- 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 Web sites:
-
- * Arecibo Observatory: <http://www.naic.edu>http://www.naic.edu
- * Center for Earth and Planetary Studies: <http://www>http://www.
nasm.si.edu/ceps/
-
- Editor's Note: The original news release can be found
<http://www.news.cornell.edu/releases/Nov03/radar.moonpoles.deb.html>here.
-
- This story has been adapted from a news release issued
by Cornell University.
-
- <http://www.sciencedaily.com/copyright.htm>Copyright
© 1995-2003 ScienceDaily Magazine
- http://www.sciencedaily.com/releases/2003/11/031119080609.htm
|
