Water Discovered in Moon Samples
By Jeremy Hsu
09 July 2008
Water has been found conclusively for the first time inside ancient moon samples brought back by Apollo astronauts. The discovery may force scientists to rethink the lunar past and future, although uncertainty remains about how much water exists and whether future explorers could extract it.
The water was found inside volcanic glass beads, which represent solidified magma from the early moon's interior. The news swept through much of the scientific community even before being detailed in the journal Nature this week.
"This really appears to have changed the rules of the game," said Robin Canup, astrophysicist and director of the Southwest Research Institute in Boulder, Colo., who was not part of the team that made the discovery. "The assumption has been that the moon is dry."
Revising lunar history
Scientists have long assumed the moon was dry because of its violent birth roughly 4.5 billion years ago. The leading theory holds that a Mars-sized planet smashed into Earth and tore off molten pieces that eventually formed into the moon. Most scientists thought that any water in the developing lunar body would have vaporized and been lost to space.
"If there was a lot of water in the early moon, then that is new for sure," said Ben Bussey, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory who also was not involved in the new study. "People will have to think about that when they think about how the moon evolved."
The earlier thinking about the moon's lack of water meant researchers struggled to even get funding to search for counter-evidence.
"I thought that if we were really lucky we would get to see it," said Alberto Saal, a geochemist at Brown University and lead author on the Nature study. "Like everybody else, I was thinking our chances were low."
A delicate proposal
Saal's group examined lunar samples brought back from the Apollo missions of the 1960s and 1970s. The glass beads range in color from green to yellow-brown to red, depending on their elemental chemistry.
Such beads formed from droplets of molten lava that spewed from fire fountains reaching down deep within the primitive lunar interior. Saal's group measured the beads' elemental makeup to ensure they came from lunar volcanic activity and not from the impact event that formed the moon.
The researchers also ruled out the chance that such beads could have become contaminated by outside forces such as hydrogen — an element of water — from the solar wind.
Others had tried and failed earlier to find water in similar samples, but one of Saal's collaborators had developed improved detection methods using a technique called secondary ion mass spectrometry (SIMS).
"For the past four decades, the limit for detecting water in lunar samples was about 50 parts per million (ppm) at best," said Erik Hauri, geochemist at the Carnegie Institution in Washington, D.C. and co-author on the study. "We developed a way to detect as little as 5 ppm of water."
The group found up to 46 ppm of water within the glass beads. Saal and his collaborators then used modeling to estimate how much water originally existed in the magma within the moon's interior, knowing some water would have escaped the molten droplets as a gas on the surface.
That led to estimates that the glass beads may contain 745 ppm of water — strikingly similar to solidified lava that came up from the Earth's upper mantle through undersea vents. However, Saal's group gives 260 ppm of water as the most certain figure for now.
Follow the water
Just finding water at all could lead to a sea-change in how scientists view the early moon — either the moon held onto water from Earth during its violent creation, or else water gathered from elsewhere within 100 million years of the impact event as the moon solidified.
Modeling done on the Earth impact event suggests that our planet would have held onto much of its water, Canup explained. But such models say little about how much the moon could have held onto, and other questions remain unanswered even from this latest study.
"The major uncertainty I see is whether they're sampling something that tells us about the bulk composition of the moon, or whether they have sampled materials produced by a more limited water-rich part of the moon's interior," Canup said.
Knowing whether water is highly abundant or relatively scarce within the moon could also have implications for lunar exploration, but not for near-future missions such as NASA's Lunar Reconnaissance Orbiter (LRO) and Lunar Crater Observation and Sensing Satellite (LCROSS). The latter mission is slated to crash two spacecraft into the moon's south pole in early 2009, in an attempt to find evidence of water ice hidden in the lunar craters.
Any surface water ice likely formed from comets and other external bodies crashing into the moon and releasing their water, Bussey said, although he acknowledged the chance that some water vapor drifted to the poles during the moon's early history.
Saal's group will attempt to clear up some of those questions as they examine samples from more Apollo missions. For now, their work stands as an example of continuing to squeeze science out of an unexpected link between the past and future.
"I think it's exciting that you keep getting results out of the Apollo samples." Bussey said.