Geology of Other Planets: The Dwarfs
A dwarf planet is a planet in the solar system that has enough mass and gravity to become spherical, but is not massive enough to clear its orbital path of other astronomical objects. Dwarf planets include Ceres in the asteroid belt, Pluto in the Kuiper belt, and Eris within the scattered disc. There are others, and new ones are being discovered all the time.
Ceres, the largest astronomical body in the main asteroid belt between Mars and Jupiter, accounts for about a third of the belt's mass. It appears to have a rocky core surrounded by an icy mantle, and has a number of permanent light and dark features on its surface. Some of those might be craters. The planet as a whole is probably not geologically active as it would've lost all of its original heat by now.
Pluto was the 9th planet in the Solar System until astronomers discovered lots of other astronomical objects that looked just like it in the same area, collectively called the Kuiper belt. The best images (from the Hubble Telescope) show a surprisingly varied surface of light and dark areas, covered with frozen nitrogen and trace amounts of frozen methane and frozen carbon monoxide. Based on its density, it's assumed to have a rocky core with an icy mantle.
Eris, slightly larger than Pluto and farther away, is assumed to be similar.
Compared to the geology of Mars, Venus, and Mercury, we don't know much about the dwarf planets because none have yet been visited by our space probes. However, NASA's Dawn spacecraft is currently en route to the asteroid belt and will reach Ceres in 2015. Likewise, NASA's New Horizons spacecraft is scheduled to make a flyby of Pluto in 2015 on its way into the Kuiper belt.