Cassini finds 101 geysers and more on Enceladus
Sen—The Cassini spacecraft has identified 101 distinct geysers erupting on Saturn's icy moon Enceladus. Their analysis suggests it is possible for liquid water to reach from the moon's underground sea all the way to its surface.
The findings, and clues to what powers the geyser eruptions, are published in the current online edition of the Astronomical Journal.
Cassini's cameras have surveyed the south polar terrain of the small moon, a unique geological basin renowned for its four prominent "tiger stripe" fractures and the geysers of tiny icy particles and water vapour first sighted there nearly 10 years ago.
The result of the survey is a map of 101 geysers, each erupting from one of the tiger stripe fractures, and the discovery that individual geysers are coincident with small hot spots.
This artist's rendering shows a regional cross-section of the ice shell underlying Enceladus' south polar terrain. Image credit NASA JPL-Caltech SSI
After the first sighting of the geysers in 2005, scientists suspected that repeated flexing of Enceladus by Saturn's tides as the moon orbits the planet had something to do with their behaviour.
Before this new study, it was not clear whether back-and-forth rubbing of opposing walls of the fractures generated frictional heat, or whether the opening and closing of the fractures allowed water vapour from below to reach the surface. Nor was it certain whether excess heat emitted by Enceladus was everywhere correlated with geyser activity.
3-D model of 98 geysers whose locations and tilts were found by triangulation. Image credit: NASA/JPL-Caltech/SSI
Triangulation was used to determine the surface locations of the geysers. When the researchers compared the geysers' locations with low-resolution maps of thermal emission, it became apparent the greatest geyser activity coincided with the greatest thermal radiation.
Comparisons between the geysers and tidal stresses revealed similar connections. However, these correlations alone were insufficient to answer the question: "What produces what?"
Comparison of the survey results with high-resolution data collected in 2010 by Cassini's heat-sensing instruments provided the answer.
Individual geysers were found to coincide with small-scale hot spots, only a few dozen feet (or tens of metres) across, which were too small to be produced by frictional heating, but the right size to be the result of condensation of vapor on the near-surface walls of the fractures. This immediately implicated the hot spots as the signature of the geysering process.
"Once we had these results in hand, we knew right away heat was not causing the geysers, but vice versa," said Carolyn Porco, leader of the Cassini imaging team from the Space Science Institute and lead author of the first paper reporting the research. "It also told us the geysers are not a near-surface phenomenon, but have much deeper roots."
This artist's rendering shows a cross-section of the ice shell immediately beneath one of Enceladus' geyser-active fractures. Image credit: NASA/JPL-Caltech/SSI
The researchers concluded the only plausible source of the material forming the geysers is the sea now known to exist beneath the ice shell. They also found that narrow pathways through the ice shell can remain open from the sea all the way to the surface, if filled with liquid water.
The brightness of the plume formed by all the geysers also changes periodically as Enceladus orbits Saturn. The authors compared the observations with the expected venting schedule due to tides.
They found the simplest model of tidal flexing provides a good match for the brightness variations but it does not predict the time when the plume begins to brighten.