An enhanced color view of Enceladus. Credit: NASA/JPL/Space Science Institute
A large coalition of scientists working with NASA‘s Cassini spacecraft have located and analyzed a huge geyser of water vapor spurting from the surface of Saturn’s moon Enceladus, opening the possibility that this distant body could support life.
Their work was published in a special March 10th issue of Science largely devoted to Enceladus.
“Not in our wildest imaginations did we imagine that we would find a plume coming out of Enceladus almost as big as it is,” said Candice Hansen. She is with the Jet Propulsion Laboratory at Caltech and is the principle author of a paper that confirmed the existence of the geyser using data from Cassini’s ultraviolet spectrometer.
Although Saturn has 35 moons, Enceladus gained the interest of astronomers thanks to images taken by the 1980 Voyager 1 mission, which suggested the moon was undergoing intense geological activity.
“There had been a lot of speculation and perhaps even hope that Enceladus might be active—even as far back as after Voyager 1 imaged this moon in 1980—because it had a very smooth surface without many craters, which suggested that it was being resurfaced,” said Jonathan Lunine, chair of the theoretical astrophysics program at the University of Arizona. He helped analyze the plume’s content using Cassini’s Ion and Neutral Mass Spectrometer.
Active geological systems that are constantly reshaping themselves for a smoother surface are characteristic of younger planetary surfaces. Older surfaces, like the four billion-year-old face of Earth’s moon, cease to resurface at a point, collecting pockmarks and craters from meteorites that crash into them over time. Scientists hoped that Cassini’s observations would prove conclusively that Enceladus, whose surface is a mere few million years old, has an active system.
Their proof came after a long period of suspense: On its first flyby, Cassini detected a change in the magnetospheric environment around Enceladus, which suggested that the moon had an atmosphere—a discovery that JPL’s Hansen called “a tease.” A second flyby was inconclusive, causing scientists to request that Cassini be repositioned and sent by Enceladus at a closer angle. On the vessel’s third flyby, it literally collided with the data the scientists sought.
“At that point we didn’t know there was this plume coming out of Enceladus, but we clipped it,” Hansen said. “We actually flew the spacecraft through it.”
Cassini had drifted into something remarkable and completely unanticipated: a 435-km high geyser of water vapor and ice crystals emanating from Enceladus’s south pole. The height is startling, especially given the fact that Enceladus is only 504 km wide. This eruption is likely the result of cryovolcanic activity, or volcanic activity involving molten ice instead of lava.
More exciting for extraterrestrial obsessives is the presence of water vapor in the plume, which suggests that liquid water may exist under Enceladus’ smooth, icy surface.
“In order to pump out that quantity of ice grains, we think it has to be coming off a liquid,” said Hansen. “It cannot just be ice that’s sublimating.”
The presence of liquid water, in which life-forming chemicals can float and combine in novel ways, is a primary precondition for life. Cassini scientists also found nitrogen, carbon dioxide and several hydrocarbons in the plume, all materials that led to the formation of life on Earth.
“If you just have a sterile liquid water pond, you’re not going to get any hope for life,” Lunine said. “But if you’ve got the nitrogen, the carbon, the other elements that are a part of life, then there’s the possibility of something more intriguing.”
The Cassini mission began on October 15, 1997, with the vessel’s launch from Cape Canaveral. On its way to the Saturnian system, the spacecraft photographed Jupiter in 2000 and has been orbiting Saturn and its moons since the summer of 2004. In December of 2004, it deployed the Huygens Probe to the surface of Titan, Saturn’s largest moon. The craft made flybys of Enceladus in January, February and November of 2005, as well.
