In 2005, the Cassini spacecraft, which has been exploring the system of Saturn and its satellites since 2004, discovered enormous plumes of water spewing out into space from the satellite Enceladus. These geysers are so powerful and pump out such enormous volumes of water that they are clearly visible from space. But until recently, it wasn’t clear where all the water was coming from, or what force was propelling it upwards so violently.The water is expelled from a geographical feature near the satellite’s South pole, called the Tiger Stripes because of their appearance. They are a series of deep fissures in the surface, which suggested that the water might come from underground reservoirs. Different images collected over the years showed the plumes varying in strength and size, so planetery scientists theorised that these variations must follow a predictable pattern. If they could find the pattern, it might unlock the mystery of the geyser’s source. Unfortunately,nobody had ever managed to gather enough evidence to show that any patterns existed.
This all changed earlier this year when a team of planetary scientists led by Matt Hedman analysed over 200 historical historical images from Cassini’s visual and infrared mapping spectrometer (VIMS) to observe the behaviour of the jets over a period of seven years. The VIMS is a sensitive instrument which allows astronomers to determine not only the chemical composition of the plumes, but their velocity and even seismic activity related to the plumes. They found that the plumes vary according to Enceladus’s position in its orbit around Saturn, with the jets at their weakest when it was at its closest point to Saturn, and strengthening to between three and four times more powerful at it’s furthest.
The timing strongly suggests that tidal forces are responsible for the variation in strength of the jets. As Enceladus moves closer to Saturn, the stresses between the gravitational attraction pulling it in and the centrifugal forces pushing it out stretch the moon apart. When Enceladus is further out, these stresses are relieved. The team of scientists performing the study now believe that there is an underground ocean of water beneath the surface of Enceladus, which sprays out through the Tiger Stripes fissures under the pressure of the weight of kilometers of surface rock. As tidal forces flex the moon, the fissures open and close, varying the intensity of the streams, according to a paper published in Nature, July 2013.
“The jets of Enceladus apparently work like adjustable garden hose nozzles,” said Matt Hedman, the paper’s lead author and a Cassini team scientist based at Cornell University in Ithaca, N.Y. “The nozzles are almost closed when Enceladus is closer to Saturn and are most open when the moon is farthest away. We think this has to do with how Saturn squeezes and releases the moon with its gravity.”
The jets themselves are composed of water vapour and small ice crystals. Although the flow begins as liquid water, it simultaneously freezes and boils when it hits the vacuum of space. “The way the jets react so responsively to changing stresses on Enceladus suggests they have their origins in a large body of liquid water,” said Christophe Sotin, a co-author and Cassini team member at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Liquid water was key to the development of life on Earth, so these discoveries whet the appetite to know whether life exists everywhere water is present.”
“Cassini’s time at Saturn has shown us how active and kaleidoscopic this planet, its rings and its moons are,” said Linda Spilker, Cassini project scientist at JPL. “We’ve come a long way from the placid-looking Saturn that Galileo first spied through his telescope. We hope to learn more about the forces at work here as a microcosm for how our solar system formed.”