Mysterious silica ejected in large quantities from Saturn’s icy moon Enceladus is powerful new evidence pointing to heat vents on the floor of a global ocean.
According to a new analytical model, internal heating from the moon’s core creates ocean currents that transport the silica particles, which are ejected from deep-sea hydrothermal vents that also release heat into the surrounding water.
It’s a tantalizing discovery that teases a real possibility for the existence of life, deep beneath an alien ocean on an alien world.
As the Cassini probe spent its years orbiting and studying Saturn, it made a surprising discovery. The planet’s E ring – the second outermost of the extensive ring system – has a composition rich in microscopic grains of silica, along with the ice of water, ammonia and carbon dioxide.
We have also detected silica particles coming from Enceladus in the form of icy plumes erupting from cracks in the moon’s thick icy shell; scientists have determined that the E ring’s composition is supplied by Enceladus, from its rocky core. And the chemistry and size of the grains suggest high temperatures.
But how silica gets from Enceladus’ core, up through the deep global ocean, to be ejected through the ice in plumes has been something of a mystery.
Enceladus is a bit of a wonder. The Moon is covered with a thick shell of ice, averaging between 18 and 22 kilometers (11 and 14 miles) in thickness. But Saturn’s orbit is not perfectly round, but elliptical, meaning that the distance from the planet varies – as does the gravitational force between them. This varying gravity stretches and compresses Enceladus, creating heating at the core.
Beneath the icy shell therefore lies a global liquid ocean over 10 kilometers deep, and the heat coming from the core prevents the water from freezing. This also raises the possibility of hydrothermal vents, cracks in the ocean floor where heat escapes from the moon’s interior.
Previous research suggested that the heat from Enceladus’ interior should generate vertical convection currents in the ocean, similar to those seen on Earth. Now, a team of planetary scientists led by Ashley Schoenfeld of the University of California, Los Angeles, has created a model involving these flows to try to understand silica transport on Enceladus.
“It’s like boiling a pot on a stove. Tidal friction adds heat to the ocean and causes upwelling currents of warm water,” explains Schoenfeld.
“What our study shows is that these currents are strong enough to pick up materials from the ocean floor and bring them to the ice shell that separates the ocean from the vacuum of space. The tiger stripe fractures that cut through the ice shell into this subsurface ocean can act as direct conduits for trapped materials that can be ejected into space. Enceladus is giving us free samples of what’s hidden deep below.”
The implications are quite intriguing. As previous research has found, the silica and other materials detected by Cassini in Enceladus’ clouds are consistent with what can be found in and around hydrothermal vents.
Here on Earth, hydrothermal vents are crawling with life, even far beyond the reach of sunlight. Entire ecosystems thrive on a chemosynthetic food web, utilizing chemical reactions of elements interacting at high temperatures to produce energy, rather than the more common photosynthetic processes that rely on sunlight.
This has led astrobiologists to hypothesize that icy moons like Enceladus may harbor life, even though they are far from the sun and the ocean floor receives no life-giving sunlight whatsoever.
The new study adds to a growing body of evidence that suggests there are hydrothermal vents on Enceladus, and if there is life there, we might be able to detect it without having to try to penetrate the ice. An orbiter or lander – several of which are under consideration – might be able to find biomolecules right there on the moon’s icy surface.
“Our model,” says planetary scientist Emily Hawkins of Loyola Marymount University, “gives further support to the idea that convective turbulence in the ocean efficiently transports important nutrients from the ocean floor to the ice shell.”
And isn’t that an exciting thought? We may have to change the name to Enticingeladus.
The research is published in Communication Earth and the environment.