First, scientists are pretty sure the Earth's core is about 80 percent iron. How they know that is a series of educated guesses.For starters, they can be reasonably sure of the planet's mass based on its gravitational pull. The material on the surface isn't dense enough to match up with that mass, so the rest of the Earth has to be much denser. Iron, meanwhile, is one of the most prevalent elements in the universe, but it isn't all that evident in the Earth's crust. Since scientists would expect more iron to be in our planet and it's a fairly dense element, that leads to the conclusion that the Earth's core is mostly iron. They theorize that it was pulled to the core over millions of years. By examining different seismic waves, they know that the inner part of the core is solid and the out