Water dampens Moon formation theory
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: July 15, 2008

An important conclusion of the Apollo and Luna programmes was that the Moon is deficient in highly volatile elements – including water – compared to the Earth, but in a new analysis of lunar rocks, scientists have detected enough water to require the classical models of lunar formation and evolution to be reconsidered.

It is widely believed that the Moon formed from debris that was flung into space following the collision of a Mars-sized object with the Earth around 4.6 billion years ago. In this scenario it is easy to imagine that the heat from the impact would have been so extreme that all the light volatile elements, including water, would have been completely vapourised. But the results of a new study, which took advantage of new methods for sniffing out miniscule amounts of water trapped inside volcanic glass beads, suggest this well established theory may not tell the whole story.

“For the past four decades, the limit for detecting water in lunar samples was about 50 parts per million (ppm) at best,” says Erik Hauri of the Carnegie's Institution for Science. “We developed a way to detect as little as 5 ppm of water. We were really surprised to find a great deal more in these tiny glass beads, up to 46 ppm.”

Hundreds of craters at the lunar north (left) and south (right) poles could spend much of their time in permanent shadows (marked by yellow dots), remaining at ice-friendly temperatures of less than -200 degrees Celsius. Image: Bussey et al.

Furthermore, the results suggest that the water originated in the Moon’s interior and was delivered to the surface via volcanic eruptions over three billion years ago. In one glass bead, the volatile content decreased from the sphere’s core to its rim, such that the difference in content indicates some 95 percent of the water was lost through volcanic activity. The finding may also have implications for the origin of possible water reservoirs at the Moon’s poles.

“We looked at many factors over a wide range of cooling rates that would affect all the volatiles simultaneously and came up with the right mix,” says James Orman of Case Western Reserve University. “A droplet cooling at a rate of about 3 to 6 degrees per second over 2 to 5 minutes between the time of eruption and when the material was quenched (rapidly cooled) matched the profiles for all the volatiles, including the loss of about 95 percent of the water.”

The researchers estimated that there was originally about 750 ppm of water in the magma at the time of eruption. “Since the Moon was thought to be perfectly dehydrated, this is a giant leap from previous estimates,” says Hauri. “It suggests the intriguing possibility that the Moon's interior might have had as much water as the Earth's upper mantle. But even more intriguing, if the Moon’s volcanoes released 95 percent of their water, where did all that water go?”

The researchers speculate that since the Moon's gravity is too feeble to retain an atmosphere, some of the water vapour from the eruptions was probably forced into space, but some may also have drifted toward the frigid poles of the Moon where ice may be stable in permanently shadowed craters, protected from the glare of the Sun that in the absence of an atmosphere, heats the lunar surface to temperatures beyond the boiling point of water.

Artist impression of the LCROSS mission heading towards the Moon. Two heavy impactors will plunge into the permanently dark floor of a south polar crater in the hopes that water ice will be thrown up in the impact event. Image: NASA.

Many scientists believe that icy reservoirs in the lunar poles could have been delivered by water-rich comets, but the new research suggests that some of this water could have come from lunar volcanic eruptions. Verifying the presence of water at the Moon’s poles will help develop the theories for water having played an integral role in the Moon’s development, and is the aim of several missions destined for the Moon over the next few years, including NASA’s Lunar Reconnaissance Orbiter, due to launch later this year, and the Lunar Crater Observation and Sensing Satellite (LCROSS) which currently has a 2009 launch date. Determining the Moon’s water inventory will be a crucial step towards an eventual manned lunar outpost.

As well as provoking scientists to rethink their ideas about the thermal and geochemical evolution of the Moon, the new research also teaches an important lesson in retesting well-established theories. Alberto Saal of Brown University, and lead author of the study comments: “Beyond the evidence for the presence of water in the interior of the Moon, which I found extremely exciting, I learned that the contributions from scientists from other disciplines has the potential to produce unexpected results. Such a scientist is able not only to ask questions that no one has asked before, but also can challenge hypotheses that are embedded in the thinking of the scientists working in the field for many years. Our case is a typical example. When I suggested we measure volatiles in lunar material, everyone I talked to thought that such proposal was a futile endeavor. We ‘knew’ the Moon was dry.”