Cassini data proposes new explanation for Enceladus’ active ocean
A new study that incorporates various findings by NASA’s Cassini mission regarding Saturn’s moon Enceladus proposes that the moon has a porous core in which rocks flex and rub together, producing sufficient heat via friction to power its global subsurface ocean.
Based on a complex 3-D computer model, the study contradicts earlier theories that attributed Enceladus’ heat source to the tidal pull of Saturn.
While the model confirmed tidal friction from the giant planet can act as a heat source for Enceladus’ core, it alone would not be sufficient to keep the moon’s interior ocean from freezing after 30 million years.
Over recent years, Cassini scientists have suspected the possibility that a porous core could be keeping the interior of Enceladus warm.
The latest study drew on several lines of evidence, all collected by Cassini during its 13 years at the Saturn system.
“This powerful research makes use of newer details – namely that the ocean is global and has hydrothermal activity – that we just didn’t have until the past couple of years,” explained Cassini Project Scientist Linda Spilker of the Jet Propulsion Laboratory (JPL) in California. “It’s an insight that the mission needed time to build, one discovery upon another.”
Hydrothermal activity, which occurs on Earth as well as on several Solar System worlds, is the movement of subsurface heated water, which interacts with the rocks through which it passes and sometimes leaves behind mineral deposits.
In its 13 years orbiting Saturn, Cassini discovered geysers emanating from fractures near Enceladus’ south pole and determined that the geysers, which contain water vapor, icy particles, and simple organic compounds, are being vented from a global underground ocean.
The spacecraft also gathered compelling evidence of hot water interacting with rock on the ocean floor.
Cassini identified tiny grains of rock on the ocean floor with temperatures as high as 194 degrees Fahrenheit (90 degrees Celsius). The decay of interior radioactive elements alone could not produce this level of heat, scientists determined.
According to a computer simulation used in the study, the porous nature of the core, of which 20 to 30 percent is empty space, allows water to filter downward, where it is heated and subsequently rises, interacting chemically with rocks along the way. As it gushes upward, the mineral-filled water thins Enceladus’ icy shell and breaks through surface fissures, producing geysers.
The model indicates this activity would be most prominent at Enceladus’ poles, which is exactly what has been observed.
While the moon’s icy shell is estimated to have an average thickness of 12 to 16 miles (20 to 25 kilometers), at the south pole, it is just half a mile to three miles (one to five kilometers) thick.
“Where Enceladus gets the sustained power to remain active has always been a bit of a mystery, but we’ve now considered in greater detail how the structure and composition of the moon’s rocky core could play a key role in generating the necessary energy,” said Gael Choblet of the University of Nantes in France, lead author of a study on the findings published in the journal Nature Astronomy.
Having a subsurface ocean and internal heat source means Enceladus is a top potential location for extraterrestrial microbial life.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.