Saturn’s moon Enceladus has become a focal point of scientific inquiry due to the discovery of massive plumes of water vapor and ice erupting from its southern polar region. These jets, venting material from beneath the icy shell, suggest the presence of a liquid water ocean. This finding has transformed our understanding of the moon, positioning it as a primary target in the search for life beyond Earth.
Discovery of Enceladus’ Water Jets
The first definitive observations came from NASA’s Cassini spacecraft in 2005, which imaged plumes of icy particles rising from long, parallel fissures in the south polar region known as “tiger stripes.” These eruptions are more like geysers than volcanoes, blasting materials into space at over 2,000 kilometers per hour. The ejected material is the primary source of Saturn’s vast E Ring, a faint ring in which Enceladus orbits.
The Cassini mission made several close flybys, at times passing directly through the plumes to analyze their composition. Instruments like the Ion and Neutral Mass Spectrometer (INMS) and the Cosmic Dust Analyzer (CDA) provided detailed data on the chemical makeup of the ejected material. These direct measurements offered a glimpse into the environment hidden beneath the moon’s frozen surface.
Composition of the Plumes
Analysis of the plumes provides a chemical inventory of the moon’s interior. Cassini’s instruments detected various salts, including sodium chloride, indicating the water has been in contact with a rocky core. This confirmed the source is a large body of liquid saltwater, not just isolated pockets of melting ice.
The plumes also contain simple organic molecules, which are carbon-based compounds. These include abundant methane, carbon dioxide, ammonia, and hydrogen cyanide, a molecule that can be a precursor for synthesizing amino acids. More recently, analysis identified sodium phosphates in ice grains, marking the first time this element for life has been found in an ocean beyond Earth.
Further analysis revealed molecular hydrogen. This gas is thought to be produced by hydrothermal reactions between hot water and rock on the floor of the hidden ocean, similar to processes at hydrothermal vents on Earth.
The Hidden Ocean Source
The existence of a global, subsurface ocean is supported by multiple lines of evidence. While plume observations hinted at a liquid reservoir, gravity measurements from Cassini provided definitive proof. Variations in the moon’s gravitational pull revealed that the icy outer shell is not frozen to the core, indicating a global ocean between the crust and rocky interior.
Evidence for ongoing hydrothermal activity at the bottom of this ocean comes from silica nanoparticles found in the plumes. These tiny rock grains form when hot, mineral-rich water from the core mixes with colder ocean water, requiring temperatures of at least 90°C. This points to hydrothermal vents on the seafloor, similar to those on Earth.
The plumes are likely driven by tidal forces from Saturn. As Enceladus orbits the planet, gravitational flexing creates heat, keeping the ocean liquid and opening the tiger stripe fissures. This action pressurizes the ocean water, forcing it up through the cracks and into space.
Implications for Life Beyond Earth
Enceladus’ plumes have significant implications for the search for life, as they offer a direct sample of a subsurface ocean. This allows study of a potentially habitable environment without needing to drill through kilometers of ice, making Enceladus a high-priority target for future missions. The necessary ingredients for life have all been detected: liquid water, a source of energy, and the right chemical building blocks.
The inferred hydrothermal activity offers a source of chemical energy independent of sunlight. This energy could fuel microbial metabolism, much like the ecosystems found around hydrothermal vents on Earth’s ocean floor. While Cassini was not equipped to detect life, its findings have laid the groundwork for future missions designed for that purpose.