Far from the Sun’s heat, orbiting the outer planets of the solar system, are moons with oceans of liquid water beneath their frozen surfaces. Keith Cooper finds out how planetary scientists are unravelling the mysteries of these frigid systems to see if they hold life beyond our planet
Our blue planet is a Goldilocks world. We’re at just the right distance from the Sun that Earth – like Baby Bear’s porridge – is not too hot or too cold, allowing our planet to be bathed in oceans of liquid water. But further out in our solar system are icy moons that eschew the Goldilocks principle, maintaining oceans and possibly even life far from the Sun.
We call them icy moons because their surface, and part of their interior, is made of solid water-ice. There are over 400 icy moons in the solar system – most are teeny moonlets just a few kilometres across, but a handful are quite sizeable, from hundreds to thousands of kilometres in diameter. Of the big ones, the best known are Jupiter’s moons, Europa, Ganymede and Callisto, and Saturn’s Titan and Enceladus.
Yet these moons are more than just ice. Deep beneath their frozen shells – some –160 to –200 °C cold and bathed in radiation – lie oceans of water, kept liquid thanks to tidal heating as their interiors flex in the strong gravitational grip of their parent planets. With water being a prerequisite for life as we know it, these frigid systems are our best chance for finding life beyond Earth.
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