of Brown. The research was funded by NASA's Lunar Advanced Science and Exploration Research and Cosmochemistry Programs, the NASA Lunar Science Institute (NLSI), and the Astrobiology Institute.
Image 1: Close-up of the orange soil discovered on the Apollo 17 mission in 1972. The color is caused by microscopic glass beads created by volcanic processes earlier in the Moon's history. Credit: NASA
Image 2: Scientists at Brown University found super-tiny melt inclusions in lunar soil samples that opened the door for measurements that revealed the magnitude of water inside the moon. Credit: Saal lab, Brown University
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