If space scientists hoping to send manned missions to distant planets were given one wish, many would ask for a convenient source of water outside the tug of Earth's gravity. And on March 5 they may have gotten it. Last Thursday, jubilant National Aeronautics and Space Administration researchers announced that the Lunar Prospector spacecraft had confirmed earlier indications that ice exists in potentially extractable quantities in the dark, cold regions at both of the moon's poles.
Image: Lunar Prospector
"We are certain we have found water," Alan B. Binder, Lunar Prospector principal investigator from the Lunar Research Institute told a packed press conference at NASA's Ames Research Center. Based on the present preliminary data, the researchers estimate that anywhere from 11 to 330 million tons of frozen water are mixed in the lunar soil, or regolith. The water appears to be distributed over 3,600 to 18,000 square miles across the northern pole, and an additional 1,800 to 7,200 square miles in the southern polar region. Moreover, the concentration of water near the South pole appears to be twice as great in the northern region.
If the water can be mined, it will become an enabling factor for establishing permanent or semi-permanent moon bases and, possibly, for setting up a staging area for future manned missions to Mars. The water could be used for domestic purposes, but also broken into its component hydrogen and oxygen to provide breathable air. The two elements could also be recombined as fuel for spacecraft or machinery on the base itself. "This is a tremendous resource for further exploration," said Binder. Indeed, the NASA scientists estimate that the supply of water would reduce the cost of a mission to Mars by about 50 percent.
The first evidence of water on the moon was sent back by a joint Defense Department-NASA spacecraft called Clementine in 1994. Clementine used radar to detect the signature of water in the permanently-shadowed South polar region, which scientists thought was the most likely place to look for water. Although tantalizing, these data were not conclusive.
Lunar Prospector, which was launched on January 6 as part of NASA's low budget Discovery program to study the composition of the moon, has apparently put doubts to rest in the first weeks of its planned two-year mission. In the following months, Prospector will continue to send back more detailed data, eventually dropping from its present orbit 62 miles above the moon's surface to a mere six miles for an even closer look.
Scientists think that impacting comets delivered any water that now exists on the moon. "The moon was born hot," said Binder, "all the atmosphere and water boiled away." Even the water in a comet striking the lighted regions of the moon would quickly evaporate in the daytime temperatures, which exceed 250 Fahrenheit. The only places in which water molecules could remain is in the shade of craters and perpetually dark areas of the moon, where temperatures stay at a chilling minus 280 Fahrenheit.
The new findings, made by Prospector's neutron spectrometer, confirm that water is trapped in the frigid regions of the moon. Neutrons from the solar wind continually bombard the moon. But if a neutron collides with an atom of hydrogen, it looses speed.
The spectrometer on Prospector detects regions on the ground that reflect increased numbers of these cool, or so-called thermal neutrons, compared with their higher energy counterparts, known as epithermal neutrons. According to Binder, graphs of data ratios from the neutron spectrometer "reveal distinctive 3.4 percent and 2.2 percent dips in the relevant curves over the northern and southern polar regions, respectively--this is the kind of data 'signature' one would expect to find if water ice is present."
The next question is just how much. Were the ice present in the form of a solid sheet or chunks, the dips in the numbers of epithermal neutrons would be much greater. The present data suggest that the ice is in the form of small crystals, comprising about 0.3 percent to 1 percent of the moon's rocky soil. At such concentrations, a cubic yard of soil would contain as much as five gallons of water.
The key issue is how deeply the water extends into the lunar regolith. The present estimate is based on the depth to which the neutron spectrometer's signal can penetrate--about a foot and a half. But James Arnold of the University of California at San Diego estimates the amount of lunar regolith that could have been "gardened" by all impacts in the past two billion years extends to a depth of about 6.5 feet. If he is correct, Lunar Prospector's estimate of water ice would have to be increased by a factor of up to four, to the range of 44 million to 1.3 billion tons.
Image: Lunar Prospector
Still, the amount of water in current estimates represents a potential bonanza. "Our data are consistent with the presence of water ice across a significant number of craters," said William C. Feldman, co-investigator and spectrometer specialist at the Los Alamos National Laboratory. "There is enough there to support a modest amount of colonization for centuries."
According to NASA, an area the size of a football field would yield enough water to provide for the drinking, food preparation, bathing and washing needs of a crew of six persons; generate 100 megawatts of electrical power for a year; or produce enough propellant to transport two crews of four people each from the Earth to the moon.
Taking that amount of water along to the moon would be a costly proposition. Currently, it costs about $10,000 to put one pound of material into orbit. NASA is conducting research with the goal of reducing that figure by a factor of 10, to only $1,000 per pound, but even at that rate the cost of providing water to a permanent base--even with recycling--would be trillions of dollars.
All the same, some scientists think that it may still be less expensive to ship water into space than to extract it from the lunar soil. But the NASA scientists strongly disagree, pointing out that gold is economically mined at the rate of a few grains per ton. According to Feldman, the water could be extracted simply by heating the soil in a closed chamber, much like a still; energy could be provided by solar panels located above the craters. "It'll be like making moonshine," he quipped. "We have paved the way for future missions."