Deep beneath the icy surface of Saturn’s moon Enceladus, something’s happening that causes particles of icy silica to spew out to space. They eventually end up in Saturn’s E ring. Planetary scientists knew that this was happening, but didn’t have a good explanation for why or how. Now, they do.
A new study done by a team at the University of California Los Angeles offers some answers. Their work shows that tidal heating in Enceladus’ rocky core creates currents (or flows) that transport the silica. Then, it’s probably released by deep-sea hydrothermal vents over the course of just a few months.
“Our research shows that these flows are strong enough to pick up materials from the seafloor and bring them to the ice shell that separates the ocean from the vacuum of space,” said Ashley Schoenfeld, a doctoral student at UCLA. “The tiger-stripe fractures that cut through the ice shell into this subsurface ocean can act as direct conduits for captured materials to be flung into space. Enceladus is giving us free samples of what’s hidden deep below.”
Why Enceladus Spews its Secrets
Data from the missions that visited the Saturn system keep revealing surprises about the Saturnian moons. We now know that Enceladus is an ocean world, for example. That’s because it has a large volume of liquid water mostly locked away beneath the icy surface. The surface itself is extremely reflective and has cracks that allow fountains of icy particles to escape into space. Those so-called “tiger stripes” provide an exit point for the icy silica.
Those particles start out on the sea floor deep beneath the surface. The tidal forces that squeeze Enceladus under the grip of Saturn’s strong gravity deform this little moon. That creates friction in the surface as well as the rocky core underneath the ocean. That sets up currents in the watery ocean.
Hydrothermal Activity Plays a Role Inside Enceladus
Although the Voyager mission first revealed the strange surface of Enceladus, it wasn’t until Cassini made its long-term studies that planetary scientists found the plumes jetting out from the tiger stripes. The spacecraft measured large amounts of hydrogen gas in those plumes. That’s pretty strong evidence of hydrothermal activity on the ocean floor.
Hydrothermal heating on Earth happens near volcanically active places beneath the sea, particularly in mid-ocean ridges. Those are where tectonic plates are spreading apart. That action allows volcanic material to spew up from beneath and superheat the water. On Enceladus, the friction caused by tidal heating creates hotspots that feed the currents carrying silica particles to space.
The UCLA team led by Schoenfeld created a model to simulate that process. It also allowed them to estimate a timeframe for it. Their model also explains why the currents are transporting other materials to the surface in addition to the silica. “Our model provides further support to the idea that convective turbulence in the ocean efficiently transports vital nutrients from the seafloor to ice shell,” said second author Emily Hawkins, a UCLA alumna who is now an assistant professor of physics at Loyola Marymount University.
Of course, the presence of heat and water raises the question of whether Enceladus is hospitable to life. On Earth, hydrothermal vents support an amazing variety of life forms. It remains for future missions to Enceladus to pin that down. They could study places inside that moon to see if it could support life. Those efforts would require landers to gather more information both on the ice and deep in the subsurface ocean.
For More Information
UCLA-led study explains how one of Saturn’s moons ejects particles from oceans beneath its surface
Particle entrainment and rotating convection in Enceladus’ ocean
I think the gist is this, which also notes that observed particle sizes are predicted by the new model:
“The new results we present for transport times through Enceladus’
ocean have important differences from predictions made by other
models. Reference 10 use the precipitation chemistry of silica to
estimate transport times through and out of the ocean; the detected
nano-silica particles with radii <10 nm imply fast and continuous
upward transport of hydrothermal products, likely within months
to years. … Ocean circulation models
similarly predict vertical flow speeds of a few mm s?1 when driven
by thermal convection, strongly constrained by rotation across
the entire ocean domain28, again similar to our estimates. Our
results imply that relatively fresh (i.e., more recently expelled)
materials from hydrothermal vents (and possible biosignatures) are
capable of being transported directly from the core to the south
polar plumes and can be sampled and analyzed with a suitable
instrument suite."
Re “Of course, the presence of heat and water raises the question of whether Enceladus is hospitable to life. On Earth, hydrothermal vents support an amazing variety of life forms.”
Remember that hydrothermal vents will only give habitable conditions for chemotrophs, so prokaryote analogs. Eukaryotes around these vents may rely on them for most nutrients, but their metabolism is still mitochondrial bound and the ocean oxygen they use derives from oxygenating photosynthesizers dependent on insolation (and today, those oxygen sources are dominated by land plants).