Two new studies fail to find any sign of previously reported ice deposits cached in the deep chill of the moon's shadows. Much of NASA's planned scientific exploration is geared to the search for lunar ice, which astronauts could drink and convert to rocket fuel, but now it appears that nothing short of a dedicated rover mission--as yet unplanned--can settle the question.
Radar signals bounced off the moon in 1996 by the orbiting Clementine spacecraft hinted at massive ice deposits in craters near the moon's south pole, parts of which never see the sun. Presumably, water delivered by impacting comets over the eons could have made its way to these 80 degree kelvin (80K) cold traps and been preserved as ice just beneath the surface of the loose soil.
These ice signatures seemed to be confirmed by planetary scientist Donald Campbell of Cornell University, who--with colleagues--used two giant Earth-based radio dishes to bounce radar signals off the moon. But thanks to the 20-meter resolution of the new combination of instruments, the researchers could see that the "ice" signals were coming from all the wrong places. Instead of being in the permanently shaded wall of Shackleton crater, for example, they showed up in well-lit Schomberger crater and many smaller young craters, the team reports tomorrow in Nature. Such areas are roughened by crater ejecta and slumping rock. "Right now, the explanation for the [radar signals] has something to do with reflection between rocks and boulders rather than ice at the poles," says planetary scientist Bruce Hapke of the University of Pittsburgh in Pennsylvania.
Radar can't rule out a light frost in lunar soils, but another analysis, presented at the annual meeting of the Division for Planetary Sciences last week in Pasadena, California, raises questions about even a trace of lunar ice. Planetary scientist David Paige of the University of California, Los Angeles, and colleagues performed the first complete calculations of lunar temperatures, starting with the shadowing by topography and including heating by reflected sunlight and by adjacent sun-warmed rock.
There are indeed crater-hosted cold traps near the poles as cold as 50K, Paige and his colleagues found. But, again, the signs of water--in this case data from the orbiting Lunar Prospector--mostly do not come where the calculated cold spots are. "We can probably say with some confidence that not all cold traps are filled with ice," Paige says.
As new work weakens the case for lunar ice, NASA is mounting an ice-oriented first stage of scientific exploration meant to pave the way for humans' return to the moon. Most of the instruments on the Lunar Reconnaissance Orbiter (LRO) to be launched in October 2008 can measure properties relevant to lunar ice. And the piggyback experiment to LRO--a crash landing into a shadowed crater intended to kick any ice into view--is all about water. Not everyone is optimistic. "I don't think you can" prove ice is there by remote means, says planetary scientist William Feldman of the Planetary Science Institute in Tucson, Arizona, who led the Lunar Prospector investigation. "You have to go there in a rover. That's hard, especially if it's 80K."