THE MOON’S ‘DIRTY LAKE’ MAKE WAVES
Wednesday, December 4 1996
Source: The Washington Post
Analysis of '94 Data Meets With Skepticism
By Curt Suplee
Washington Post Staff Writer
Wednesday, December 4 1996; Page A03
The Washington Post
The moon may harbor a vast "dirty lake" of frozen water and dust covering 38 square miles
to a depth of 50 feet, scientists announced yesterday.
The researchers analyzed evidence gathered by the Clementine lunar mapping mission in
1994, and found that one area of the moon reflected radar waves in the same telltale way
that water ice does on other planets. Their conclusions suggest that an ice field, presumably
deposited by water vapor from hundreds of comet impacts over the past few billion years,
exists in the perpetual dark of the moon's deep-cratered south pole.
If the tentative discovery is confirmed, it would make the site "possibly the most valuable
piece of real estate in the solar system," Paul D. Spudis of the Lunar and Planetary Institute at
Rice University told a news conference at the Pentagon, which funded the unmanned
Clementine spacecraft's mission.
Spudis, who with colleagues reported the find in the Nov. 29 issue of the journal Science,
said that the ice lake could serve as a sort of "filling station" for future lunar explorers,
providing not only drinking water, but a source of hydrogen and oxygen for rocket fuel.
The announcement met with mixed reactions and widespread skepticism in the scientific
community. "I find this tantalizing, mesmerizing, but I'm not convinced," said James B. Garvin,
a geophysicist at NASA's Goddard Space Flight Center and an expert on planetary radar.
There are several other materials, including rock with a high metal content, that might
produce the same kind of radar signature, he said.
"I'd love it to be the answer," Garvin said. "We all would," because it would answer the
question, first voiced in 1961, of whether the vaporized debris of comets (which are about
90 percent water ice by mass) could somehow remain on the lunar landscape. Rock samples
taken by Apollo astronauts around the moon's equator indicated that the surface was
surpassingly arid.
The 900-pound, 4-by-6-foot Clementine made hundreds of passes around the moon. The
data for the possible ice detection came from only one -- when the craft was looking straight
down on the south pole. As a result, said Garvin, "admittedly there aren't a lot of data here."
Study co-author Stewart Nozette of Lawrence Livermore Laboratory said yesterday that the
observation was limited to the single pass in which "we could use the spacecraft antenna like
a flashlight" pointing directly down into a shadowed region.
When it did, he said, the radar returned the same kind of characteristic wave pattern -- "sort
of like a roadside reflector" that bounces much of the original illumination back just the way it
was sent -- that other astronomers have found characteristic of water ice deposits on
Mercury, Mars and some of Jupiter's moons.
Other materials, such as certain kinds of sulfur deposits, might give off the same radar
signature, Nozette said. But he and Spudis emphasized that they regard an ice lake as the
most likely explanation for the anomalous radar signal.
The reason, the scientists suggested, is that the lunar south pole lies within the huge South
Pole-Aitken basin area some 1,500 miles wide and seven miles deep. The temperature in
that abyss, they estimate, is about 40 degrees Centigrade above absolute zero -- far too cold
for water ice to evaporate, even in a vacuum.
Much of the moon's ice, the Clementine researchers said yesterday, may be spread across
this broad area, which is in permanent darkness. Within that region, they estimated, perhaps
one-half of 1 percent of the surface may be ice. That would mean a volume of pure water
equivalent to four football fields filled to a depth of 16 feet -- a large amount of ice for an
otherwise waterless world.
Many space scientists believe the enormous impact force of a comet colliding with the moon
at 30 or 40 miles per second would fragment water into its constituent atoms of hydrogen
and oxygen, which would easily escape the moon's modest gravitational pull and be lost in
space. And some scientists say if the moon had any water, it would have been squeezed out
of the rock billions of years ago when the nascent satellite was in its volcanic epoch.
Spudis called that scenario "unlikely," and the Clementine researchers suspect that, as comets
struck the moon and vaporized over billions of years, enough water molecules found their
way into this frigid south polar well to create a permanent formation of dust and ice particles
that could be as much as 50 feet thick in some places.
Although the data announced yesterday are insufficient by themselves to make the case for a
lunar lake, the scientists are hoping that subsequent probes -- including the unmanned Lunar
Prospector that is scheduled for launch next year -- will add to the evidence. The Prospector
will have an onboard instrument designed to detect the distinctive atomic signature of
hydrogen. If the element shows up around the south pole at any appreciable volume, then it
will probably be in the form of water ice, because no other source is likely.
If the ice hypothesis is confirmed, Spudis said, it would revolutionize prospects for lunar
exploration. "Water is one of the most valuable strategic materials we can find in the solar
system," and the lake data suggest that "it is apparently accessible and ready to use."
Fortunately, he noted, the Clementine maps show that there is a relatively small elevated
ridge area that rises above the incessant murk of the south pole. That ridge -- which is large
enough to accommodate a lunar lander, he said -- is exposed to sunlight 85 percent of the
time. So humans landing there could use "solar panels" for power generation.
@CAPTION: Col. Pedro Ruston, left, mission director for Clementine probe, and Dwight
Duston of the Ballistic Missile Defense Organization answer questions by a model of the
craft, which gathered key data during pass over moon's south pole.
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