Instead they are shooting out from regions that are no larger than 0.2 square miles (0.6 square kilometers), although the vents may be even smaller, depending on the temperature of the water vapor, the researchers note.
Overall, about 440 pounds (200 kilograms) of water are being blasted into space every second—roughly the equivalent of a residential swimming pool every few minutes.
But the amount observed in 2007 was less than that seen in 2005.
That was a surprise, the University of Colorado's Esposito said, because in 2007, Enceladus was at a point in its orbit where scientists had calculated that tidal forces from Saturn would stretch the vents wider, increasing their flow.
Instead, the opposite appears to have happened, the scientists report in this week's issue of the journal Nature.
"Sometimes you learn more by being wrong than by being right," study leader Hansen said.
She added that theoretical physicists are already looking at ways to refine their models of the moon to account for the new finding.
"It is the natural back-and-forth ping-pong [of science]," she said.
"You make observations. People have hypotheses and theories about what's going on. [We] take more observations and figure out where we're on the right track and where we probably are not."
The finding does, however, bolster the case for subterranean liquid water, strengthening the argument that Enceladus might be one of the better places in the solar system to look for extraterrestrial life.
In a March 2008 flyby, Cassini dipped into the plume and detected not only water vapor but also methane, carbon dioxide, and other organic molecules.
"We cannot say whether there is life or not," Hansen said. "But if we can conclude that there is liquid water, then we can say at least the ingredients are there."
SOURCES AND RELATED WEB SITES