In 2005, a NASA spacecraft flew past Enceladus, an icy moon of Saturn, snapping pictures and recording observations as it went. When scientists processed the data, they saw plumes of mist erupting from the cracked surface of the moon’s south pole and into the emptiness of space. The plumes, the spacecraft’s instruments had found, were made of water vapor.
Scientists were stunned. Enceladus is small, just 300 miles wide, and its surface reflects sunlight rather than absorbs it. For these reasons, they had expected the moon to be frozen solid, yet here was some evidence to the contrary. Later observations found more proof that Enceladus was alive, geologically speaking, and hiding a liquid ocean between an icy crust and a rocky core.
Close flybys of Enceladus revealed that the moon’s plumes contained a smorgasbord of chemicals, including methane, carbon dioxide, ammonia, nitrogen, hydrogen, ammonia, and formaldehyde. Enceladus quickly became an extremely attractive candidate in the search for other life in the solar system. Life, at least life as we know it on Earth, arises from some molecules and a little water, and this Saturnian moon had both.
But the NASA spacecraft, Cassini, couldn’t tell us much more than that. It was equipped to sample the contents of the moon’s geysers, not determine whether they showed signs of microbial life. (Also, Cassini plummeted into Saturn’s atmosphere and disintegrated last September #RIP.) To investigate that tantalizing possibility, scientists had to bring Enceladus closer to Earth.