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 frigid temperatures far from the sun. Even small amounts of internal heat can turn that ice to liquid. Beneath their protective icy shells, these worlds might collectively hold 100-fold the volume of Earth’s oceans, calculates geologist Kevin Hand of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.

Scientists have yet to understand these strange ice balls’ surface processes or internal compositions entirely, and NASA and other space agencies are interested in further investigating their secrets. Last year, NASA started putting together an Ocean Worlds program; the earliest targets will be Europa, Enceladus, and Titan, which have the best evidence for liquids on or near their surfaces. Within a couple of decades, these places might tell us if life is widespread in our solar system, and perhaps elsewhere.

“We know that physics works beyond Earth; we know that chemistry works beyond Earth. We know that geology works,” says Hand. “But we have yet to determine whether or not biology works beyond Earth. These liquid water environments in our cosmic backyard present the opportunity to answer that question.”

Ice on Top Our knowledge of icy ocean worlds came about gradually. Back in the 1960s, some scientists argued that the outer solar system satellites would be nothing more than dusty gray worlds not too different from our own moon. But several researchers pointed out that a number of outer solar system bodies contained enough ice and rock—which would be studded with radioactive elements—that the heat from decay could melt their interiors, leading to large pockets of liquid water topped with a frozen crust (1). When Voyager 1 and 2 swept past Jupiter in 1979, they beamed back startling images. Rather than an inert dust ball, the moon Io turned out to be the most volcanically active body in the solar system, whereas its nearby sister Europa had a surface of broken chaotic terrain similar to tectonic plates. “Once the Voyager probes saw that these moons had few craters and huge cracks on them, it gave us a clue that they weren’t just places of boring rock and ice,” says atmospheric, oceanic, and space scientist Catherine Walker, also of the JPL. “They were floating laboratories of geology.” The full scope of oceanic incidence in the solar system became clearer when the Galileo mission arrived at Jupiter in 1995. The spacecraft measured odd anomalies in Jupiter’s magnetosphere near the moons Callisto and Europa (2). Because Jupiter’s magnetic axis is tilted relative to its poles, its moons experience a periodic magnetic oscillation as the giant planet rotates. Basic electromagnetic theory states that a changing magnetic field will induce a current in a conductive material, generating a secondary, weaker magnetic field. Galileo was seeing this induced magnetic field at Europa and Callisto, suggesting their interiors were made from some electrically conductive material. The most likely culprit? Seawater. “That result was surprising because Callisto had this heavily cratered surface and looked as if it had never done anything interesting in its life,” says planetary scientist David Stevenson of the California Institute of Technology in Pasadena. “It was the ugly stepsister and, look, it had an ocean.” By the time the Cassini spacecraft reached Saturn in 2004, researchers were ready to hunt for ice-capped ocean moons. Although Titan’s methane lakes and thick nitrogen atmosphere initially drew the most interest, tiny Enceladus soon stole the show. Cassini’s cameras spotted the source of a faint and unique Saturnian ring—incredible plumes of water ice, silica, and ammonia that were shooting hundreds of kilometers from Enceladus’ surface (3). The tally of frozen ocean worlds continues to grow. The same magnetic effect pointing to salty water on Europa and Callisto suggests that Jupiter’s moon Ganymede possesses an interior liquid layer (4). When it flew past Pluto in 2015, the New Horizons spacecraft spotted a smooth 1,000-kilometer-wide plain thought to house an underground ocean (5). Surface features on Pluto’s moon Charon hint that it, too, once had liquid water, a characteristic that the dwarf planet Ceres, the largest member of the asteroid belt, seems to share (6). When prompted, planetary scientists throw out a half-dozen other worlds that might be worth investigating, including Saturn’s moons Mimas, Dione, and Tethys, and Uranus’ moons Ariel, Umbriel, and Miranda. Researchers hope to explore the oceans of Neptune’s moon Triton; Saturn’s moons Enceladus and Titan; and Jupiter’s moons Europa, Callisto, and Ganymede (seen here to scale with Earth). Image courtesy of Kevin P. Hand/JPL/NASA.