There was a time when Earth had no Moon. About 4.5 billion years ago, when our ancient Solar System was still forming, the dark night sky above our primordial planet was moonless. At this time, the Earth was about 60 percent formed, although it did have a differentiated crust, mantle, and core. This was a very chaotic and violent era in our Solar System's past, with planets first forming out of blobs of primordial dust, gas, and rock. During this era, frequently likened to a "cosmic shooting gallery", collisions between the still-forming planets were commonplace. Orbits were not as orderly as they are now.
The ring around the Earth eventually began to condense into blobs that then proceeded to merge and create a large and brightly glowing sphere--our primordial Moon. Our Moon would have appeared to be ten times larger than it does today in Earth's ancient sky--if anyone had been around to see it.
Ganymede is larger than Mercury, which is the innermost--and smallest--major planet in our Solar System. The surface area of Ganymede is more than half that of the land area of Earth, and it provides scientists with a wealth of data concerning a great variety of surface features.