Many experts were shocked by the recent discovery of water
on the moon, which was long thought to be bone-dry.
But not everyone was
surprised.
Astrophysicist Arlin Crotts of Columbia University has been
working for years on research that he says predicted this finding. In a paper
he submitted recently to the Astrophysical Journal with his graduate student
Cameron Hummels, Crotts hypothesizes the existence of widespread water on the
lunar surface, and offers an idea for how it got there.
"I am predicting something that just happened, that
nobody else was predicting," Crotts said. "I hope people recognize
that this is a true prediction of the spatial distribution of water around the
moon."
Until recently, many scientists thought the lunar surface
was almost completely dry, and that shadowed
craters near the poles offered the only chance for small stores of water.
But new
data from the NASA-built Moon Mineralogy Mapper (M3) on India's
Chandrayaan-1 satellite, NASA's Cassini spacecraft and NASA's Deep Impact probe
uncovered tantalizing evidence of water molecules all over the moon's surface.
These findings were detailed in three papers in the Sept. 25 issue of the
journal Science.
Some more details, especially about the possible water at
the poles, are likely to come when NASA's
LCROSS impactor slams into a crater on the moon's south pole Friday morning
in search of signs of water.
Where did it come from?
The experts behind the new findings said they don't yet know
the source
of this water. According to one hypothesis, charged hydrogen ions carried
from the sun to the moon by the solar wind could combine with oxygen on the
moon to form water molecules. Another idea is that the water is left over from
comets that have impacted the moon.
"There are many models out there," said Roger
Clark of the U.S. Geological Survey in Denver, who is a team member for the
Cassini spacecraft and a co-investigator for Chandrayaan-1. "Probably to
some degree they all are in play. It's too early to tell."
But Crotts has a different idea in mind.
Previous research has uncovered some water trapped in
minerals deep inside the moon, Crotts said. According to his model, this water
is likely to travel up through fissures to the lunar surface along with other
gases that are escaping the pressure of the moon's dense interior.
"We now know that there's water in the interior,"
Crotts told SPACE.com. "There's no particular reason to think that it
doesn't get out."
Buried water
One piece of evidence for interior water - a 2008 Nature
study by Brown University's Alberto Saal and colleagues - identified water
(between 260 and 745 parts per million, or ppm) in pebbles of hardened moon
lava brought back by Apollo astronauts. Other work on similar samples by
Francis McCubbin of the Carnegie Institution for Science in Washington D.C.
also indicates the moon could harbor water beneath its surface.
While Crotts thinks those amounts are enough to produce the
observed surface water, other experts are skeptical.
"I feel that it is highly unlikely that there are
significant amounts of water remaining in the moon's interior at this
time," said Darby Dyar of Mount Holyoke College in Massachusetts, who was
a co-author on the recent Science papers announcing the surface water
discovery. "The amounts of water found are at the parts per million level,
and as such constitute only a very small amount of water as a resource."
Other scientists echo this thinking.
"The moon interior is believed to be very dry, with
less water than what we observed on the surface," Olivier Groussin, a
scientist at the Laboratoire d'Astrophysique de Marseille in France and another
co-author on the Science papers, wrote in an e-mail. "Apollo samples
indicate less than 50 ppm of water in the interior, while we detected about
1000 ppm on the surface."
Wet moon
However, Denton Ebel, curator of meteorites at the American
Museum of Natural History in New York, said the trace amounts of interior moon
water so far identified could be enough to produce the signature found at the
surface.
"I think the amounts of water that are inferred for the
lunar interior from the work of Alberto Saal and the work of Francis McCubbin,
coupled with what we know about the lunar core, implies that degassing is a
viable cause of the hydrogen signal that's observed," Ebel said in a phone
interview.
"I think that [Crotts'] scenario of seepage - slow
degassing - is consistent with the findings," Ebel said. "And I think
it's more encouraging than the idea of hydrogen implantation by the solar wind.
The bottom line is, he could turn out to be right."
Crott's paper outlining his hypothesis has been submitted to
an academic journal, and is in the process of being peer-reviewed before
possible publication. Some scientists are waiting to reserve judgment until
then.
"I am delighted that scientists have been thinking
along these lines, but we must wait to see if it holds up to the test of peer
review," said Jim Green, director of the Planetary Science Division of the
Science Mission Directorate at NASA Headquarters in Washington, D.C., another
co-author on the Science papers.
To get to the bottom of the issue, more data will be needed,
scientists say.
In fact, the signature of water seen on the surface could
easily result from a combination of multiple processes, Crotts said, adding that
his explanation might only account for some of the water on the surface.
To find out for sure, more lunar expeditions will be
required, Crotts said.
'"We've got to have another polar orbiter mission, and
it's got to have some instruments on it that study this question," he
said.
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