L9. Moons and Rings

Unit 2 · The Solar System

R-report

L9. Moons and Rings

One of Jupiter’s moons has more active volcanoes than Earth. Another hides a saltwater ocean under kilometres of ice, holding more water than every ocean on our planet combined. Moons are not just rocks going round in circles.

Moons are worlds too

A moon is simply an object that orbits a planet. Earth has one; Jupiter and Saturn each have dozens of confirmed moons, plus many small ones still being catalogued. Mercury and Venus have none. Some of the largest are genuinely planet-like. - Ganymede, orbiting Jupiter, is the biggest moon in the solar system — larger than the planet Mercury. - Titan, orbiting Saturn, has a thick nitrogen atmosphere and lakes and rivers of liquid methane. It is the only moon with a substantial atmosphere. - Io, another moon of Jupiter, is the most volcanically active body we know, constantly resurfacing itself with sulphur. - Europa has a smooth, cracked crust of ice with a liquid ocean underneath. Moons form in different ways. Large ones usually condensed from the disc of material around a forming planet. Small, irregular ones are often captured asteroids — Mars’s two lumpy moons, Phobos and Deimos, are likely examples.

Squeezed by gravity, warmed from inside

Io and Europa are so far from the Sun that sunlight alone would leave them frozen solid. Something else is heating them: tidal heating. As these moons orbit Jupiter, the giant planet’s gravity pulls harder on the near side than the far side, and the pull changes strength as the moon moves along its slightly stretched orbit. The moon is squeezed and released over and over. That constant flexing generates heat inside, exactly the way a paperclip warms up when you bend it back and forth. On Io the heating is so intense that the interior melts and drives hundreds of volcanoes. On Europa it is gentler, but enough to keep an ocean liquid beneath the ice. This matters because it changes where life might be possible. Astronomers once assumed a world needed to sit at the right distance from its star to have liquid water. Tidal heating shows that a moon far outside that zone can still hold an ocean — which is why Europa and Saturn’s geyser-spouting moon Enceladus are among the top targets in the search for life.

What rings really are

Saturn’s rings look like solid discs in photographs. They are nothing of the kind. A ring is a swarm of countless separate pieces — mostly water ice, ranging from dust grains to chunks the size of a house — and every single piece follows its own orbit around the planet. There is nothing holding them together. The proportions are extraordinary. Saturn’s main rings stretch more than 280,000 kilometres across but are typically only tens of metres thick. Scaled down to the width of a football pitch, they would be far thinner than a sheet of paper. When Saturn’s rings turn edge-on to us every 15 years or so, they very nearly disappear from view. Jupiter, Uranus, and Neptune have rings too; they are simply dark and faint compared with Saturn’s bright ice. Rings probably form when a moon or comet strays too close and is torn apart by the planet’s gravity, or when impacts blast material off a small moon.

Putting the investigation together

Moons range from captured lumps of rock a few kilometres across to worlds bigger than Mercury with atmospheres, volcanoes, and hidden oceans. Rings, meanwhile, are not solid at all — they are vast swarms of orbiting ice and rock, spread wide but astonishingly thin.

Tidal heating is the idea worth carrying forward. It shows that energy for a warm, wet environment does not have to come from a star, and it widened the search for life from a narrow band around each sun to include icy moons far out in the cold.