New observations reported this week in the journal Nature have cast doubt on the theory that thick deposits of ground ice lie conveniently close to the surface in permanently shadowed crater floors at the lunar poles.
Researcher Bruce Campbell of the Center for Earth and Planetary Studies at the Smithsonian Institution and his collaborators used the Arecibo radio observatory in Puerto Rico to delve into the depths of the lunar poles. They aimed 70-cm-wavelength radar signals at the Moon. Thick deposits of ice up to five meters below the surface would have reflected bright signals to the observatory. But there was nothing bright to be seen; nothing in the crater floors looked any different from the surrounding highlands.
These new results appear to contradict conclusions drawn from Lunar Prospector's Neutron Spectrometer. Researchers reported in the journal Science in 1998 that a lack of neutrons flowing from areas in permanent shadow near the poles indicated the presence of large amounts of hydrogen in the lunar soil. They suggested that the hydrogen they detected was most likely found in the form of water because of its tendency to be confined to permanently shadowed areas. At the end of the mission, engineers deliberately crashed Lunar Prospector into the Moon's south pole in the hopes that water would be spotted in the spray of debris. It wasn't.
Thick ice deposits have, however, been discovered at the poles of Mercury using ground-based radar methods. "Mercury's polar craters are radar-bright in every wavelength from 3 to 12 to 70 centimeters," Campbell said. The Moon had already proved to be different from Mercury, however: "We hadn't seen those bright signatures at 3 or 12 centimeter wavelengths, but there was still a possibility that there was thick ice down more than a meter below the dust and it simply wasn't being detected by the shorter wavelengths. By using the 70-centimeter radar, we know we have probed below the depth to which the Lunar Prospector measurements were sensitive."
Does this mean that there is no ice at the lunar poles? No, says Campbell. "All that we're ruling out is thick slab-like ice deposits more than a meter thick. The 70-centimeter radar is not sensitive to ice distributed as small crystals through the soil. You could certainly have water ice that was responsible for the Lunar Prospector measurement, but that ice is not in the form that it is on Mercury." If the Arecibo team's conclusions are correct, ice at the lunar poles, if it exists, is not the easily accessible resource that the Lunar Prospector results once promised.
The European Space Agency's SMART-1 spacecraft will take a closer look at the poles when it goes into orbit in early 2005.
The Arecibo observations of the lunar surface were not performed merely to check the Lunar Prospector results. Campbell and his collaborators are remapping the near side of the Moon using Arecibo's 70-centimeter radar system as a test of a possible future imaging radar mission to Mars.
"We're proposing a radar system that would look down 5 to 10 meters into the Martian surface," Campbell said. So far, the imaging systems sent to Mars have used visible light to thermal infrared wavelengths, which can penetrate at most a few centimeters into the soil. "But 30 percent of Mars is obscured by a meters-thick dust layer. There's tremendous benefit to be gained by using a radar system that could look below that." His proposal to NASA for the Mars Scout 2007 launch opportunity to do just that was not successful, but he's going to keep trying, and in the meantime, he'll map the Moon. "The Moon is a great testbed for looking at a dry surface where you get a possible penetration of many meters into the soil," Campbell said. "We'll do the Moon, then go to Mars."