More specifically, they looked like rocks found in the continental crust here on earth. The Earth's crust is divided up into solid tectonic plates that move around on a melty, squishy mantle deeper inside the planet. Some of these plates are thinner (approximately 4-6 miles thick), darker, and heavier. These plates tend to make up the oceanic crust, or all the rocks underlying the oceans. Then there are the plates that are thicker (approximately 15-40 miles thick) and generally more light-colored, and also made of more light-weight rocks. Scientists thought for a long time that Earth was the only planet with a continental crust, because typically it takes a very long time on a very active planet for the lighter rocks to rise to the surface and form into the chunks that make up the continental crust.