Cassini proves complex chemistry in Enceladus ocean

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Scientists digging through data collected by the Cassini spacecraft have found new complex organic molecules spewing from Saturn’s moon Enceladus. This is a clear sign that complex chemical reactions are taking place within its underground ocean. Some of these reactions could be part of chains that lead to even more complex, potentially biologically relevant molecules.

This artist’s impression shows Saturn’s icy moon Enceladus. With a frozen surface covering a deep ocean, Enceladus is a fascinating target to search for signs of habitability elsewhere in our Solar System. Jets of water burst from cracks close to the moon’s south pole, shooting ice grains into space. These ice grains carry organic molecules from Enceladus’s underground ocean, some of which are complex enough to reveal that advanced chemistry is taking place inside this ice world. Some of Enceladus’s secrets were already revealed by the NASA/ESA/ASI Cassini-Huygens mission. ESA is now studying a dedicated mission to Enceladus. Planned for launch in the 2040s, the mission would fly through the jets and even land on the moon’s south polar terrain to collect samples. Image credit: Graphic composition: ESA; Surface: NASA/JPL-Caltech/Space Science Institute/Lunar and Planetary Institute

Published today in Nature Astronomy, this discovery further strengthens the case for a dedicated European Space Agency (ESA) mission to orbit and land on Enceladus.

In 2005, Cassini found the first evidence that Enceladus has a hidden ocean beneath its icy surface. Jets of water burst from cracks close to the moon’s south pole, shooting ice grains into space. Smaller than grains of sand, some of the tiny pieces of ice fall back onto the moon’s surface, whilst others escape and form a ring around Saturn that traces Enceladus’s orbit.

Lead author Nozair Khawaja explains what we already knew: “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.

The ice grains in the ring can be hundreds of years old. As they have aged, they may have been ‘weathered’ and therefore altered by intense space radiation. Scientists wanted to investigate fresh grains ejected much more recently to get a better idea of what exactly is going on in Enceladus’s ocean.

Saturn’s E ring is a huge, fuzzy, donut-shaped circle of particles around Saturn, that puzzled scientists for a long time. Thanks to the Cassini-Huygens mission, scientists realised that the ring is actually created by particles left behind in the wake of Saturn’s icy moon Enceladus. This view, taken by Cassini on 15 September 2006, at a distance of about 2 million km from Enceladus, shows a plume emanating from the moon’s south polar region. This plume is created as water from Enceladus’s underground ocean spews out of cracks in its icy surface. Image credit: NASA/JPL/Space Science Institute

Fortunately, we already had the data. Back in 2008, Cassini flew straight through the icy spray. Pristine grains ejected only minutes before hit the spacecraft’s Cosmic Dust Analyzer (CDA) instrument at about 18 km/s. These were not only the freshest ice grains Cassini had ever detected, but also the fastest.

The speed mattered. Nozair explains why: “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.”

It took years to build up knowledge from previous flybys and then apply it to decipher this data. But now, Nozair’s team has revealed what kind of molecules were present inside the fresh ice grains.

They saw that certain organic molecules that had already been found distributed in the E ring were also present in the fresh ice grains. This confirms that they are created within Enceladus’s ocean.

They also found totally new molecules that had never been seen before in ice grains from Enceladus. For the chemists reading, the newly detected molecular fragments included aliphatic, (hetero)cyclic ester/alkenes, ethers/ethyl and, tentatively, nitrogen- and oxygen-bearing compounds.

“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,” says Nozair. “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.”

Co-author Frank Postberg adds: “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.”

Nicolas Altobelli, ESA Cassini project scientist adds: “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.”

The inside of Enceladus

Returning to Enceladus
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Discoveries from Cassini are valuable for planning a future ESA mission dedicated to Enceladus. Studies for this ambitious mission have already begun. The plan is to fly through the jets and even land on the moon’s south polar terrain to collect samples.

A team of scientists and engineers is already considering the selection of modern scientific instruments that the spacecraft would carry. This latest result made using CDA will help guide that decision.

Concept for ESA’s next mission to orbit and land on Enceladus. Image credit: ESA

Enceladus ticks all the boxes to be a habitable environment that could support life: the presence of liquid water, a source of energy, a specific set of chemical elements and complex organic molecules. A mission that takes measurements directly from the moon’s surface, seeking out signs of life, would offer Europe a front seat in Solar System science.

“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,” says Nozair.

Citation
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The study Detection of Organic Compounds in Freshly Ejected Ice Grains from Enceladus’s Ocean by N. Khawaja et al. is published today in Nature Astronomy. DOI: 10.1038/s41550-025-02655-y

Lead author Nozair Khawaja conducted the research at Freie Universität Berlin and the University of Stuttgart, both in Germany. Frank Postberg is also affiliated with Freie Universität Berlin.

Cassini-Huygens was a cooperative project of NASA, ESA and the Italian Space Agency. It comprised two elements: the Cassini orbiter and the Huygens probe.

Cassini’s Cosmic Dust Analyzer (CDA) was led by the University of Stuttgart in Germany.

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