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Clementine's lunar gold.

On December 3rd a scientific press conference was held in an unlikely setting - a Pentagon briefing room - to announce something equally unlikely. At issue was the Clementine spacecraft's 1994 study of the Moon. "We think we have found ice," intoned Paul D. Spudis (Lunar and Planetary Institute). "We are not positive, but we see signals consistent with ice and we think it's there."

"Lunar ice" might sound like an oxymoron, yet in 1961 three Caltech researchers offered plausible arguments for its existence. The Sun never deviates more than 1.6 [degrees] from the Moon's equatorial plane, so Kenneth Watson, Bruce C. Murray, and Harrison Brown theorized that some crater floors near the lunar poles might lie in constant shadow. Hovering near 40 [degrees] to 50 [degrees] Kelvin, these "cold traps" keep ice so solidly frozen that almost none of it can escape to space.

All the Caltech trio needed was a source for the water, like an occasional comet crash. Some of the ephemeral atmosphere created by the comet's vaporized remains would condense onto the poles. Water is by far the most likely denizen of these dim niches, they pointed out, but carbon dioxide and other volatile compounds could be there too.

A few years later Apollo's rocks and soils proved to be dry as dust. But in 1979 James R. Arnold (University of California) revisited the cold-trap idea and found it more viable than ever. Arnold estimated that in two billion years up to 100 billion tons of water could have accumulated in the Moon's polar prisons. But proving it was there would require more than mere photography.


Sharing the Pentagon podium last December was Stewart Nozette, now assigned to the Air Force's Phillips Laboratory. It was Nozette who in 1989 talked the Ballistic Missile Defense Organization into using its Star Wars technology to reconnoiter the Moon and then fly off to "attack" asteroid 1620 Geographos. Conceived on the back of a cocktail napkin, the high-risk, no-frills Clementine craft proved spectacularly successful in lunar orbit (S&T: August 1994, page 20). But en route to Geographos a massive propellant leak scuttled the close-range flyby. Much like its namesake, the miner's daughter of folk legend, the spacecraft ended up "lost and gone forever" in heliocentric orbit.

Clementine was designed for imaging and altimetry, not prospecting for water. But early in its four-month lunar run Nozette hatched another scheme. He proposed using the craft's transmitter to "shine" radio waves into the perpetually darkened floors of craters at the Moon's poles. The reflected energy, if detected on Earth, might settle the debate over whether patches of ice lie hidden there.

A crucial first step was showing that sunlight-free real estate really does exist on the Moon. By chance, the point at 90 [degrees] south lies 200 kilometers inside the rim of a gargantuan, 2,500-km-wide excavation known as South Pole-Aitken Basin, putting it some 4 to 7 km below lunar sea level. In the pole's vicinity Clementine images reveal some 15,000 square kilometers of crater floors and valleys, a Hawaiian Islands' worth of territory, never touched by sunlight. The north pole has much less area in continual shadow.

Clementine's makeshift radar experiment was searching for two telltale indicators of ice. Steven J. Ostro (Jet Propulsion Laboratory), who conducted radar studies of the Galilean satellites 20 years ago, explains that an icy surface becomes especially reflective when the angle between the transmitter and receiver (as seen from the target) nears 0 [degrees]. This peculiar effect arises in part due to a property of light termed coherent backscatter. Two photons that enter the ice in phase, ricocheting along the same complex path once inside but traveling in opposite directions, will combine coherently and amplify the energy reflected toward the receiver. For this reason, radar echoes from icy Europa are 30 times stronger than those from the slightly larger but soil-covered Moon.

Coherent backscatter alone would not raise the "ice is here" flag, because the same physics is at work in the rough lunar regolith and helps explain why the full Moon looks brighter than expected based on geometry alone (S&T: April 1993, page 14). More diagnostic is what happens to a circularly polarized radar beam. The sense of polarization reverses during a mirrorlike bounce off a rocky surface, Ostro says, but when passing through ice the beam emerges with its initial polarization largely intact. Greenland's ice sheet, the icy Galilean satellites, and the polar caps of Mars and Mercury all share this property.

Clementine's first foray into radar astronomy failed in March 1994, but during a second trial in April one radar track swept directly over the south pole. "We lucked out immensely," Nozette says. Not only did the echo prove stronger than expected, but the all-important polarization ratio also showed a modest peak precisely when the shadowed areas were within the radio beam. The echo from the next orbit, which passed about 200 km from the pole, showed no enhancement, nor did a pair of scans over the north pole.

If ice really exists at the Moon's bottom, there's not much of it. As Nozette and his team point out in Science for November 29th, if collected in one spot it would form a slab the size of a football field and 10 to 20 meters high. Don't expect glistening skating rinks tucked between craters, either - more likely are isolated small patches mixed with big helpings of rocky material.

Despite two years of tedious analysis and critical appraisal, the Clementine team realizes other interpretations (like a rough, ice-free surface) are possible. A skeptical Ostro doesn't find the blip in the polarization ratio particularly convincing, and radar veteran Gordon Pettengill (MIT) worries that since the peak appears in just one pass it could be merely a statistical fluke. Others, however, are encouraged because the ratio rises to a maximum exactly where it should - over the polar darkness.

One independent check on Clementine's result is a 1993 radar sounding made from Earth with the giant Arecibo radio dish in Puerto Rico. According to Donald B. Campbell (Cornell), who conducted the study with Nicholas J. S. Stacy, Arecibo's radar system did find high polarization ratios near the south pole in several craters. However, he warns, some of these areas are in sunlight at least part of the time, and he doubts that the observed polarization effects seen by Arecibo are due to ice.

A stable cache of lunar permafrost would be far more valuable than gold to planetary scientists. "For the first time," Spudis explains, "we have a preserved record of the cometary impact rate over time." Probing the polar layers could yield critical insights on how frequently comets have hit the Earth-Moon system, whether that rate changes with time, and what comets are made of.

But there's more underlying Clementine's provocative data than the usual scientific give-and-take. Exploration proponents envision using the ice (and its component hydrogen and oxygen) to sustain a lunar colony or as a source of rocket fuel. One pinnacle overlooking the south pole is bathed in sunlight 85 percent of the time - the perfect spot for an ice-processing plant. "You may be looking at the most valuable piece of real estate in the solar system," ventures Spudis.

Fortunately, all the uncertainty should be short-lived. This September NASA hopes to launch Lunar Prospector, a mission in its Discovery series. Aboard will be a neutron spectrometer able to sense the hydrogen in the ice below. This is considered an almost foolproof test for water down to 0.01 percent by mass, says principal investigator William C. Feldman (Los Alamos National Laboratory). "We'll know one month after we arrive whether any ice is there."
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Title Annotation:ice on the moon
Author:Beatty, J. Kelly
Publication:Sky & Telescope
Date:Feb 1, 1997
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