A new study reveals that the water within the Apollo Moon rocks – and within the Moon itself — likely came from comets bombarding the nascent lunar surface, shortly after it formed following an impact event with a young Earth and Mars-sized protoplanet. The recent findings of abundant water at the lunar poles by the LCROSS impactor and across the Moon’s surface by various spacecraft have turned the long-standing notion of a dry Moon on its head, and the past year and a half, researchers have been trying to determine where this unexpected water came from.
“The water we are looking at is internal,” said Larry Taylor from the University of Tennessee, Knoxville, a member of an international team. “It was put into the moon during its initial formation, where it existed like a melting pot in space, where cometary materials were added in at small yet significant amounts.”
Using secondary ion mass spectrometry, the researchers measured the water signatures within rocks returned from the Apollo 11, 12, 14, and 17 missions that landed on the moon between 1969 and 1972. They found the chemical properties of the lunar water were very similar to signatures seen in three different comets: Hyakutake, Hale-Bopp and Halley.
The team found significant water in the lunar mineral apatite from both mare and highlands rocks, which indicates “a role for water during all phases of the Moon’s magmatic history,” the team wrote in their paper. “Variations of hydrogen isotope ratios in apatite suggest sources for water in lunar rocks could come from the lunar mantle, solar wind protons and comets. We conclude that a significant delivery of cometary water to the Earth–Moon system occurred shortly after the Moon-forming impact.”
Even though comet impacts may also have created the Earth’s oceans, Taylor said the water signatures from the mass spectrometer show that the water on the Earth and Moon are different, as apatite has a ratio of the deuterium and hydrogen that are distinctive from those in normal Earth water.
“The values of deuterium/hydrogen (D/H) that we measure in apatite in the Apollo rock samples is clearly distinguishable from water from the Earth, mitigating against this being some sort of contamination on Earth,” said James Greenwood of Wesleyan University, who led the research team.
Initially after the Apollo program, the Moon was believed to extremely dry. Many of the rocks returned by the astronauts and also the Soviet Luna program contained trace water or minor hydrous minerals, but those signatures were attributed to terrestrial contamination since most of the boxes of the Apollo program used to bring the Moon rocks to Earth leaked. This led the scientists to assume that the trace amounts of water they found came from Earth air that had entered the containers. The assumption remained that, outside of possible ice at the moon’s poles, there was no water on the moon.
Forty years later, a trio of spacecraft found evidence of water across the surface of the Moon: The Chandrayaan-1 spacecraft’s Moon Mineralogy Mapper (M Cubed) found that infrared light was being absorbed near the lunar poles at wavelengths consistent with hydroxyl- and water-bearing materials. A spectrometer on the re-purposed Deep Impact probe showed strong evidence that water is ubiquitous over the surface of the moon, and archival data from a Cassini Moon flyby also agreed with the finding that water appears to be widespread across the lunar surface.
“This discovery forces us to go back to square one on the whole formation of the Earth and moon,” said Taylor. “Before our research, we thought the Earth and moon had the same volatiles after the Giant Impact, just at greatly different quantities. Our work brings to light another component in the formation that we had not anticipated — comets.”
Taylor added that the existence of hydrogen and oxygen – water – on the moon can literally serve as a launch pad for further space exploration.
“This water could allow the moon to be a gas station in the sky,” said Taylor. “Spaceships use up to 85 percent of their fuel getting away from Earth’s gravity. This means the moon can act as a stepping stone to other planets. Missions can fuel up at the moon, with liquid hydrogen and liquid oxygen from the water, as they head into deeper space, to other places such as Mars.”
Their paper, “Extraterrestrial Hydrogen Isotope Composition of Water in Lunar Rocks” was published in the journal, Nature Geoscience.