Located around Saturn is the icy moon Enceladus — one of the most interesting and potentially habitable moons in the solar system. NASA’s Cassini mission extensively studied Saturn and its moons from 2004 to 2017 and, in 2005, revealed large geyser-like jets extending from Enceladus’ south pole.
Later studies would suggest that a massive subsurface ocean was responsible for the jets, which are primarily comprised of water ice. Now, in a new study using archival Cassini data, a team of scientists has discovered a plethora of new complex organic molecules within these jets. The presence of the new molecules in the jets proves that complex chemical reactions are occurring in Enceladus’ subsurface ocean.
“Cassini was detecting samples from Enceladus all the time as it flew through Saturn’s E ring. We had already found many organic molecules in these ice grains, including precursors for amino acids,” explained Nozair Khawaja, the study’s lead author.
Saturn’s E ring is the faint ring of ice grains created by the ice grains spewed out of Enceladus’ south pole. The ring traces the moon’s orbit, and as the ice grains orbit Saturn, they are subjected to intense radiation from the Sun and Saturn’s magnetic field.
In this image from Cassini, Enceladus can be seen in the center, with the massive E ring extending above and below the moon. (Credit: NASA/JPL-Caltech/Space Science Institute)
However, these ice grains are hundreds of years old, and given their continuous radiation exposure, are often significantly weathered from their initial states. To better assess the processes within Enceladus’ ocean, scientists needed to investigate the ice grains immediately after their ejection from the moon’s south pole.
Fortunately, in 2008, Cassini flew directly through the plume from the south polar jets, using its Cosmic Dust Analyzer (CDA) to collect data on ice grains just minutes after their ejection. The ice grains hit the CDA at around 18 km/s. The speed at which the ice grains impacted the CDA proved to be important in Khawaja et al.’s analysis.
“The ice grains contain not just frozen water, but also other molecules, including organics. At lower impact speeds, the ice shatters, and the signal from clusters of water molecules can hide the signal from certain organic molecules. But when the ice grains hit CDA fast, water molecules don’t cluster, and we have a chance to see these previously hidden signals,” Khawaja said.
Cassini’s Cosmic Dust Analyzer. (Credit: ESA/NASA/JPL/Space Science Institute)
Several of the molecules detected by the CDA during Cassini’s 2008 plume flyby were also detected in later flybys of the E ring, confirming that the E ring’s ice grains originate from Enceladus’ subsurface ocean.
“These molecules we found in the freshly ejected material prove that the complex organic molecules Cassini detected in Saturn’s E ring are not just a product of long exposure to space, but are readily available in Enceladus’s ocean,” said Frank Postberg, a co-author of the study.
What’s more, the team found new molecules, including aliphatic esters/alkenes, heterocyclic esters/alkenes, ethers/ethyl, and nitrogen- and oxygen-bearing compounds. These complex organic molecules, when involved in chains of chemical reactions, can produce other complex molecules required for life.
“There are many possible pathways from the organic molecules we found in the Cassini data to potentially biologically relevant compounds, which enhances the likelihood that the moon is habitable. There is much more in the data that we are currently exploring, so we are looking forward to finding out more in the near future,” says Nozair.
Future missions to Enceladus are expected to further explore the moon’s subsurface ocean and habitability. The European Space Agency (ESA) announced in 2024 that Enceladus was one of the agency’s top targets for a future mission to Saturn. Studies for the mission have begun, with the spacecraft expected to land on and collect samples from the moon’s south polar region.
Enceladus hosts some of the most habitable conditions in the solar system — liquid water, a source of energy, and complex reactions and molecules. As space technology advances and current missions like ESA’s Jupiter Icy Moons Explorer (JUICE) further inform scientists on icy moons and their nature, Enceladus is becoming a clear target for humanity’s search for life outside of Earth.
“Even not finding life on Enceladus would be a huge discovery, because it raises serious questions about why life is not present in such an environment when the right conditions are there,” Khawaja said.
“It’s fantastic to see new discoveries emerging from Cassini data almost two decades after it was collected. It really showcases the long-term impact of our space missions. I look forward to comparing data from Cassini with data from ESA’s other missions to visit the icy moons of Saturn and Jupiter,” said ESA Cassini project scientist Nicolas Altobelli.
(Lead image: Enceladus’ south polar jets from Cassini. Credit: NASA/JPL-Caltech/Space Science Institute)
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