In general, the farther a satellite is from the sun, the higher is its percentage of ice. The majority of satellites consist mostly of ice. Many satellites appear rough, with craters dating from more than 3 billion years ago. Some satellites have changed geologically since then. These moons have a variety of surface features, including active volcanoes.
Some satellites have an atmosphere.
Titan, which orbits Saturn, and Neptune's moon Triton have atmospheres that are dense enough to measure accurately. Triton's atmosphere is much thinner. Scientists use telescopes and spacecraft to study satellites and their parent planets. Spacecraft have flown close to all the planets except Pluto. 1 Formation of satellites Many scientists believe that satellites formed at about the same time, and in the same way, as planets did.