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An illustration showing the stream of charged hydrogen ions carried from the sun to the moon by the solar wind. Scientists think this process might explain the possible presence of hydroxyl or water on the moon. Credit: University of Maryland/F. Merlin/McREL |
Editor's Note: See also this newer story: 'Significant Amount' of Water Found on Moon, published Nov. 13, 2009.The story below, about scant amounts of water found across the lunar surface, was published in September, 2009:
This story was updated at 10:49 p.m. EDT on the date of initial publication.
Since man
first touched the moon and brought pieces of it back to Earth, scientists have
thought that the lunar surface was bone dry. But new observations from three
different spacecraft have put this notion to rest with what has been called
"unambiguous evidence" of water across the surface of the moon.
The new
findings, detailed in the Sept. 25 issue of the journal Science, come in the
wake of further evidence of lunar
polar water ice by NASA's Lunar Reconnaissance Orbiter and just weeks
before the planned
lunar impact of NASA's LCROSS satellite, which will hit one of the
permanently shadowed craters at the moon's south pole in hope of churning up
evidence of water ice deposits in the debris field.
The moon remains drier than any desert on Earth, but the water is said to exist on the moon in very small quantities. One ton of the top layer of the lunar surface would hold about 32 ounces of water, researchers said. ?
"If the water molecules are as mobile as we think they are ? even a fraction of them ? they provide a mechanism for getting water to those permanently shadowed craters," said planetary geologist Carle Pieters of Brown University in Rhode Island, who led one of the three studies in Science on the lunar find, in a statement. "This opens a whole new avenue [of lunar research], but we have to understand the physics of it to utilize it."
Finding
water on the moon would be a boon to possible future
lunar bases, acting as a potential source of drinking water and fuel.
Apollo turns up dry
When Apollo
astronauts returned from the moon 40 years ago, they brought back several
samples of lunar rocks.
The moon
rocks were analyzed for signs of water bound to minerals present in the rocks;
while trace amounts of water were detected, these were assumed to be
contamination from Earth, because the containers the rocks came back in had
leaked.
"The
isotopes of oxygen that exist on the moon are the same as those that exist on
Earth, so it was difficult if not impossible to tell the difference between
water from the moon and water from Earth," said Larry Taylor of the
University of Tennessee, Knoxville, who is a member of one of the NASA-built
instrument teams for India's Chandrayaan-1 satellite and has studied the moon
since the Apollo missions.
While
scientists continued to suspect that water
ice deposits could be found in the coldest spots of south pole craters that
never saw sunlight, the consensus became that the rest of the moon was bone dry.
But new
observations of the lunar surface made with Chandrayaan-1, NASA's Cassini
spacecraft, and NASA's Deep Impact probe, are calling that consensus into
question, with multiple detections of the spectral signal of either water
or the hydroxyl group (an oxygen and hydrogen chemically bonded).
Three spacecraft
Chandrayaan-1,
India's first-ever moon probe, was aimed at mapping the lunar surface and
determining its mineral composition (the orbiter's mission ended 14 months
prematurely in August after an abrupt malfunction). While the probe was still
active, its NASA-built Moon Mineralogy Mapper (M3) detected wavelengths of
light reflected off the surface that indicated the chemical bond between
hydrogen and oxygen ? the telltale sign of either water or hydroxyl.
Because M3
can only penetrate the top few millimeters of lunar regolith, the newly
observed water seems to be at or near the lunar surface. M3's observations also
showed that the water signal got stronger toward the polar regions.
Cassini,
which passed by the moon in 1999 on its way to Saturn, provides confirmation of
this signal with its own slightly stronger detection of the water/hydroxyl
signal. The water would have to be absorbed or trapped in the glass and
minerals at the lunar surface, wrote Roger Clark of the U.S. Geological Survey
in the study detailing Cassini's findings.
The Cassini
data shows a global distribution of the water signal, though it also appears
stronger near the poles (and low in the lunar maria).
Finally,
the Deep Impact spacecraft, as part of its extended EPOXI mission and at the
request of the M3 team, made infrared detections of water and hydroxyl as part
of a calibration exercise during several close approaches of the Earth-Moon
system en route to its planned flyby of comet 103P/Hartley 2 in November 2010.
Deep Impact
detected the signal at all latitudes above 10 degrees N, though once again, the
poles showed the strongest signals. With its multiple passes, Deep Impact was
able to observe the same regions at different times of the lunar day. At noon,
when the sun's rays were strongest, the water feature was lowest, while in the
morning, the feature was stronger.
"The
Deep Impact observations of the Moon not only unequivocally confirm the
presence of [water/hydroxyl] on the lunar surface, but also reveal that the
entire lunar surface is hydrated during at least some portion of the lunar
day," the authors wrote in their study.
The
findings of all three spacecraft "provide unambiguous evidence for the
presence of hydroxyl or water," said Paul Lucey of the University of
Hawaii in an opinion essay accompanying the three studies. Lucey was not
involved in any of the missions.
The new
data "prompt a critical reexamination of the notion that the moon is dry.
It is not," Lucey wrote.
Where the water comes from
Combined,
the findings show that not only is the moon hydrated, the process that makes it
so is a dynamic one that is driven by the daily changes in solar radiation
hitting any given spot on the surface.
The sun
might also have something to do with how the water got there.
There are potentially
two types of water on the moon: that brought from outside sources, such as
water-bearing comets striking the surface, or that that originates on the moon.
This
second, endogenic, source is thought to possibly come from the interaction of
the solar wind with moon rocks and soils.
The rocks
and regolith that make up the lunar surface are about 45 percent oxygen
(combined with other elements as mostly silicate minerals). The solar wind ?
the constant stream of charged particles emitted by the sun ? are mostly
protons, or positively charged hydrogen atoms.
If the
charged hydrogens, which are traveling at one-third the speed of light, hit the
lunar surface with enough force, they break apart oxygen bonds in soil
materials, Taylor, the M3 team member suspects. Where free oxygen and hydrogen
exist, there is a high chance that trace amounts of water will form.
The various
study researchers also suggest that the daily dehydration and rehydration of
the trace water across the surface could lead to the migration of hydroxyl and
hydrogen towards the poles where it can accumulate in the cold traps of the
permanently shadowed regions.
- Video
- Target Moon: NASA's New Lunar Scouts, Part
2
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of Water Ice Spurs Excitement for Moon Exploration
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