For those who hunt for life on other worlds, water in its liquid form is perhaps the leading indicator. Life as we know it on Earth is based on water and carbon. And if organisms can prosper here in nasty environments — in geysers, in the depths of the sea, in toxic waste, in water that is too hot, too cold, too acidic or too alkaline — why could they not prosper out there?

Scientists for years regarded liquid water as a solar system rarity, for there was no place apart from Earth that seemed to have the necessary physical attributes, except perhaps Jupiter’s ice-covered moon, Europa, which probably concealed a subterranean ocean.

The past 20 years of space exploration, however, have caused what the astrobiologist David Grinspoon calls a sea change in thinking. It now appears that gravity, geology, radioactivity and antifreeze chemicals like salt and ammonia have given many “hostile” worlds the ability to muster the pressures and temperatures that allow liquid water to exist. And research on Earth has shown that if there is water, there could be life.

On Mars and Venus, on Saturn’s moons Enceladus and Titan, and even on two outer-belt asteroids, researchers have shown that the presence of liquid water is possible and even likely. Proof of life, of course, will come only when something — or someone — puts a drop of alien water under a microscope and sees a microbe.