IN DEPTH: No Ice at Lunar Pole

Oct. 20, 2006 — Donald Campbell has some advice for colleagues debating where to send a robotic probe to look for water ice on the moon: don't expect much at the south pole.

High-resolution radar images taken by ground-based observatories of Shackleton crater, a 15-mile-wide pit on the moon's south pole, show no sign of thick water-ice deposits, says Campbell, a planetary scientist with Cornell University in New York.

That's not to say scientists planning to smash a pair of spacecraft into the moon's surface won't find small grains of ice mixed in the dust, added Campbell, lead author of a research paper published in this week's issue of the journal Nature.

Intrigued by long-standing theories that the moon's caps harbor ice deposits, Campbell and colleagues used the Arecibo Observatory in Puerto Rico and a second radar telescope in Green Bank, W. Va., to probe Shackleton crater in hopes of finding ice sheets similar to what has been found at Mercury's poles.

"It had been a reasonable suggestion since the 1960s that there were deposits of water ice," Campbell said, "but we didn't see anything that looks like that."

Campbell and his colleagues found the cause of radar echoes noted in previous studies which had led to speculation of lunar ice deposits. Instead of bouncing off ice, the radar reflected off small rocks that had been ejected during an asteroid or comet impact and the craggy walls of the crater's inner surface.

The findings were published as scientists wrapped up a meeting in California to discuss potential impact sites for a two-part robotic lunar probe. The spacecraft is scheduled to be launched along with NASA's Lunar Reconnaissance Orbiter in two years.

The Lunar Crater Observation and Sensing Satellite, known as LCROSS, consists of a pair of satellites designed to smash into the permanently shadowed floor of a lunar crater. The impacts are expected to send up a plume of material which will be analyzed by space- and ground-based observatories for signs of water vapor and other elements.

The existence of water on the moon is far more than scientific curiosity. NASA plans to return astronauts to the moon by 2020 and eventually build a habitat as a testbed for future manned missions to Mars.

Tapping lunar resources for life support systems, rocket fuel and other critical needs should not only cut the cost of the moon exploration program, but also the time needed to develop similar technologies for Mars expeditions, said NASA scientist Jennifer Heldmann, with the Ames Research Center in Moffett Field, Calif.

Depending on the type of material hit, the probes are expected to carve out a hole in the crater floor about 16 feet deep and 33 yards in diameter, Heldmann said.

In addition to considering several south pole sites, scientists are looking at sending LCROSS to north pole locations. A list of potential targets is expected to be delivered to project managers by Nov. 22.

A final site selection, however, may not be determined until after the probe's companion satellite, the Lunar Reconnaissance Orbiter, returns its initial mapping data in late 2008. The LCROSS impacts are targeted for late January 2009.

Scientists also hope to use radar data collected by India's first lunar probe, Chandrayaan-1, which is scheduled for launch in February 2008, eight months before the NASA's satellites fly.

"I suspect it doesn't make a great deal of difference whether it's the north or south pole," said Campbell, who attended this week's planning meeting in California.

A previous mission found roughly the same concentration of hydrogen at the poles, though scientists do not know if the hydrogen is bound with oxygen as water or exists in some other form.

The south pole has more shadowed areas than the north, as well as bigger craters, Campbell added. "It may be easier from a practical standpoint."

Campbell said he hopes his research does not take the shine off NASA's hunt for lunar ice.

"There is always the possibility that concentrated deposits exist in a few of the shadowed locations not visible to radars on Earth, " he said. "But any current planning for landers or bases at the lunar poles should not count on this."