Far out in the distant reaches of the solar system, the dwarf planet Pluto lies in a neighborhood of ice and rock known as the Kuiper Belt. Frigid temperatures mean that the tiny body contains a great deal of ice.

Surface of Pluto

Lying 30 to 50 times as far from the sun as Earth, Pluto's composition bears a greater resemblance to the rocky terrestrial planets than the gas giants that are its neighbors. The surface of the dwarf planet appears to be dominated by ices, with some rock mixed in.

Pluto's distance and small size makes studying it a challenge, but astronomers have relied on advanced optics such as the Hubble Space Telescope to examine the dwarf planet. The powerful telescope has found new moons around Pluto in the last few years. Scientists can use Pluto's largest moon, Charon, which is close to the size of the dwarf planet and similar in composition, to study the surface of Pluto. As Charon passes between Pluto and Earth, the eclipse blocks light from the surface, emphasizing the brightness changes on Pluto.

Scientists have also used Charon to calculate the mass and size of Pluto more precisely. After calculating its density, scientists determined that the planet is somewhere between 50 to 70 percent rock, with ices making up the rest. The surface is composed of exotic ices such as methane and nitrogen frost, with water ice lying underneath. Carbon monoxide is also present on the surface. Recent studies by the Hubble Space Telescope suggest the presence of complex organic molecules.

The surface of the plane varies, with bright and dark spots, and black, orange, and white colors. But because it is ice, the surface also constantly changes with temperature. As it travels closer to the sun, the ices warm and melt into gases, and the atmosphere of the planet expands; as it move away, the gases resolidify. The surface also shifts due to seasonal changes, as the planet is tilted at an extreme angle of 122 degrees.

Inside Pluto

Pluto likely boasts a rocky core. Scientists cannot measure the inside of planets and moons, but they can study the density and use that to determine the composition of its interior. They determined that Pluto's interior has most likely separated out, with rock falling to the center while the lighter ices remained at the exterior.

Pluto lies in the distant Kuiper Belt, with similar rock-and-ice bodies. These pieces were left over at the beginning of the solar system. The Kuiper Belt hosts not only dwarf planets and asteroids, but also comets; scientists think that if Pluto traveled close enough to the sun, it would develop a tail. The core of a planet is the first to form, but Pluto's core failed to collect enough mass during its formation to help it escalate into a full-scale planet.

Pluto contains potassium under its surface. If enough of the potassium undergoes radioactive decay, Pluto could contain a subsurface ocean between its rocky core and icy shell.

When NASA's New Horizons spacecraft reaches Pluto and Charon in 2015, it will collect more information about the surface of the dwarf planet, and will be able to glean more details about its interior.

— Nola Taylor Redd, SPACE.com Contributor