The Apollo astronauts used seismometers during their visits to the moon and discovered that the gray orb isn't a totally dead place after all, geologically speaking. Small moonquakes, originating several kilometers below the surface, are thought to be caused by the gravitational pull of Earth. Sometimes tiny fractures appear at the surface, and gas escapes.

Scientists think the moon probably has a core that is hot and perhaps partially molten, as is the Earth's core. But data from NASA's Lunar Prospector spacecraft in 1999 showed that the moon's core is small—probably between two percent and four percent of its mass. This is tiny compared with Earth, where the iron core makes up about 30 percent of the planet's mass.