In 2005, NASA’s Cassini spacecraft spied jets of water ice and vapor erupting into space from fissures on Enceladus, evidence of a salty ocean beneath the saturnian moon’s placid icy surface. Now, the discovery of hydrogen in the water plumes suggests that although the moon is just 505 kilometers in diameter, it may harbor under its surface the chemicals and processes capable of supporting microbial life.
In 2014, with the help of a network of massive radio antenna back on Earth, Cassini confirmed the existence of a large ocean of salty water beneath the ice. This was determined based on inconsistencies measured in Enceladus's libration, or wobbling in the moon's rotation as it orbits Saturn. The heat keeping the ocean liquid, scientists believe, comes from the stretching and pulling Enceladus experiences from Saturn’s gravity, a phenomenon known as tidal heating.
Once they knew there was a large ocean and likely a rocky core, NASA's Cassini team speculated that if the moon is warm enough for liquid water, then it might have enough geologic activity for hydrothermal vents. This week, the team announced that an abundance of hydrogen gas (H2) was detected during a final flyby in 2015, when Cassini flew deeper into the geysers than it ever had before, skimming just 48 kilometers above the surface.
In previous passes through the plumes, the Cassini team had already found organic molecules like methane, carbon dioxide and ammonia, while in 2015, they discovered tiny glass beads. These particles were found when they studied Saturn’s E ring, which is known to be fed by Enceladus’s plumes, and could have only come from hydrothermal reactions. The even more tantalizing discovery of hydogen gas further adds to the chain of evidence for the existence of hydrothermal vents on Enceladus's ocean floor.
These hydrothermal vents are fissures in the rocky core of Enceladus. They release water that has been heated by geothermal activity, and the jets of hot water carry minerals and nutrients that could support life. On Earth, when the silica grains of a hydrothermal vent condense upon hitting the colder water, we can see it as a white plume of material called a white smoker. Compounds associated with these white smokers, such as an excess of hydrogen and methane, were also detected by the Cassini spacecraft.
To analyze samples from Enceladus's geysers, Cassini used two science instruments, the Cosmic Dust Analyzer (CDA) and Ion and Neutral Mass Spectrometer (INMS).
"You have the tiny nano-silica grains detected by the Cosmic Dust Analyzer that had come out with some of the ice grains in the plumes," says Cassini project scientist Linda Spilker. "These nano-silica grains could only form in water that's near the boiling point. So we think that the water goes into the seafloor, into the rocky core of Enceladus, it gets heated up, picks up things like silica, and then as the water comes back out and hits the cold water, those silica grains condense."
The results of the 2015 flyby also suggest that the geysers contain a surprising ratio of molecular hydrogen, carbon dioxide and methane. The molecules are in “thermodynamic disequilibrium,” the researchers say; that is, they're chemically out of whack. Hydrogen is a very volatile gas, and is not easily trapped on a small, icy world like Enceladus. Its presence in the geyser plume indicates that there are processes beneath the surface constantly replenishing the supply of molecular hydrogen.
The most likely explanation is a process called serpentinization. As hot water from Enceladus's ocean flows through cracks in the seafloor, it reacts with the iron-rich rock to form molecular hydrogen, a compound made of two hydrogen atoms. This exact phenomenon is known to happen around the hydrothermal vents in Earth's oceans, where it fuels a thriving community of microbes, algae, tube worms, crabs, and even fish.
The hydrothermal vents are surrounded by total darkness, so instead of deriving energy from the light of the sun, these creatures power themselves by getting hydrogen to react with carbon dioxide to form methane, a process called methanogenesis. Methanogenesis is one of the oldest metabolic processes on the planet, predating photosynthesis. A similar alien ecosystem might exist on the seafloor of Enceladus, which makes Enceladus the top candidate for hosting life elsewhere in the solar system—besting even Jupiter’s Europa, another icy moon with an ocean.
“We didn’t see microbes,” says Hunter Waite, a planetary scientist at the Southwest Research Institute in San Antonio, Texas, and the lead author of the study. “But we saw their food. We're going to have to go back with new missions and more focused instrumentation.”
Cassini won't have any more opportunities to sample the geyser plumes. After orbiting Saturn for more than a decade, the spacecraft is scheduled to start dives between the planet and its rings next week. In September, Cassini will plunge straight into Saturn, burning up almost as soon as it hits the gas giant's atmosphere.
It's Enceladus's fault that Cassini must die — NASA doesn't want to risk the spacecraft inadvertently contaminating the moon, so they cannot leave it hanging out in space after it runs out of fuel. After all, one of the most intriguing questions to be answered would be whether life on Enceladus, if it exists, is based on a form of DNA or something else entirely.
Fortunately, Enceladus no longer appears to be the only ocean world spitting its contents into space. Europa, one of Jupiter's four Galilean moons and slightly smaller than Earth's Moon, also has geysers spewing from an icy crust. The jets are so powerful that they extend 50 kilometers above the moon's surface.
Since NASA is currently pinning its hopes for life on Europa, work has begun on the Europa Clipper mission. Set for a launch in the 2020s (around 2022), the $2 billion spacecraft is being developed to study the icy moon through a lander and a series of flybys while in orbit around Jupiter.
When the Clipper arrives in the mid-2020s, it will carry instruments specifically designed to sample Europa's plumes and test for organic molecules. Unlike Cassini, which had no idea what it would encounter when it detected Enceladus's geysers for the first time, the spacecraft should be well-equipped to detect life — if there's any life to be found.
We now know Enceladus has not only a salty ocean but also direct evidence of hydrothermal vents and large geysers that provide easy access to samples from the moon's seafloor. Maybe we should soon return to Saturn's small moon, follow up on Cassini's discoveries, and perhaps answer the question: Are we alone?
Image: Enceladus to scale (NASA/JPL/Space Science Institute)