Scientists using data from NASA’s Cassini orbiter have found evidence of active hydrothermal vents gushing from the interior of Saturn’s moon Enceladus, increasing the odds that alien life could await discovery in the watery depths of that icy world. Enceladus’s hydrothermal vents appear remarkably similar to some vents found on Earth.
One of the leading theories for the origin of life on Earth postulates that it began in hydrothermal vents at the bottom of the ocean, where seawater percolating through hot rocks created energy- and nutrient-rich environments favoring the formation of the first cells. Today, Earth’s active hydrothermal vents are seafloor oases, harboring ecosystems that flourish in the darkness, isolated from the surface world. Find someplace else beyond Earth where hot rock and water intermingle, and even if it’s far from the sun life might flourish there, too. Such systems may have been common early in the solar system’s history, when rocky planets and icy moons were still relatively hot and wet from their initial formation. But until now scientists had no evidence of ongoing hydrothermal activity anywhere beyond Earth.
That evidence remains circumstantial, and somewhat tentative, despite slowly building up over the past decade. In 2005, Cassini spied plumes of water vapor gushing from mysterious warm fissures near Enceladus’s south pole. In subsequent flybys of the moon—including several that plunged through the plumes—the spacecraft all but confirmed that a 10-kilometer-deep ocean exists beneath 30 to 40 kilometers of ice around the south pole. The vapor Cassini encountered while diving through the plumes was salty, like seawater, and the spacecraft measured small variations in Enceladus’s gravitational field suggestive of an ocean lying directly atop the moon’s rocky core.
But no one knew how this ocean was connected to the water-gushing surface fissures, and many researchers thought the moon’s core would be too cold to sustain hydrothermal activity. Roughly the size of England, Enceladus is a relative runt as far as icy moons go, lacking enough mass to retain the heat of its formation or to have large quantities of heat-generating radioactive elements. By these basic estimates, the moon should be frozen solid. Instead, much of Enceladus’s ocean-sustaining heat is thought to come from its orbit around Saturn. As the moon circles the ringed planet, gravitational interactions between the two bodies cause Enceladus’s interior to flex, generating heat through tidal friction.
Perhaps, some scientists posited, the moon’s plumes and even its whole ocean were just transient phenomena created by minor variations in Enceladus’s orbit—momentary sparks of warmth and activity on an otherwise inert and frozen world. Others supposed Enceladus’s ocean and its plumes could be ancient, persistent features of the moon, boosting the chances for finding life there.
In fact, astronomers had data to help solve some of these mysteries in-hand even before Cassini stumbled across Enceladus’s ocean. In January 2004, as it was approaching the Saturnian system through interplanetary space, Cassini’s instruments registered the spacecraft flying through a diffuse shower of nanometer-scale dust particles that had somehow been ejected from the system. Subsequent encounters with the dust as well as modeling work suggested this material came from icy particles confined in the planet’s E ring, a tenuous torus of material fed by Enceladus’s plumes. In the new study, published this week in Nature, the planetary scientist Sean Hsu of the University of Colorado in Boulder worked with an international team to trace the origins and dynamics of this dust through laboratory experiments, computer modeling and more detailed analysis of the original Cassini data. (Scientific American is part of Nature Publishing Group.)
Previous analyses of the Cassini data showed that the dust particles are mostly made of silicon. Hsu and his colleagues argue that the silicon-rich dust is specifically silica—the main constituent of quartz—rather than pure silicon or silicon carbide, which are thought to be more difficult for a moon like Enceladus to produce. There are only two ways to make such small particles of silica – “top-down,” via grinding collisions of larger grains, or “bottom-up” through some microscopic chemical reaction. The silica dust particles Cassini encountered all appear to be between two and eight nanometers in size, a distribution so narrow it essentially rules out top-down formation. Assuming the particles Cassini observed are made of silica, the only plausible bottom-up source is Enceladus’s rocky core, where silica could be leached out by seawater and then vented to the surface.
Hsu acknowledges that other bottom-up formation methods exist, but notes they only work under well-controlled laboratory conditions. “So unless there is something really bizarre happening, we think our interpretation is solid,” he says.
In a series of months-long laboratory experiments designed to simulate the plausible interior conditions of Enceladus, Hsu and his colleagues were only able to make similarly sized silica particles through very specific thermal and chemical conditions. Extrapolated to Enceladus’s interior, the experiments suggest the moon’s core-ocean interface must be nearly hot enough to boil water, and that this water is slightly saltier and more alkaline than Earth’s oceans. Once leached from the rocky core, silica would crystallize in seconds out of the enriched water near the ocean floor, forming nanoparticles that then flow upward with the warm, convecting fluid to reach the surface fissures in a matter of several months to a few years.
If all this is true, then the dusty, icy plumes of Enceladus are not a superficial surface phenomenon, but rather a deep expression of processes taking place throughout the entire moon. Cassini is scheduled for three more encounters with Enceladus, including one final plunge through a plume, before it is sent to a fiery death in Saturn’s atmosphere to avoid crashing into and contaminating the ringed planet's icy moons. Probing the plumes to find out more about prospects for life on Enceladus—past and present—will be a task for future missions. NASA is presently pondering a mission in the 2020s to Jupiter’s Europa, another icy moon with a much larger and even more mysterious subsurface ocean. But the latest news from Enceladus could sway that decision—expect some scientists to soon advocate for this diminutive moon of Saturn as their preferred, disruptive dark-horse target for humanity’s next emissary into the outer solar system.
A new mission could also tackle other mysteries raised by the latest findings. The inferred alkalinity and salinity of Enceladus’s ocean aligns with Cassini’s previous measurements of the plume, but the estimated temperature of its core is a surprise. Even with substantial tidal friction, such sustained high temperatures are hard to explain, given that the cold overlying ocean could efficiently quench much of the core’s heat. The most likely scenario is that the core is in fact fractured and porous, with its heat supplied by some combination of tidal friction and serpentinization, a heat-generating chemical reaction between water and rock. That is, Enceladus’s sizzling core may actually be a bit like a broken heart, kept alive by tidal forces continually pumping seawater through its fractured veins. But only further observations of the moon can test this idea.
According to the dynamical models of Hsu and his collaborators, after the dust particles erupt with water in the plumes, they freeze into ice grains, and the fastest of them escape Enceladus’s gravity to reach the E ring. There they linger for years before collisions with plasma ions trapped in Saturn’s powerful magnetic field scatter them into interplanetary space. When Cassini detected the ejected dust on the outskirts of Saturn, it was really seeing what Hsu calls “the silicon footprint of Enceladus.” From there, he says, the particles may fly on the solar wind to “become interstellar dust—the silicon footprint of our solar system.”
Enceladus’s silicon footprint, Hsu says, shows that dust deserves consideration alongside light itself as one of astronomy’s most vital tools. Someday, he says, we may have a “dust telescope” on a space-based “dust observatory,” gathering scattered flecks of flotsam and tracing them back mind-bogglingly far to other worlds and times.