Moon Water
Discovered: Dampens Moon-Formation Theory
(9 July 2008) Using new techniques,
scientists have discovered for the first time that tiny beads of volcanic
glasses collected from two Apollo missions to the Moon contain
water.
The researchers found that, contrary to previous thought,
water was not entirely vaporised in the violent events that formed the Moon.
The new study suggests that the water came from the Moon's interior and was
delivered to the surface via volcanic eruptions over 3 billion years ago. The
finding calls into question some critical aspects of the "giant impact" theory
of the Moon's formation and may have implications for the origin of possible
water reservoirs at the Moon's poles. The research is published in the July 10,
2008, issue of Nature.
It is believed that the Moon was formed when a
Mars-size body collided with Earth some 4.5 billion years ago. This "giant
impact" melted both objects and sent molten debris into orbit around the Earth,
some of which coalesced to form the Moon. Under this scenario, the heat from
the giant impact would have vaporised the light elements.
Over the past
forty years there have been significant efforts to determine the content and
origin of the volatile contents in the lunar samples. There is reliable
evidence that the Moon's interior contains sulphur, some chlorine, fluorine,
and carbon. Yet the evidence for indigenous H2O has remained elusive,
consistent with the general consensus that the Moon is dry.
The research
team, with scientists from Brown University, Carnegie Institution for Science,
and Case Western Reserve University, took advantage of new methods for
analysing lunar samples to detect tiny amounts of water. Co-author of the
paper, Erik Hauri of the Carnegie's Department of Terrestrial Magnetism,
developed new techniques that can detect extremely minute quantities of water
in glasses and minerals by the technology called secondary ion mass
spectrometry (SIMS). These technical advances were made in collaboration with
engineers from Cameca Instruments (France), who manufactured the NanoSIMS
instrument used to make these challenging measurements.
"For the past
four decades, the limit for detecting water in lunar samples was about 50 parts
per million (ppm) at best," explained Hauri. "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."
One glass bead told the tale of
what happened. The researchers found that the volatiles decreased from the tiny
sphere's core to its rim--a difference that indicates that some 95% of the
water was lost during the volcanic activity. James Van Orman, a former Carnegie
postdoc now at Case Western Reserve University, was one of the team members who
wrote the numerical model. "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. A droplet cooling at a rate of about 3° F to 6° F
per second over 2 to 5 minutes between the time of eruption and when the
material was quenched or rapidly cooled matched the profiles for all the
volatiles, including the loss of about 95% of the water," he said.
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," continued 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% of their water, where did all that water
go?"
Since the Moon's gravity is too feeble to retain an atmosphere, the
researchers speculate that some of the water vapour from the eruptions was
probably forced into space, but some may also have drifted toward the cold
poles of the Moon where ice may be present in permanently shadowed craters.
Several previous lunar missions have suggested the presence of ice at both
poles. Unless it is very deep, lunar groundwater is unlikely to exist since the
Sun heats most of the Moon's surface to over 200°F (100°C).
Lead
author of the study, Alberto Saal of Brown University remarked: "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 endeavour. We 'knew' the Moon was
dry."
Many scientists have believed the Moon's polar ice, if there,
originated from impacts of water-rich meteoroids and comets that struck the
Moon's surface over its history. The new study suggests that some of this water
could have come from lunar volcanic eruptions. Verifying that water is at the
Moon's poles is one goal of the NASA Lunar Reconnaissance Orbiter
(http://lro.gsfc.nasa.gov/) mission, due to launch later this year. And it is
the primary objective of the Lunar Crater Observation and Sensing Satellite
with a 2009 launch date. Verification of water on the Moon's surface is an
important step in progress toward an eventual manned lunar
outpost.
(source: Carnegie Institution)
