Arecibo Radar Shows No Evidence of Thick Ice At Lunar Poles

http://www.news.cornell.edu/releases...poles.deb.html

Arecibo radar shows no evidence of thick ice at lunar poles, despite
data from previous spacecraft probes, researchers say

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ARECIBO, P.R. -- Despite evidence from two space probes in the
1990s, radar astronomers say they can find no signs of thick ice at
the moon's poles. If there is water at the lunar poles, the
researchers say, it is widely scattered and permanently frozen inside
the dust layers, something akin to terrestrial permafrost.

Using the 70-centimeter (cm)-wavelength radar system at the National
Science Foundation's (NSF) Arecibo Observatory, Puerto Rico, the
research group sent signals deeper into the lunar polar surface --
more than five meters (about 5.5 yards) -- than ever before at this
spatial resolution. "If there is ice at the poles, the only way left
to test it is to go there directly and melt a small volume around the
dust and look for water with a mass spectrometer," says Bruce
Campbell of the Center for Earth and Planetary Studies at the
Smithsonian Institution.

Campbell is the lead author of an article, "Long-Wavelength Radar
Probing of the Lunar Poles," in the Nov. 13, 2003, issue of the
journal Nature. His collaborators on the latest radar probe of the
moon were Donald Campbell, professor of astronomy at Cornell
University; J.F. Chandler of Smithsonian Astrophysical Observatory;
and Alice Hine, Mike Nolan and Phil Perillat of the Arecibo
Observatory, which is managed by the National Astronomy and
Ionosphere Center at Cornell for the NSF.

Suggestions of lunar ice first came in 1996 when radio data from the
Clementine spacecraft gave some indications of the presence of ice on
the wall of a crater at the moon's south pole. Then, neutron
spectrometer data from the Lunar Prospector spacecraft, launched in
1998, indicated the presence of hydrogen, and by inference, water, at
a depth of about a meter at the lunar poles. But radar probes by the
12-cm-wavelength radar at Arecibo showed no evidence of thick ice at
depths of up to a meter. "Lunar Prospector had found significant
concentrations of hydrogen at the lunar poles equivalent to water ice
at concentrations of a few percent of the lunar soil," says Donald
Campbell. "There have been suggestions that it may be in the form of
thick deposits of ice at some depth, but this new data from Arecibo
makes that unlikely."

Says Bruce Campbell, "There are no places that we have looked at with
any of these wavelengths where you see that kind of signature."

The Nature paper notes that if ice does exist at the lunar poles it
would be considerably different from "the thick, coherent layers of
ice observed in shadowed craters on Mercury," found in Arecibo radar
imaging. "On Mercury what you see are quite thick deposits on the
order of a meter or more buried by, at most, a shallow layer of dust.
That's the scenario we were trying to nail down for the moon," says
Bruce Campbell. The difference between Mercury and the moon, the
researchers say, could be due to the lower average rate of comets
striking the lunar surface, to recent comet impacts on Mercury or to
a more rapid loss of ice on the moon.

What makes the lunar poles good cold traps for water is a temperature
of minus 173 degrees Celsius (minus 280 degrees Fahrenheit). The limb
of the sun rises only about two degrees above the horizon at the
lunar poles so that sunlight never penetrates into deep craters, and
a person standing on the crater floor would never see the sun. The
Arecibo radar probed the floors of two craters in permanent shadow at
the lunar south pole, Shoemaker and Faustini, and, at the north pole,
the floors of Hermite and several small craters within the large
crater Peary. In contrast, Clementine focused on the sloping walls of
Shackleton crater, whose floor can't be "seen" from Earth. "There is
a debate on how to interpret data from a rough, tilted surface," says
Bruce Campbell.

The Arecibo radar probe is a particularly good detector of thick ice
because it takes advantage of a phenomenon known as "coherent
backscatter." Radar waves can travel long distances without being
absorbed in ice at temperatures well below freezing. Reflections from
irregularities inside the ice produce a very strong radar echo. In
contrast, lunar soil is much more absorptive and does not give as
strong a radar echo.

Related World Wide Web sites: The following sites provide
additional information on this news release. Some might not be part
of the Cornell University community, and Cornell has no control over
their content or availability.

o Arecibo Observatory:
http://www.naic.edu

o Center for Earth and Planetary Studies:
http://www.nasm.si.edu/ceps/

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