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One of Jupiter’s largest moons. Ganymede, the largest satellite in our solar system, is about 5300 kilometers (3300 miles) wide and larger than the planet Mercury.


The innermost of Jupiter’s four large moons. Due to Jupiter's gravitational might, Io is geologically active; its surface is peppered with volcanoes that send sulfurous eruptions into its thin atmosphere. Io appears to have the most active volcanoes in the solar system.

Io Plasma Torus

A bagel-shaped region of trapped sulfur ions around Jupiter that originates from the surface of Io, one of Jupiter’s moons. Gravitational tidal forces between Jupiter, other Galilean moons, and Io cause tidal friction in Io’s interior, producing geysers that spew sulfur at tremendous speeds. Some of the sulfur ions leave Io’s surface and become trapped around Jupiter.

Lunar Eclipse

A darkening of the Moon, as viewed from Earth, caused when our planet passes between the Sun and the Moon.


A dark, flat, large region on the surface of the Moon. The term is also applied to the less well-defined areas on Mars. Although maria literally means “seas,” watery regions do not exist on the Moon or Mars. Marias on the Moon may be evidence of past volcanic lava flows.


A large body orbiting a planet. On Earth’s only moon, scientists have not detected life, water, or oxygen on this heavily cratered body. The Moon orbits our planet in about 28 days.


The layer of loose rock resting on bedrock (sometimes called mantle rock), found on the Earth, the Moon, or a planet. Regolith is made up of soils, sediments, weathered rock, and hard, near-surface crusts. On the surface of the Moon, regolith is a fine rocky layer of fragmentary debris (or dust) produced mainly by meteoroid collisions.

Solar Eclipse

A phenomenon in which the Moon’s disk passes in front of the Sun, blocking sunlight. A total eclipse occurs when the Moon completely obscures the Sun’s disk, leaving only the solar corona visible. A solar eclipse can only occur during a new phase of the Moon.


The largest of Neptune’s satellites. Triton has an atmosphere and is roughly the size of Earth's moon. It has an “ice cap” of frozen nitrogen and methane with “ice volcanoes” that erupt liquid nitrogen, dust, and methane compounds from beneath its frozen surface.


A break or vent in the crust of a planet or moon that can spew extremely hot ash, scorching gases, and molten rock. The term volcano also refers to the mountain formed by volcanic material.