Saturn’s moon Enceladus continuously ejects a plume of ice grains and gases originating from its subsurface ocean via fractures near its south pole. Using data from NASA’s Cassini spacecraft, a team of planetary researchers at the University of Stuttgart and Freie Universität Berlin has now chemically analyzed freshly emitted particles originating directly from Enceladus’ ocean. They were able to detect intermediates of potentially biologically relevant organic molecules — aliphatic, (hetero)cyclic ester/alkenes, ethers/ethyl and, tentatively, nitrogen- and oxygen-bearing compounds — which were thus discovered for the first time in ice particles from an ocean outside Earth.
An artist’s impression of NASA’s Cassini spacecraft flying through plumes erupting from the south pole of Enceladus; these plumes are much like geysers and expel a combination of water vapor, ice grains, salts, methane and other organic molecules. Image credit: NASA / JPL-Caltech.
Enceladus measures approximately 500 km in diameter; its surface is covered by an ice shell with an average thickness of approximately 25-30 km.
In 2005, Cassini found the first evidence that Enceladus has a hidden ocean beneath the 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’ orbit.
“Cassini was detecting samples from Enceladus all the time as it flew through Saturn’s E ring,” said Freie Universität Berlin researcher Nozair Khawaja, lead author of the study.
“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.
The scientists wanted to investigate fresh grains ejected much more recently to get a better idea of what exactly is going on in Enceladus’ ocean.
Fortunately, they 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 ice grains contain not just frozen water, but also other molecules, including organics,” Dr. Khawaja said.
“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, the authors have 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’ 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.
On Earth, these same molecules are involved in the chains of chemical reactions that ultimately lead to the more complex molecules that are essential 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,” Dr. Nozair said.
“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.”
“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’ ocean,” said co-author Dr. Frank Postberg, also from Freie Universität Berlin.
The results were published this month in the journal Nature Astronomy.
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N. Khawaja et al. Detection of organic compounds in freshly ejected ice grains from Enceladus’s ocean. Nat Astron, published online October 1, 2025; doi: 10.1038/s41550-025-02655-y
