Solar System: The Moon - Our Nearest Neighbor

Named Luna by the ancient Romans, the Moon is our planet's only natural satellite, and the only place other than Earth where you can find human footprints.

Today, many scientists believe the Moon was formed about 4.5 billion years ago when an object the size of Mars (about 2,100 miles, or 3,400 km in diameter) crashed into the young Earth. The collision smashed apart the object, and broke off a molten glob of our planet. Material from both created a debris cloud that orbited the Earth until it eventually came together, or coalesced, to form the Moon.

The Moon's crust cooled and hardened over time, and meteoroids that smashed into its surface created many craters. The largest impacts cracked through the crust and allowed molten lava to fill up the craters and create dark regions we can see from Earth. People once believed those dark areas were filled with water, and so they called them by the Latin word for sea, "mare." The last of these mare were created over two billion years ago, before the interior of the Moon cooled and hardened. The brighter regions we can see from Earth are called the lunar highlands, and from the rocks the Apollo astronauts brought back we know these areas are older than the dark mare.

Why does the Moon have so many more craters than the Earth? Because the Moon is not geologically active like the Earth, which still has erupting volcanoes and moving tectonic plates to change its surface. Also, the Moon does not have an atmosphere. Here on Earth, wind and watererode the planet's surface, and the atmosphere protects us from incoming meteoroids, which often burn up and appear as meteors before hitting the ground.