There are two sides to everything, and that is certainly the case for our Moon. The problem is that we only see one side of the Moon all the time. That's because the Moon is tidally locked to Earth due to gravitational attraction. Think of it this way: it takes the Moon the same amount of time to rotate once as it does to orbit the Earth once.
But the side of the Moon facing us is not the most dense side of the Moon. Moon's center of gravity is slightly skewed toward the far side. That side, the part we never see, is heavily cratered and more "lumpy". The side that faces us is flater and has more lava beds, those dark grey "mare" that we see.
The two sides of the Moon are very different. The side that faces Earth (left) is smoother and flatter than the side facing away from Earth (right). Courtesy of NASA.
On December 31 and January 1, two unmanned spacecraft entered orbit around the Moon to measure it's gravity. This is the GRAIL mission, an acronym that means "Gravity Recovery and Interior Laboratory". Read more about the mission HERE. The spacecrafts will eventually orbit the Moon close to each other and close to the lunar surface. By measuring the variation in distance between the spacecraft and the lunar surface, NASA scientists will be able to better map the Moon's interior. The GRAIL measurements will determine tiny changes in the Moon's gravitational force on the spacecraft.
Artist rendering of one of the GRAIL spacecraft in orbit around the Moon. Courtesy of NASA.
Does the Moon have a solid core and if so, how large is it? There is a theory that the Moon was formed from a collision between Earth and a Mars-sized planet about 4.5 billion years ago. This collision ejected material that eventually gathered back together to form the Moon. That Mars-sized planet was known as Theia. You can read more about the Theia Hypothesis HERE.
Recently a theory has come forth to explain the thicker far side of the Moon. Known as "the big splat", the theory stats that a smaller object orbiting in the same plane as the Moon, caught up to the Moon in it's orbit and slowly collided with it. This slow collision caused one side of the Moon to accrete more mass.
Who knew the beautiful Moon, bella luna, that graces our night sky, could be so interesting? The GRAIL mission will discover more fascinating things about our lunar companion.