NASA / JPL-CALTECH / MIT / GSFC

The GRAIL mission made this gravity map of the side of the Moon that faces Earth. Red shows areas with higher local gravity, and blue show areas of lower local gravity

Go outside when the Moon is full. Look up and imagine you see two Moons, instead of one. Some scientists think that long ago Earth may have had two Moons.

When Earth was very, very young, scientists are pretty sure it did not have a moon at all. Then along came a huge object—perhaps the size of Mars. This object slammed into Earth and knocked a huge amount of rocks, rubble, and dust into space. Earth's gravity kept this stuff from traveling far, though. Instead, the rubble went into orbit around Earth. Before long, the rubble started to glom together into larger and larger chunks.

So, here's the question. Did the pieces just naturally all come together into one big piece, which is now our Moon? Or was there more than one Moon for a while?

The reason we might wonder is that the Moon has some weird features that scientists can't yet explain. The side of the Moon that faces Earth (always the same side) is low and flat. The other side that faces away from Earth is high and mountainous, with a much thicker crust. Some scientists think two different moons may have formed at first.

They think maybe the smaller moon began to drift slowly toward the larger moon. The two moons finally collided in a low-speed "splat." The smaller moon spread out sort of like a pancake on the larger moon's surface and became the highlands and thicker crust now on the far side of our Moon.

A NASA mission called GRAIL sent two spacecraft to make a gravity map of the Moon. A gravity map shows which parts of an object are denser than other parts. Denser materials have stronger gravity, and that is what these spacecraft measured. So far, the data from the GRAIL mission do not support the two-moon idea. However, the GRAIL data did reveal that the Moon's crust was battered by meteor impacts much more severely than previously thought.

Science is very good at discovering the history and nature of the universe. In only a few thousand years of recorded history, we humans have found ways to investigate how our Moon formed more than 4.5 billion years ago.

Visit The Space Place website to find out more about our Moon and do a fun Moon phases activity using Oreos. Go to http://spaceplace.nasa.gov/oreo-moon.

This article was written by Diane K. Fisher and provided through the courtesy of the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.