The intensity of the jets of water ice and organic molecules that shoot out from Saturn's moon Enceladus depends on the moon's proximity to the planet, according to data obtained by NASA's Cassini spacecraft. The finding, detailed in the journal Nature this week, is the first clear observation that shows the Enceladus plume varies in a predictable manner.
Robert Brown led the University of Arizona team at the Lunar and Planetary Laboratory that controls operation of the visual and infrared mapping spectrometer, or VIMS, instrument aboard the spacecraft Cassini. The team directed the observations taken by the VIMS instrument, collected the raw data and provided data to other scientists associated with the Cassini mission.
"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 finding adds to evidence that a liquid water reservoir or ocean lurks under the icy surface of the moon.
"The way that 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, which has been in orbit around Saturn since 2004, discovered the geysers that form a bright plume emanating from Enceladus's south pole in 2005. The water ice and organic material, including hydrocarbons, nitrogen, and possibly phosphorous, spray out from several narrow fissures nicknamed "tiger stripes."
Scientists had hypothesized for years that the intensity of the jets likely varied over time, but no one had been able to show that they changed in a recognizable pattern. Hedman and colleagues were able to see the changes by examining infrared data of the plume as a whole, obtained by Cassini's VIMS instrument, and looking at data gathered over a long period of time.
The UA-operated VIMS instrument, which has analyzed a wide range of data including the hydrocarbon composition of the surface of another Saturnian moon, Titan, and the seismological signs of Saturn's vibrations in its rings, collected more than 200 images of the Enceladus plume from 2005 to 2012.
The data provided by the UA scientists show that the plume was dimmest when the moon was at the closest point in its orbit to Saturn. It lit up relatively gradually to about three to four times in brightness when Enceladus was at the most distant point. This is comparable to moving from a dim hallway to a brightly lit office.
The Cassini team put this data with previous models that suggested that the tiger stripes change shape based on the different ways Saturn's gravity puts stress on them, to deduce that it must be the squeezing and relaxation of the moon that prevents or allows the spray to escape.
"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 the Jet Propulsion Laboratory. "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."
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, manages the mission for NASA's Science Mission Directorate.