Neptune

Neptune's upper atmosphere (what we see) is a mixture of hydrogen, helium, methane and traces of acetylene (C2H2), carbon dioxide, and other gasses. Only 10% of the planet's mass is in this outermost layer (approximately 3,100 mi or 5,000 km thick). Under the upper atmosphere lies a lower atmosphere of molecular (gaseous) hydrogen and helium, plus some ices (approximately 6,200 mi or 10,000 km thick). Below the atmosphere lies the mantle, a water ice and rock mixture that perhaps contains methane ice and ammonia ice mixed in. A core is at the center of the planet's mass, and it is likely a body with a 6,200-mi radius and represents 45% of the planet's mass that is composed of silicate rock and water ice. Like the other Jovian planets (Jupiter, Saturn, and Uranus), Neptune has a distinctive structure quite different from the terrestrial planets like Earth.

The size of Neptune's Great Dark Spot (top) compared to the size of Earth (bottom). The Great Dark Spot is a huge storm of long duration in the planet's upper atmosphere. Its winds blow counterclockwise. U.S. National Aeronautics and Space Administration (NASA).