Though named for a mythical giant, Enceladus, Saturn’s sixth-largest moon, had claimed little distinction in the annals of astronomy since its discovery in 1789. That changed in early 2008, when observations from NASA’s Cassini spacecraft made it a leading candidate among potential hosts of extraterrestrial life in the solar system.
In March Cassini flew within 30 miles of Enceladus’s icy surface, sampling the chemical composition of water-vapor plumes streaming from fissures near its south pole. In addition to detecting carbon dioxide, carbon monoxide, and volatile gases, Cassini’s instruments also noted hydrocarbons—the primitive components of life.
Why was the discovery surprising? “First, we didn’t expect to find any plumes at all, and second, we didn’t expect to find any organic matter in those plumes,” says Cassini project scientist Dennis Matson of NASA’s Jet Propulsion Laboratory in Pasadena, California. Cassini first spotted the vapor plumes rising more than 900 miles above Enceladus’s surface in 2005. To explain the geysers, some researchers posit the presence of liquid water trapped 6 to 20 miles beneath a frozen crust. The new Cassini data reveal that temperatures at the polar fissures are more than 200 degrees Fahrenheit warmer than the rest of the moon, and Matson’s analyses suggest that subsurface temperatures there are high enough to support chemical reactions among hydrocarbons.
Heat, hydrocarbons, and water: These are essential ingredients of life. The Cassini mission had been scheduled to end last July. Due in large part to this year’s discoveries, NASA has extended the project for another two years and plans at least five more flybys of Enceladus, some bringing Cassini to within 15 miles of its surface.