Radar shows no evidence of thick ice at lunar polesNov 14, 2003( taken from SpaceFlightNow ) |
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 NationalScience Foundation's (NSF) Arecibo Observatory, Puerto Rico, theresearch group sent signals deeper into the lunar polar surface --more than five meters (about 5.5 yards) -- than ever before at thisspatial resolution. "If there is ice at the poles, the only way leftto test it is to go there directly and melt a small volume around thedust and look for water with a mass spectrometer," says BruceCampbell of the Center for Earth and Planetary Studies at theSmithsonian Institution.
  Images of the moon's poles obtained with the Arecibo Observatory's 70-centimeter wavelength radar system. Top, the lunar south pole, showing the craters Shoemaker and Faustini. Bottom, the lunar north pole, showing crater Hermite and several small craters within the large crater Peary. Credit: Bruce Campbell/Arecibo Observatory |
Suggestions of lunar ice first came in 1996 when radio data from theClementine spacecraft gave some indications of the presence of ice onthe wall of a crater at the moon's south pole. Then, neutronspectrometer data from the Lunar Prospector spacecraft, launched in1998, indicated the presence of hydrogen, and by inference, water, ata depth of about a meter at the lunar poles. But radar probes by the12-cm-wavelength radar at Arecibo showed no evidence of thick ice atdepths of up to a meter. "Lunar Prospector had found significantconcentrations of hydrogen at the lunar poles equivalent to water iceat concentrations of a few percent of the lunar soil," says DonaldCampbell. "There have been suggestions that it may be in the form ofthick deposits of ice at some depth, but this new data from Arecibomakes that unlikely."
Says Bruce Campbell, "There are no places that we have looked at withany of these wavelengths where you see that kind of signature."
The Nature paper notes that if ice does exist at the lunar poles itwould be considerably different from "the thick, coherent layers ofice observed in shadowed craters on Mercury," found in Arecibo radarimaging. "On Mercury what you see are quite thick deposits on theorder 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," saysBruce Campbell. The difference between Mercury and the moon, theresearchers say, could be due to the lower average rate of cometsstriking the lunar surface, to recent comet impacts on Mercury or toa more rapid loss of ice on the moon.
What makes the lunar poles good cold traps for water is a temperatureof minus 173 degrees Celsius (minus 280 degrees Fahrenheit). The limbof the sun rises only about two degrees above the horizon at thelunar poles so that sunlight never penetrates into deep craters, anda person standing on the crater floor would never see the sun. TheArecibo radar probed the floors of two craters in permanent shadow atthe lunar south pole, Shoemaker and Faustini, and, at the north pole,the floors of Hermite and several small craters within the largecrater Peary. In contrast, Clementine focused on the sloping walls ofShackleton crater, whose floor can't be "seen" from Earth. "There isa debate on how to interpret data from a rough, tilted surface," saysBruce Campbell.
The Arecibo radar probe is a particularly good detector of thick icebecause it takes advantage of a phenomenon known as "coherentbackscatter." Radar waves can travel long distances without beingabsorbed in ice at temperatures well below freezing. Reflections fromirregularities inside the ice produce a very strong radar echo. Incontrast, lunar soil is much more absorptive and does not give asstrong a radar echo.
