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THIS WEEK IN
CALIFORNIA WILD

Skywatcher

Sally Stephens

It turns out that we really didn't know as much about the Moon as we thought. Driven to explore space and upstage the Soviets, NASA sent 22 missions to the Moon, culminating in six manned landings. The twelve Apollo astronauts who walked on the lunar surface brought back 850 pounds of rocks. None contained even a trace of water, so scientists concluded water had never existed there. For most people, the much-romanticized Moon became nothing but an airless, dry world, of little interest except as a rest stop on the way to Mars.

Still, NASA and most scientists decided it made more sense to explore other worlds than return to the Moon. In 1994, Clementine, a Department of Defense spacecraft designed to test "Star Wars" sensors, orbited the Moon for two months before going on to rendezvous with an asteroid (a meeting that was canceled due to a loss of fuel). Radar signals bounced off the lunar south pole by Clementine suggested that water ice might be there. Maybe the Moon wasn't so boring after all. But there were other possible explanations for the single observation of water, and most scientists weren't convinced.

Enter Lunar Prospector. For nearly a year now, the first NASA lunar mission in 25 years has been orbiting the Moon, making the first global maps of lunar elements and trying to confirm Clementine's intriguing observation. The mission's first results, published in Science this past September, delighted scientists. David Lawrence, a post-doctoral research associate at Los Alamos National Laboratory (LANL) and member of the Lunar Prospector science team, says, "The Moon is a more complex place than we thought."

Confirming Clementine's observation, Prospector discovered that at least three billion metric tons of water ice lie buried underground at each of the Moon's poles. (All the lunar ice, if gathered in one place, would form a lake 20 miles square and 20 feet deep.) The ice hides in permanently shaded craters whose tall walls keep sunlight out and parts of the crater floors always dark and cold. Scientists think the water is not native to the Moon, but was brought there by comets, which are made of ice and dust. They've bombarded the Moon throughout its history, depositing ice on its surface. But the only places where cometary ice won't evaporate are the polar craters–so cold that once water ice lands there, it stays.

"Those cold spots [are] like a cosmic deep freeze," according to Hubbard, who is also the NASA Mission Manager for Lunar Prospector. "There should be material there that has been collected from comets over the last two billion years." Oddly enough, that may provide scientists with clues about how life began on Earth. "We believe strongly that Earth's water and early organic compounds were probably brought here by comets and meteorites," Hubbard explains. "The same things undoubtedly hit the Moon. So we may have a record of [those] early organic molecules left in our lunar deep freeze."

To be fair, Prospector's instruments didn't actually detect water. What they did find was lots of hydrogen. Prospector carries no cameras; instead it has detectors that look for neutrons and gamma rays coming off the Moon's surface. Such detectors have been used to monitor compliance with nuclear weapons treaties, but, says Lawrence, "This is the first time anyone has used a neutron detector to map the composition of another planet."

On the Moon, neutrons and gamma rays are produced when cosmic rays–fast-moving, energy-packed particles that travel throughout the galaxy–smash into surface rocks. The collision produces neutrons, which bounce around inside the rocks and excite atoms to give off their own neutrons and gamma rays. Some eventually escape the lunar surface and get detected by the spacecraft.

Ironically, it's a lack of detectable neutrons that gives away the presence of water at the poles. There, the neutrons appear to have been absorbed by hydrogen. "The most likely form [for hydrogen], due to what we know of the temperature," says Lawrence, "is in the form of water ice." At temperatures that never rise higher than minus 280¼F, the shadowed craters are cold enough to keep water as ice, but other hydrogen-containing molecules, like methane or ammonia, would evaporate. "Going from hydrogen to water ice is an interpretation," Hubbard concedes, "but we think it's a very plausible one."

"Water" wasn't Prospector's only scientific discovery. By carefully studying subtle shifts in radio signals from the tiny spacecraft, scientists charted local variations in the Moon's gravity. "We've learned that the Moon has more of these so-called mass concentrations, unusual areas of higher gravity, than were anticipated," Hubbard says. These "mascons" pull unevenly on orbiting spacecraft, turning circular orbits into elliptical ones. If not properly accounted for, they could perturb the orbit so much that the spacecraft crashes into the Moon. Says Hubbard, "Nobody [has] ever mapped the Moon's gravity field with this precision."

The gravity experiment also provided clues about the Moon's internal structure. "We now believe there is a small iron core," Hubbard explains, "about 360 miles [in diameter], which is...something nobody knew about before."

In addition, Prospector found a few isolated and unexpected areas on the lunar surface with a strong magnetic field. Unlike the Earth, the Moon has no overall magnetic field, no magnetic north and south poles. But its surface rocks carry some residual magnetism, suggesting that the Moon may have had an earthly magnetic field early in its history. Curiously, the magnetic areas found are on the Moon's dark side, directly opposite Mare Imbrium and Serenitatis, two of the larger impact basins on the near side, which formed when large objects hit the Moon billions of years ago. "We're still not quite sure how these [areas of strong magnetism] came to be," Hubbard cautions.

Mare Imbrium also plays a role in some unusual chemical observations made by Lunar Prospector. "What you see when you map thorium and potassium over the Moon is this huge concentration on the near side," Lawrence says. Thorium and potassium are tracers for a kind of material called KREEP–an acronym made up of the chemical symbols for potassium (K), Rare-Earth Elements, and phosphorus (P)–first identified in Moon rocks. KREEP is thought to have been among the last rock types to solidify on the Moon, sandwiched underground between the lunar crust and mantle until impacts and volcanism lifted it to the surface.

Scientists knew there was an excess of KREEP on the surface where Apollo 14 landed, but didn't know if there were other areas with high levels of kreep as well. Now they know there's only one. The area forms a rim around the Imbrium basin, with the highest concentration south of Mare Imbrium, where Apollo 14 landed. Because thorium is radioactive, or "hot," some scientists have suggested calling this area "The Great Lunar Hot Spot."

