A new analysis of Cassini spacecraft data points to the distinct possibility that Saturn's icy moon Enceladus exhibits hydrothermal activity on its seafloor. It's a discovery that significantly bolsters the moon's status as a potentially habitable object.
Enceladus has been an intriguing object of study ever since plumes of escaping water vapor were discovered in 2005. A year ago, gravity measurements were used to confirm that a large reservoir of liquid water exists underneath the moon's icy crust. What's more, astronomers have learned that Enceladus is a differentiated body consisting of an external icy layer and an internal rocky core comprised of silicates. Taken together, these factors make Enceladus an excellent candidate for astrobiological study.
Now, with the discovery of present-day hot water activity on the moon's seafloor, its status as a potentially habitable object just went up a significant notch.
To make the discovery, mission scientists used Cassini's Cosmic Dust Analyzer to identify clusters of tiny dust grains in orbit around Saturn. These dust grains, which measure a mere 2-to-8 nanometers across, are rich in silicon, thus distinguishing it from the water-ice particles that dominates Saturn and majestic ring system, especially its E-ring.
After conducting lab experiments and computer simulations, the scientists concluded that these silicon-rich grains (SiO2) originated on the seafloor of Enceladus where hydrothermal processes have been at work, both in the past and present.
At the bottom of the moon's subterranean ocean, 90 degree Celsius (194 degree F) water is dissolving minerals from the moon's rocky interior. As this heated water travels upward, it hits the cooler water, causing the minerals to condense out and form nano-grains of "silica" floating in the water. These bits of grains spend a few months to several years rising to the surface (otherwise they'd get too big), where it eventually joins larger ice grains in the vents that connect the ocean to the surface of the moon. From there, they get ejected into space via the moon's geysers. Once in space, the ice grains erode, freeing the tiny rocky inclusions.
The precise mechanics behind the hydrothermal energy process is not well understood, but it likely arises from a combination of tidal heating, and owing to radioactive decay in the core and other chemical reactions.
"It's very exciting that we can use these tiny grains of rock, spewed into space by geysers, to tell us about conditions on — and beneath — the ocean floor of an icy moon," noted lead author Sean Hsu, a postdoctoral researcher at the University of Colorado at Boulder, in an ESA report.
Here on Earth, grains of silica can be found in sand and the mineral quartz. Geologists know that small silica grains form through hydrothermal activity involving a strict set of conditions. For example, these grains can form when slightly alkaline water with modest salt content and super-saturated silica experiences a significant drop in temperature.
"We methodically searched for alternative explanations for the nanosilica grains, but every new result pointed to a single, most likely origin," noted Frank Postberg, a Cassini Cosmic Dust Analyser scientist at the University of Heidelberg in Germany, and a co-author on the paper. The researchers were helped by a team in Tokyo whose lab experiments verified the conditions under which silica grains form at the same size detected by the Cassini spacecraft.
"These findings add to the possibility that Enceladus, which contains a subsurface ocean and displays remarkable geologic activity, could contain environments suitable for living organisms," added NASA's John Grunsfeld in a statement. "The locations in our solar system where extreme environments occur in which life might exist may bring us closer to answering the question: are we alone in the Universe?"
Excitingly, Cassini's gravity measurements from last year suggest that Enceladus's rocky core is quite porous, which would allow the ocean water to percolate into the interior. This means that the moon has a huge surface area where rock and water can interact. Fascinatingly, much of this hot-water chemistry can occur inside the moon's core, and not just at the seafloor.
"This moon has all the ingredients – water, heat, and minerals – to support habitability in the outer Solar System, confirming the astrobiological potential of Enceladus," said Nicolas Altobelli, ESA's Cassini project scientist. "Enceladus may even represent a very common habitat in the Galaxy: icy moons around giant gas planets, located well beyond the 'habitable zone' of a star, but still able to maintain liquid water below their icy surface."
Read the scientific study here.