A new modeling study led by the University of Nantes is the first to explain several key characteristics of Saturn’s moon Enceladus observed by NASA’s Cassini spacecraft: a global ocean underneath the moon’s ice shell, internal heating, thinner ice at the south pole, and hydrothermal activity.
This graphic illustrates how water might be heated inside Enceladus. Over time, cool ocean water seeps into the moon’s porous core. Pockets of water reaching deep into the interior are warmed by contact with rock in the tidally heated interior and subsequently rise owing to the positive buoyancy, leading to further interaction with the rocks. The heat deposited at the boundary between the seafloor and ocean powers hydrothermal vents. Heat and rocky particles are transported through the ocean, triggering localized melting in the icy shell above. This leads to the formation of fissures, from which jets of water vapor and the rocky particles from the seafloor are ejected into space. In the graphic, the interior ‘slice’ is an excerpt from a new model that simulated this process. The orange glow represents the parts of the core where temperatures reach at least 194 degrees Fahrenheit. Tidal heating owing to the friction arising between particles in the porous core provides a key source of energy, but is not illustrated in this graphic. The tidal heating results primarily from the gravitational pull from Saturn. Image credit: NASA / JPL-Caltech / Space Science Institute / LPG-CNRS / University of Nantes / University of Angers / ESA.
“Although we had suspected for years that a porous core might play an important role in the mystery of Enceladus’ warm interior, this study brings together several more recent lines of evidence in a very elegant way,” said Cassini project scientist Dr. Linda Spilker, who was not involved in the study.
“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. It’s an insight that the mission needed time to build, one discovery upon another.”
In 2005, Cassini found that Enceladus sprays towering, geyser-like jets of water vapor and icy particles, including simple organics, from warm fractures near its south pole.
Additional investigation revealed the moon has a global ocean beneath its icy crust, from which the jets are venting into space.
Multiple lines of evidence indicate that hydrothermal activity is taking place on the seafloor.
One of those lines was the detection of tiny rock grains inferred to be the product of hydrothermal chemistry taking place at temperatures of at least 194 degrees Fahrenheit (90 degrees Celsius).
The amount of energy required to produce these temperatures is more than planetary researchers think could be provided by decay of radioactive elements in the interior.
“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 lead author Dr. Gaël Choblet, from the Laboratory of Planetology and Geodynamics at the University of Nantes.
The scientists found that a loose, rocky core with 20 to 30% empty space would do the trick.
Their simulations show that as Enceladus orbits Saturn, rocks in the porous core flex and rub together, generating heat.
The loose interior also allows water from the ocean to percolate deep down, where it heats up, then rises, interacting chemically with the rocks.
The models show this activity should be at a maximum at Enceladus’ poles.
Plumes of the warm, mineral-laden water gush from the seafloor and travel upward, thinning the moon’s ice shell from beneath to only half a mile to 3 miles (1 to 5 km) at the south pole. The average global thickness of the ice is thought to be about 12 to 16 miles (20 to 25 km). And this same water is then expelled into space through fractures in the ice.
“Our simulations can simultaneously explain the existence of an ocean at a global scale due to large-scale heat transport between the deep interior and the ice shell, and the concentration of activity in a relatively narrow region around the south pole, thus explaining the main features observed by Cassini,” said co-author Dr. Gabriel Tobie, also from the University of Nantes.
The efficient rock-water interactions in a porous core massaged by tidal friction could generate up to 30 GW of heat over tens of millions to billions of years, according to the study.
“Future missions capable of analyzing the organic molecules in the Enceladus plume with a higher accuracy than Cassini would be able to tell us if sustained hydrothermal conditions could have allowed life to emerge,” said ESA’s Cassini project scientist Dr. Nicolas Altobelli, who was not involved in the study.
A future mission equipped with ice-penetrating radar would also be able to constrain the ice thickness, and additional flybys — or an orbiting craft — would improve models of the interior, further verifying the presence of active hydrothermal plumes.
“We’ll be flying next-generation instruments, including ground-penetrating radar, to Jupiter’s ocean moons in the next decade with ESA’s Juice mission, which is specifically tasked with trying to understand the potential habitability of ocean worlds in the outer Solar System,” Dr. Altobelli said.
The team’s results are published in the journal Nature Astronomy.
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Gaël Choblet et al. Powering prolonged hydrothermal activity inside Enceladus. Nature Astronomy, published online November 7, 2017; doi: 10.1038/s41550-017-0289-8
