This important measurement was made using Cassini's INMS instrument, which detects gases with the goal of determining their composition. INMS was designed to sample the upper atmosphere of Saturn's large, smoggy moon Titan. However, after Cassini's surprising discovery of a tall plume if icy spray erupting from cracks on Enceladus in 2005, planetary scientists turned its detectors to that small moon.
Dr. Rodriguez and his colleagues also spotted a trio of odd equatorial brightenings in infrared images obtained by Cassini during Titan's 2009 northern equinox. At the time, the scientists speculated that the brightenings might also be the same kind of methane clouds observed in tropical areas. However, this proved not to be the case. A later investigation conducted by the astronomers revealed that these brightenings were caused by something entirely different.
But what truly makes Enceladus so remarkable is that its habitable zone can be observed with relative ease by astronomers. Dr. Porco told the press on March 27, 2012 that "It's erupting out into space where we can sample it. It sounds crazy but it could be snowing microbes on the surface of this little world. In the end, it's the most promising place I know of for an astrobiology search. We don't even need to go scratching around on the surface. We can fly through the plume and sample it. Or we can land on the surface, look up and stick our tongues out. And voila... we have what we came for."