Read More: Scientific American
It’s not something NASA likes to advertise, but ever since its creation in 1958, the space agency has only conducted one direct, focused hunt for extraterrestrial life—and that was more than 40 years ago. It happened in 1976, when the twin Viking landers touched down at separate sites on Mars to look for any signs of life lurking on the planet’s desolate, freeze-dried surface. The Viking mission was—and still is—the most expensive planetary science mission ever launched as well as a technical tour de force that laid the foundations for all future interplanetary exploration. Both landers came up empty in their search for life, however, and ever since NASA has favored a series of missions—most of them to Mars—that transformed our understanding of our neighboring worlds as they tiptoed around the central question of whether any of them harbor life. Now, after decades of wandering in Martian deserts, NASA’s astrobiologists are at last preparing to rekindle a direct search for a “second genesis” of life in our solar system—but not where one might think. This time, they will look well beyond Mars, the most Earthlike of our planetary neighbors, to the dark reaches of the outer solar system, where probes and space telescopes have revealed ever-more tantalizing signs of oceans hidden inside icy moons and dwarf planets. Warmed by tidal forces rather than sunlight, those environments could harbor life, scientists say. “These oceans may be close to the surface, or may be deeper, with thicker ice crusts, but there must be water in liquid or slush form there—even all the way out to Pluto,” says James Green, director of NASA’s Planetary Science Division and an architect of the agency’s embryonic Ocean Worlds exploration program. The program’s central focus is Europa, a moon of Jupiter that despite being slightly smaller than Earth’s lunar companion is thought to contain an ocean twice as voluminous as all our planet’s seas combined. Data from previous spacecraft flybys hint Europa’s ocean is billions of years old and in direct contact with the moon’s hot, rocky core, offering life sufficient time and energy to get started somewhere within. Locked below a crust with an average thickness of at least dozens of kilometers, any Europan biosphere might have remained forever out of reach. Occasionally seawater wells up through fissures in the crust to freeze at or near the surface, however, and recent observations by the Hubble Space Telescope suggest the ocean may even be venting vast amounts of water vapor into space through geyserlike plumes erupting from beneath the surface. If astronomers could collect the frozen material or the vapor they might learn what, if anything, lurks within Europa.