Moons and Rings

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Moons are natural satellites orbiting planets; there are hundreds in the Solar System. Earth's Moon likely formed from debris after a Mars-sized body collided with Earth. Notable moons: Jupiter's Io (volcanic) and Europa (subsurface ocean), Saturn's Titan (thick atmosphere) and Enceladus (water geysers).

Rings, around all four giant planets, are made of countless ice and rock particles, from dust to house-size chunks, perhaps the remains of moons or comets broken up by gravity. Saturn's are the brightest and thinnest relative to width.

A good example is Io, which is squeezed by Jupiter and its neighbouring moons, so tidal heating melts its interior and drives hundreds of volcanoes, the most volcanically active place known. Europa is flexed in the same way, which may keep a salty ocean liquid beneath its icy shell, and the Europa Clipper mission is designed to investigate it. Enceladus sprays water vapour into space that feeds Saturn's E ring, and Cassini flew through the plumes and found organic compounds.

Rings last only a limited time on cosmic scales, so they are either young or being resupplied. Particles in them follow their own orbits, and tiny shepherd moons keep edges sharp. Moons matter to science because several could host life, and to humans because Earth's Moon stabilises our tilt and produces tides. A frequent misconception is that only Saturn has rings; Jupiter, Uranus and Neptune have them too, just fainter. Another is that Earth has only one moon in the sense of any satellite; temporary captured asteroids exist but are not true moons.