At least that's what the chemistry says. And Hubble observations support it.
Meanwhile, here's the north pole of another dwarf planet, Ceres- which was also briefly considered a planet before we realized that it was simply the largest of a new class of objects called "asteroids." The Ceres story was in many ways a parallel to what happened when we realized that Pluto was merely a large- if typical- Kuiper Belt Object. The photo was taken by NASA's Dawn spacecraft.