Sometime in the early 2030s, a washing machine-sized robot could be carefully descending toward the icy crust of Jupiter’s moon Europa. Armed with cameras, a spectrometer, a microscope, and a scoop, the vehicle would be lowered from a UFO-like sky crane similar to the one that delivered the Curiosity rover to Mars. As the robot nears the frozen ground, its autonomous navigation system may have to take evasive action. “Maybe the surface is nice and flat and smooth,” says Curt Niebur, a program scientist at the National Aeronautics and Space Administration (NASA) headquarters in Washington, DC. “But maybe it’s covered in penitentes, which are literally six-foot-tall ice spikes.” Saturn’s moon Enceladus, shown here via a mosaic of images collected by the Cassini spacecraft in 2005, is one of many moons with oceans that could harbor signs of life. Image courtesy of NASA/JPL/Space Science Institute. The probe’s mission is a familiar one: to find signs of life beyond Earth. But its target for this investigation, a moon’s ocean, has only recently gained popularity. Europa is thought to have a vast liquid water ocean beneath its frozen crust, a potentially perfect place to find extraterrestrial organisms. The Jovian moon is merely one of many similar locations. In recent decades, exploratory spacecraft have revealed that our solar system is chock full of icy ocean worlds. Along with Europa, there are Saturn’s moons, the geyser-spewing Enceladus, and the methane-filled Titan. Then there's Neptune’s cryovolcanic Triton and the distant dwarf planet Pluto, just to name a few. “You throw a stone, and you find another ocean world,” says planetary scientist Francis Nimmo of the University of California, Santa Cruz. “They’re all over the place.” The moons of Jupiter, Saturn, Uranus, and Neptune are built largely from frozen water, which becomes hard a rock at the …