Icy moons with vast oceans are the latest candidates for alien life

ASTROBIOLOGY is a branch of science that, for now, lacks anything to actually study. Despite this lack of research subjects, however, the search for life beyond Earth has a few rules of thumb. The most important is “follow the water”. The unusual chemical properties of water make it vital for life on Earth. And, since the laws of chemistry are the same everywhere, it is not unreasonable to think that water may play the same role on other planets too.

For most of the space age that insight led scientists to Mars. Although the planet is today a frigid desert, there is plenty of evidence that it used to have oceans on its surface like Earth’s. That is one reason why, these days, Mars is lousy with landers, rovers and orbiting probes looking, among other things, for any ice or patches of liquid that may have survived.

More and more, though, planetary scientists are following the water to other places—and in particular to the so-called “icy moons” that orbit Jupiter, Saturn, Neptune and Uranus, the solar system’s quartet of giant gas planets. Many of those moons are either known or suspected to have oceans beneath their icy shells, kept liquid by gravitational squeezing from the planets they orbit.