 Eris is a tiny bit smaller than Pluto, but it's a good bit more massive. That means it's denser than Pluto. It has a higher ratio of rock to ice. But like Pluto, it's probably still geologically active. One indication of that is that the surface of Eris is almost pure white. It reflects almost all of the sunlight that strikes it. That suggests that fresh ice is erupting from its interior, repaving the surface.