Meteoroids that strike Earth's atmosphere disintegrate in the rarefied air above our stratosphere. 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. You can see the little meteoroid craters for yourself. Just as meteoroids constantly bombard the Moon so do cosmic rays, and they leave their fingerprints on Moon rocks, too.
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's an interesting twist on the conspiracy theory that TV producers might consider for their next episode of the Moon Hoax. And these aren't all NASA employees, either.