Cassini and DSN evidence suggest ocean inside Saturn’s moon Enceladus
Water, water, everywhere it seems – including the sub-surface of Enceladus, one of Saturn’s moons. Compelling evidence for an underground ocean on this remote, ice-coated world has come from NASA’s Cassini spacecraft and Deep Space Network.
With a diameter of only 314 miles (505 kilometers) Enceladus is small enough to fit inside the state lines of Arizona and is only Saturn’s sixth largest moon. Years, ago no one would have believed that it might show signs of geological activity, let alone harbor liquid water – and, perhaps, even life.
But in 2005, Cassini amazed scientists when the bus-sized spacecraft spotted water vapor spraying out of fissures near the moon’s south pole. Where the water came from, no one knew, but a vast underground reservoir of the liquid was a distinct possibility. The new data from Cassini and the Deep Space Network add powerful weight to the notion that a saltwater ocean lies concealed inside the moon and is the source of the plumes issuing from the polar region.
Tiny variations in the gravitational field of Enceladus, as well as their effect on Cassini as it flew close by, have been the key to unraveling the mystery of what may lie beneath the surface.
“The way we deduce gravity variations is a concept in physics called the Doppler effect, the same principle used with a speed-measuring radar gun,” said Sami Asmar of NASA’s Jet Propulsion Laboratory, one of the scientists involved in the study. “As the spacecraft flies by Enceladus, its velocity is perturbed by an amount that depends on variations in the gravity field that we’re trying to measure. We see the change in velocity as a change in radio frequency, received at our ground stations here, all the way across the Solar System.”
An international team of researchers, led by Luciano Less at the Sapienza University in Rome, utilized data from Cassini which the spacecraft had collected during three close flybys of Enceladus. The robotic explorer conducted these passes between April 2010 and May 2012, which brought the spacecraft within 62 miles (100 kilometers) of the surface. Thanks to the giant antennae of the Deep Space Network they were able to observe minute shifts in the frequency of radio signals coming from Cassini, indicating subtle changes in speed of as little as a few millimeters per second.
These tiny accelerations and decelerations were caused by variations in the gravitational field of the moon as a result of different densities of material under the surface. The manner in which the field varied showed that something denser than ice but less dense than rock lay far under the south pole of Enceladus. Water fit the bill perfectly as to what this substance was.
The gravity measurements suggest a body of water at least as big as Lake Superior, 19 to 25 miles (30 to 40 kilometer) below the surface and roughly 6 miles (10 kilometers) deep. Its exact extent is unknown at this time, but it may stretch at least halfway to the equator in every direction and possibly into the northern hemisphere as well.
A direct connection between the underground ocean and the plumes of water vapor that have been seen erupting from the south polar region has yet to be established. However, if it exists, it’s of great importance to those studying the prospects of astrobiology, as Linda Spilker, Cassini project scientist at JPL, points out:
“Material from Enceladus’ south polar jets contains salty water and organic molecules, the basic chemical ingredients for life.”
Additionally, because of the moon’s small size, its ocean must be more or less in contact with the rocky core, which will act as a source of elements crucial to life-as-we-know-it. These include phosphorus, sulfur, and potassium.
Within the space of a few years, Enceladus has climbed up the list of places in the Solar System that alien life hunters would like to visit, from nowhere to close to the top. Only Mars and Jupiter’s moon Europa rate higher.
Writing in the most recent edition of the journal Astrobiology, Chris McKay, based at NASA’s Ames Research Center, argues the case for an Enceladus flyby mission to capture and return samples of plume material and search for bio-molecular markers.
He stated: “With samples of the organic material from the plume, we could search in terrestrial laboratories for organic biomarkers that would be conclusive evidence for life.”
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Dr. David Darling is an astronomer and author of numerous books, including We Are Not Alone, Megacatastrophes, The Complete Book of Spaceflight, and his latest, The Rocket Man. His website, The Worlds of David Darling, is one of the largest and most visited science resources on the Internet. Darling is a renaissance man, he is a musician, noted author and journalist and serves as our science writer. Darling provides The Spaceflight Group with articles detailing what he knows best - space exploration.