Galilean Moons

Imagine pointing a telescope at the dark night sky and seeing tiny stars lined up in a row next to giant Jupiter. In 1610, the Italian scientist Galileo Galilei was studying Jupiter through a telescope he had improved himself, and he noticed something strange.

Every night he looked again, and the little dots had shifted a bit from side to side. At first he took them for background stars. But they were really moons of Jupiter, circling the planet on a steady schedule. Their names are Io, Europa, Ganymede, and Callisto.

This discovery was a huge shock. Until then, people firmly believed that everything in the sky circled the Earth alone. That was the old Earth-centered view. But now there was proof of objects orbiting another planet. That gave real strength to the idea that the Earth is not the center of the universe, and that the planets travel around the Sun.

The Galilean moons are far more than shiny dots in a telescope. Each one is a completely different world. Io, the closest to Jupiter, has hundreds of boiling volcanoes. It is the most volcanically active body in the solar system. Lava and sulfur spray out across its surface, so it looks a lot like a scene from the underworld.

Next is Europa, covered all over in a smooth shell of ice. Under that ice sits a liquid ocean holding far more water than all of Earth's oceans put together. That makes Europa a leading candidate for alien life.

The third moon, Ganymede, is the largest moon in the solar system. It is even bigger than the planet Mercury, and it has its own magnetic field. The outermost moon, Callisto, is packed with meteorite craters, which keep a record of the solar system's early history. So four extremely different worlds sit together beside Jupiter.

To be a bit more precise, how can volcanoes erupt and oceans stay unfrozen near Jupiter, where sunlight barely reaches and it is well over a hundred degrees below zero? The secret is the huge gravitational tug between Jupiter and its neighboring moons, called tidal heating.

Jupiter's strong gravity and the pull of the other moons keep squeezing and stretching each moon from the inside, like a rubber ball. All that squeezing creates intense friction heat between the layers of rock. On Io, that heat melts rock into magma and sends it out through volcanoes. On Europa, it keeps the ocean under the ice from freezing solid.

So the Galilean moons are not just a chapter of history that supported the Sun-centered view. They also point to a new kind of habitable zone, a hint that life could exist in the far reaches of space where sunlight is too weak to help.