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Two Missions Go in Search of A Watery Lunar Bonanza

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Science  15 May 2009:
Vol. 324, Issue 5929, pp. 878-879
DOI: 10.1126/science.324_878

Frustrated by long-controversial hints of water ice on the moon, researchers-turned-miners are going to blast for the mother lode in hopes that astronauts can use water to fuel a permanent moon base.

Termination.

Both the Shepherding Spacecraft (foreground) and Atlas upper stage will hit the moon.

CREDIT: NASA/NORTHROP GRUMMAN

Almost half a century ago, theoreticians began arguing that the moon—despite blistering midday temperatures—harbors eonsold permanent ice. Orbiting spacecraft eventually found hints of ice near the lunar poles, but the remote observations weren't convincing. Now, scientists are going for broke in their search for lunar water that could fuel and water a long-duration presence on the moon.

Early next month, NASA plans to launch an Atlas 5 rocket to the moon. It will carry a rather conventional spacecraft, Lunar Reconnaissance Orbiter (LRO), designed primarily to peer down in search of safe landing sites for future astronauts. But piggybacking on the Atlas launch will be the Lunar CRater Observation and Sensing Satellite (LCROSS) mission. Actually, most of LCROSS is the Atlas, as the one mission involves two craft bound for the moon. The Shepherding Spacecraft bearing LCROSS's brains, eyes, and maneuvering jets will send the spent Atlas upper stage to a 7200-kilometer-per-hour impact on the moon this fall. Minutes later, the shepherd will crash as well. The terminal encounters are intended to blast lunar water—if any—high above the surface for all the world to see.

No one is sure that the water is there, where exactly it would be, or how well the $80 million LCROSS will excavate it, but scientists are looking forward to the big splat. “I think it's highly likely that there's ice,” says lunar scientist Paul Spudis of the Lunar and Planetary Institute (LPI) in Houston, Texas. “If there is no ice there, I don't really care. But I want to know.” That urge to know—and the lure of a resource easily convertible to a high-energy fuel of oxygen and hydrogen—have driven the decades-long and often exasperating search for lunar ice.

Hints but no pay dirt

Apollo moon rocks are drier than bone dry, yet ice on the moon makes abundant sense. Icy comets and water-rich asteroids have been bombarding the moon since its formation, Spudis notes, and there are places on the moon where the explosively delivered water would be stable indefinitely. Thanks to the moon's tiny tilt on its axis, its polar regions barely lean toward the sun in “summer.” As a result, the kilometers-high walls of some near-pole impact craters cast eternal shadows across adjacent crater floors. Although the lunar surface can reach 120°C in sunny spots, temperatures in permanent shadow hover at about 50 degrees above absolute zero by most calculations, cold enough to freeze nitrogen and to lock up water ice forever.

Before finding signs of polar ice on the moon, astronomers stumbled across them on hellishly hot Mercury. Bouncing radar signals off the innermost planet to image its rocky geology in the early 1990s, radar astronomers received reflections from permanently shadowed craters near the poles. The reflections behaved electromagnetically as if they had bounced around within thick ice layers on the planet.

Inspired by the apparent discovery, in 1994 planetary physicist Stewart Nozette of LPI and colleagues jury-rigged a last-minute radar experiment that bounced signals from the Clementine lunar orbiter off the lunar surface. On the radar's one pass over the moon's south pole, the reflected signal was “suggestive of ” ice, the Clementine team reported, although other planetary radar specialists remained doubtful.

Follow-up using ground-based radars, principally by planetary scientist Donald Campbell of Cornell University and his colleagues, failed to support the Clementine observations, Campbell says. Some parts of the moon did return the distinctive reflections, but they come from rough terrain, Campbell says, not permanently shadowed areas. “I'm skeptical about significant water deposits at the lunar poles,” he says, although he adds that there could be grains of water ice too small for radar to pick up.

The next moon mission after Clementine could look for just such grains. In 1998, the neutron spectrometer aboard the orbiting Lunar Prospector spacecraft gauged the energy of neutrons that cosmic rays create on hitting the moon's surface. Such neutrons slow dramatically if they collide with hydrogen atoms in the upper meter of lunar soil before flying off to the spacecraft. By measuring the proportion of fast neutrons to slow ones, Lunar Prospector proved to everyone's satisfaction that the moon's polar regions are enriched with hydrogen—possibly from traces of ice mixed in with the soil.

Lunar Prospector's principal investigator, Alan Binder of the Lunar Research Institute in Tucson, Arizona, immediately hailed the result as proof of water in lunar polar regions at an abundance of about 1 weight percent, enough to mine (Science, 13 March 1998, p. 1628). Subsequent studies have tended to narrow the hydrogen signal to permanently shadowed regions. But “that doesn't mean it's water,” says the instrument's principal investigator, William Feldman of the Planetary Science Institute in Tucson. The hydrogen could have been beamed in on the solar wind, for example.

