Are we alone in the universe? An answer to that age-old question has seemed tantalizingly within reach since the discovery of ice-encrusted moons in our solar system with potentially habitable subsurface oceans. But looking for evidence of life in a frigid sea hundreds of millions of miles away poses tremendous challenges. The science equipment used must be exquisitely complex yet capable of withstanding intense radiation and cryogenic temperatures. What's more, the instruments must be able to take diverse, independent, complementary measurements that together could produce scientifically defensible proof of life.