Apollo astronauts brought 841 pounds of Moon rock home to Earth.
"Moon rocks are absolutely unique," says Dr. David McKay, Chief Scientist for Planetary Science and Exploration at NASA's Johnson Space Center (JSC). McKay is a member of the group that oversees the Lunar Sample Laboratory Facility at JSC where most of the Moon rocks are stored. "They differ from Earth rocks in many respects," he added.
"For example," explains Dr. Marc Norman, a lunar geologist at the University of Tasmania, "lunar samples have almost no water trapped in their crystal structure, and common substances such as clay minerals that are ubiquitous on Earth are totally absent in Moon rocks."
"We've found particles of fresh glass in Moon rocks that were produced by explosive volcanic activity and by meteorite impacts over 3 billion years ago," added Norman. "The presence of water on Earth rapidly breaks down such volcanic glass in only a few million years. These rocks must have come from the Moon!"
Right: A glass spherule (about 0.6 mm in diameter) produced by a meteorite impact into lunar soil. Features on the surface are glass splashes, welded mineral fragments, and microcraters produced by space weathering processes at the surface of the moon. SEM image by D. S. McKay (NASA Photo S71-48109).
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Fortunately not all of the evidence needs a degree in chemistry or geology to appreciate. An average person holding a Moon rock in his or her hand can plainly see that the specimen came from another world.
"Apollo moon rocks are peppered with tiny craters from meteoroid impacts," explains McKay. This could only happen to rocks from a planet with little or no atmosphere... like the Moon.
Meteoroids are nearly-microscopic specks of comet dust that fly through space at speeds often exceeding 50,000 mph -- ten times faster than a speeding bullet. They pack a considerable punch, but they're also extremely fragile. Meteoroids that strike Earth's atmosphere disintegrate in the rarefied air above our stratosphere. (Every now and then on a dark night you can see one -- they're called meteors.) But the Moon doesn't have an atmosphere to protect it. The tiny space bullets can plow directly into Moon rocks, forming miniature and unmistakable craters.
"There are plenty of museums, including the Smithsonian and others, where members of the public can touch and examine rocks from the Moon," says McKay. "You can see the little meteoroid craters for yourself."
Right: Nick-named "Big Muley," this 11.7 kg Moon rock was the largest returned to Earth by Apollo astronauts.
One side of Big Muley was peppered with meteoroid "zap pits." Below right: A close-up view of 1 mm diameter zap pits shows tiny craters lined with black glass surrounded by a white halo of shocked rock.
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Just as meteoroids constantly bombard the Moon so do cosmic rays, and they leave their fingerprints on Moon rocks, too. "There are isotopes in Moon rocks, isotopes we don't normally find on Earth, that were created by nuclear reactions with the highest-energy cosmic rays," says McKay. Earth is spared from such radiation by our protective atmosphere and magnetosphere.
Even if scientists wanted to make something like a Moon rock by, say, bombarding an Earth rock with high energy atomic nuclei, they couldn't. Earth's most powerful particle accelerators can't energize particles to match the most potent cosmic rays, which are themselves accelerated in supernova blastwaves and in the violent cores of galaxies.
Indeed, says McKay, faking a Moon rock well enough to hoodwink an international army of scientists might be more difficult than the Manhattan Project. "It would be easier to just go to the Moon and get one," he quipped.
And therein lies an original idea: Did NASA go to the Moon to collect props for a staged Moon landing? It's an interesting twist on the conspiracy theory that TV producers might consider for their next episode of the Moon Hoax.
"I have here in my office a 10-foot high stack of scientific books full of papers about the Apollo Moon rocks," added McKay. "Researchers in thousands of labs have examined Apollo Moon samples -- not a single paper challenges their origin! And these aren't all NASA employees, either. We've loaned samples to scientists in dozens of countries [who have no reason to cooperate in any hoax]."
Even Dr. Robert Park, Director of the Washington office of the American Physical Society and a noted critic of NASA's human space flight program, agrees with the space agency on this issue. "The body of physical evidence that humans did walk on the Moon is simply overwhelming."
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