Scientists assume the excess KREEP was dug up during the impact that formed Imbrium and thrown out onto the surface around the basin. But if that's the case, you would expect to see high levels of KREEP surrounding other deep impact craters. Prospector found none.

"You would imagine these [KREEP] elements would have been [distributed] all over the Moon," says William Feldman, a Prospector instrument group leader at LANL. "But something happened in the evolution of the Moon...that somehow concentrated them on the near side, toward the Earth." Scientists are still trying to figure out what that "something" was.

As researchers continue to puzzle over the Lunar Prospector results, they hope to flesh out details of how the Moon formed and evolved. The prevailing theory holds that about 4.5 billion years ago, something roughly the size of Mars hit the Earth. "As a result of this collision," explains Feldman, "an enormous amount of the surface material of the Earth was ripped off, and went into orbit. Much of this material fell back on the Earth. The rest swirled around and eventually coagulated and formed the Moon."

The theory is not without its difficulties, however. For example, there appear to be some differences in the bulk composition of the Moon and the Earth. Did the Moon form out of only terrestrial material? Or does it include large amounts from the impactor? Was only surface material from the Earth involved? Or did the collision remove a chunk from deeper inside? Lunar Prospector's chemical composition maps will help shed light on these questions and show how credible the theory is. So far, Hubbard says, "the things we're finding seem to be consistent with [the impact theory]."

"We're getting data which are better by far, by an order of magnitude, than we had anticipated," Binder, the principal investigator and driving force behind Lunar Prospector, says with pride. "It's performed absolutely flawlessly." But he sees the scientific results as icing on the cake. "Prospector, to me, is not [about] science. It's a demonstration of how to do things properly and inexpensively.

"I came to the conclusion a number of years ago that while you need complex spacecraft for some missions, there's a lot you can do with simple spacecraft," he explains. For several years he tried to sell his idea for a small lunar mission to private investors, hoping to circumvent the nasa bureaucracy and big-mission mindset. "People believed in what we were going to do," he recalls. "The Soviet Union was going to launch us for free...[But] it was very difficult to raise ten million dollars."

In the meantime, NASA had decided to shift its focus from big, multi-disciplinary missions to smaller, cheaper, and less ambitious ones through its Discovery program. In 1994, Binder joined forces with Lockheed Martin Corporation and the NASA Ames Research Center to compete against more than 20 other proposals for a shot as a Discovery mission. In February 1995, when NASA made its decision, Binder's mission was ranked at the top.

Lunar Prospector cost a total of just $63 million (a third of the budget for the movie "Titanic," as everyone involved in the project is quick to point out), and was built, tested, and ready for launch in 22 months. The spacecraft itself is shaped like a drum, roughly four-and-a-quarter feet high and four-and-a-half feet in diameter. Three masts extend eight feet out from the drum to hold its detectors. When fully fueled, it weighs a "mere" 650 pounds, an interplanetary lightweight.

"This is a very simple, stupid spacecraft," Binder says fondly. Prospector has no sophisticated onboard computer. It is controlled by commands radioed up from mission control, a small, enclosed room on the top floor of an otherwise nondescript office building at NASA Ames. In addition, the spacecraft has no back-up systems. Binder felt they weren't needed because everything put in the spacecraft was off-the-shelf, flight-proven hardware. "I went to various vendors and said, 'Those tanks you made before, make them for me. That transponder you made before, make it for me.'" Development costs for the mission were practically negligible. "I made the smallest, tiniest, easiest-to-run spacecraft possible," he adds with a smile.

While still watching over Lunar Prospector, Binder has already set his sights on the next step in lunar exploration–a privately financed rover that will roll into the permanently shadowed craters at the Moon's south pole, dig into the ground, and prove once and for all if there's any water ice there. "We cannot really talk about a lunar base without knowing for sure if that's water," he says.

And Binder is serious about a commercial lunar base. He is convinced he can build one for five to eight billion dollars and is seeking investors. "Just like Prospector, we're going to use what's available," he says. For example, using existing Spacehab modules–portable labs for experiments that fit inside the space shuttle's cargo bay–would keep development costs down.

Binder and his colleagues envision renting space on the base to NASA, the European Space Agency, and others for research. And there's always space tourism. "If one of our modules is the Lunar Hilton," he adds, "people will go. I guarantee it."

But Binder doesn't stop there. "The idea is not just to build a lunar base," he says. "What we are doing is starting a lunar colony. The idea is to start utilizing lunar resources, first the water...and then...the other resources to get metals and things and start manufacturing."

Abundant water ice on the Moon would cut the cost of any lunar base or colony. Hydrogen in the water can provide cheap fuel, and oxygen the life support, so these would not have to be sent from Earth. This amount of water, transformed into rocket fuel, equals what it takes to launch millions of shuttle missions from Cape Canaveral. "The three billion metric tons [of water] is enough to support a modest colony for a thousand years," Feldman agrees. But, he cautions, "It's taken billions of years to put it there. You use it up and that's the end of it."

Lunar Prospector made its water ice observations from an orbit 63 miles above the Moon's surface. This December, mission controllers will drop its orbit to within six miles of the surface, to increase the surface resolution of its instruments. It should continue to gather data from the lower orbit for another six months or so. Then, when either fuel or money (or both) runs out, the mission will end as the spacecraft crashes into the Moon.

Should a privately financed lunar base actually be built in the next ten or twenty years, Lunar Prospector will have helped open the doors for it. "NASA calls this 're-discovering the Moon'," Binder says. "We're starting over, not from zero, but this is the beginning again of lunar exploration."

Winter 1999

Vol. 52:1