Inky targets.

LCROSS will target permanently shadowed crater floors near the pole (center).

CREDIT: NAVAL RESEARCH LABORATORY/CLEMENTINE SCIENCE TEAM

All in all, a majority of planetary scientists remain guardedly hopeful. “There's a good probability there's water there,” says Dana Hurley of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, who has modeled the preservation of cometary water on the moon. “The issue is that none of the data is conclusive.”

Fire in the hole

When space on board LRO's launch vehicle opened up, NASA selected LCROSS as a means of conclusively testing the icy-moon hypothesis, or, as LCROSS principal investigator Anthony Colaprete of NASA's Ames Research Center in Mountain View, California, puts it, as a way to finally “reach out and touch the lunar hydrogen.” It will be one bang-up experiment. Separating from LRO, the still-coupled upper stage and Shepherding Spacecraft will swing by the moon and enter two long, looping orbits around Earth before separating just before impact.

The 10-meter-long, 2-ton upper stage will lead the way, crashing at a steep angle at 7200 kilometers per hour into the likeliest permanently shadowed region available. A flash, an upward jet of debris, and excavation of a 3-meter-deep, 20-meter-wide crater will follow. The instrument-laden shepherd and LRO, as well as Earth-based telescopes, will probe the rising debris plume for signs of ice, water vapor, hydroxyl from water, and hydrated minerals. Then the trailing 700-kilogram shepherd will fly through the plume—sending back data all the while—before blasting its own, smaller crater near the first.

At least, that's the plan. Whether the basic physics of impact probing will cooperate remains to be seen. “It's a very unproven and highly unpredictable science, impact cratering,” Colaprete told an audience at the Lunar and Planetary Science Conference (LPSC) in March. Erik Asphaug agrees. Asphaug, an impact modeler at the University of California (UC), Santa Cruz, calls LCROSS “the most challenging impact modeling I've ever done.” If too little of the right stuff rises into view above the crater rim, observations will be compromised or impossible. But calculating the depth of excavation and the amount, speed, and direction of ejecta is fraught with uncertainty, Asphaug notes.

The uncertainties start with the LCROSS impacter. “By planetary standards, it's pretty slow,” says Asphaug. Making assumptions that work in simulations of faster comet and asteroid impacts may give a misleading idea of what to expect in slower impacts. And the upper stage is a far cry from models' usual solid spheres. “A good model of it would be a soda can,” says Asphaug, a hollow shape that's tough to model. In laboratory experiments reported at LPSC by impact specialist Peter Schultz of Brown University, hollow projectiles fired into targets similar to lunar soil splash out high-speed ejecta at lower angles than solid projectiles do. As a result, models may underestimate the chances that ejecta will hit the crater rim instead of rising into view, Schultz says.

Then there's the target, the dirt and rubble of the upper few meters of the moon. As much empty space as rock, this “regolith” will be highly compressible, another challenge for modeling. All things considered, “it's going to be interesting,” Schultz says.

Even if there's ice on the moon and LCROSS kicks up plenty of debris high into the sky, it could still miss striking it rich. Clementine radar and Lunar Prospector neutrons painted broad-brush pictures of where ice might be, including sunlit areas surely too hot to harbor ice. Planetary scientist Richard Elphic of Ames Research Center and colleagues have recently sharpened the Lunar Prospector picture by discarding the sunlit areas and confining the detected hydrogen to nearby, permanently shadowed areas as mapped recently by Japan's Kaguya orbiter. Assuming the hydrogen is bound up in water, the analysis boosts its abundance well above 1% in some craters, Elphic says. “But some permanently shadowed features still do not appear to have any hydrogen.”

LCROSS will avoid apparently dry places, of course, but the absence of hydrogen in locations where simple theory would call for it suggests to some that any lunar ice could be patchy. “I didn't like LCROSS from the get-go,” says Spudis. “It is highly possible that it will miss a deposit of ice that is there.”

That's where the $550 million LRO comes in. Although its primary mission is to scout out safe landing sites, four of its seven instruments are dominantly or entirely devoted to the search for water ice. It carries another neutron detector to map hydrogen at higher spatial resolution. Radar will get the best view yet inside permanently shadowed craters. (A similar instrument is already flying on board the Indian Chandrayaan-1 orbiter.) A laser altimeter will map topography. And a radiometer will gauge actual temperatures in permanent shadow.

“We have hope, but that's not the same as data,” says David Paige of UC Los Angeles, the principal investigator of LRO's radiometer. “Is the LCROSS approach going to work? We don't know. Even LCROSS, LRO, and the international effort combined may in fact not be enough to solve this problem. There's no guarantee the moon will cooperate.” And no country has a mission in the works to land in eternal darkness to force the issue.

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