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The hottest spots in the search for alien life are a few frigid moons in the outer solar system, each known to harbor a liquid-water ocean beneath its icy exterior. There is Saturn’s moon Titan, which hides a thick layer of briny water beneath a frozen surface dotted with lakes of liquid hydrocarbon. Titan’s sister Saturnian moon Enceladus has revealed its subsurface sea with geyserlike plumes venting from cracks near its south pole. Plumes also emanate from a moon that is one planet closer to the sun: Jupiter’s Europa, which boasts a watery deep so vast that, by volume, it dwarfs all of Earth’s oceans combined. Each of these aquatic extraterrestrial locales might be the site of a “second genesis,” an emergence of life of the same sort that occurred on Earth billions of years ago.

Astrobiologists are now pursuing multiple interplanetary missions to learn whether any of these ocean-bearing moons actually possess more than mere water—namely, habitability, or the nuanced geochemical conditions required for life to arise and flourish. NASA’s instrument-packed Europa Clipper spacecraft, for example, could begin its orbital investigations of Jupiter’s enigmatic moon by 2030. And another mission, a nuclear-fueled flying drone called Dragonfly, is scheduled to touch down on Titan as early as 2036. As impressive as these missions are, however, they are only preludes to future efforts that could more directly hunt for alien life itself. But in those strange sunless places so unlike our own world, how will astrobiologists know life when they see it?

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