National Geographic : 2006 Dec
"WE'RE LOOKING FOR PLACES WHERE WE MIGHT FIND BUGS. WE DON'T EXPECT ANYTHING INTELLIGENT, OR HIGHLY DEVELOPED, BUT HERE YOU HAVE A PLACE WHERE LIFE IS POSSIBLE." -BOB BROWN, UNIVERSITY OF ARIZONA pleaded with spacecraft controllers to set a course that would take Cassini close to the moon's south pole, where her team had measured the strongest disturbances. On July 14, 2005, the spacecraft descended to a hundred miles above Enceladus's south polar region. Working in con cert, its many instruments probed the enigmatic moon, monitoring surface heat, chemical traces, and magnetic fields. The data indicated that plumes of material were erupting near the south pole. Four months later, as the distant sun sil houetted Enceladus, Cassini made images that showed geyserlike eruptions of water vapor and ice particles shooting far into space. The temperature near the south pole was at least 100 degrees F higher than expected-warm enough to melt ice just below the surface and feed the plumes, which erupt from long fissures that cut across the ice, dubbed "tiger stripes." In freshly fallen snow around the fissures, Cassini detected simple carbon compounds. One mystery was solved. The E ring bulges near the moon because the plumes are pump ing ice particles into it. Now a new puzzle arose: the source of the heat. It could be generated by radioactive elements trapped inside Enceladus or by Saturn's powerful gravity as it squeezes and flexes the moon. A greater question: Could this modest moon harbor life? Life as we know it requires liquid water, energy, and organic molecules, says Bob Brown of the University of Arizona. "Evidence of all three are here," he says. "We have the cock tail." That same cocktail, Brown says, may exist on Jupiter's moon Europa, in a briny ocean shielded under miles of ice. It may have existed long ago on Mars, when that planet was warm enough to harbor open water. It was present on Earth as early as 3.8 billion years ago. "But we 56 NATIONAL GEOGRAPHIC * DECEMBER 2006 know it exists right now on Enceladus." Life might be hiding just a few dozen feet be low the ice in pockets of warm water, living off dissolved organic com pounds and reproducing using some alien version of DNA-or an entire ly different kind of genetic material. "We're look ing for places where we might find bugs," says Brown. "We don't expect anything intelligent, or highly developed, but here you have a place where life is possible." Cassini is scheduled to revisit Enceladus once more and, if space budgets allow, may extend its mission beyond 2008 to make more flybys of Enceladus, Titan, and other key targets. But sci entists are already thinking ahead to future space probes that could actually look for life on Ence ladus and study the precursors of life on Titan exploration that would take us closer to under standing our own origins. Some dream of a robot that would land at the south pole of Enceladus and drop a probe through the vents to look for life. Others picture a satellite that would orbit Titan and launch blimp-like rovers into its atmosphere for a leisure ly survey of its hills and plains. Jonathan Lunine of the University of Arizona, a Cassini-Huygens scientist who also studies planets around other stars, sees the quest in the biggest terms. "These places," he says, "will write new chapters in the book on how life began in the universe." D Water vapor and ice particles erupt hundreds of miles from Enceladus (naturalandfalse color, above). Fallingto the surface, the ice smooths the moon's southern hemisphere (right). The jets emergefrom fractures (falsecolor)fed by subsur face reservoirs.Says CarolynPorco, Cassiniimag ing team leader: "We have found an environment potentiallysuitablefor living organisms."