Organic Molecules from Enceladus's Subsurface Ocean

An international research team has analysed data from the Cosmic Dust Analyser (CDA) instrument aboard the Cassini spacecraft, obtained during its flyby of Saturn's moon Enceladus. Fresh ice particles ejected from the subsurface ocean were found to contain organic compounds — including aromatic, ether, ester, and alkyl compounds, as well as compounds containing nitrogen and oxygen. This discovery strengthens the hypothesis of hydrothermal activity on the ocean floor and Enceladus's potential habitability.

How the Research Was Conducted

The icy gas jets erupting from fractures near Enceladus's south pole originate from a global subsurface ocean.
During the Cassini–Huygens mission (2004–2017), the probe repeatedly flew through these jets, analysing the composition of microscopic ice particles.

The fifth flyby (E5) in 2008 proved to be the most informative: the spacecraft passed through the plume at a speed of 17.7 km/s. This high velocity caused intense ionisation of material upon impact, revealing molecular fragments previously undetected during slower flybys (4–12 km/s).

Findings

Several types of organic structures were identified in the freshly ejected ice crystals:

• Aromatic (aryl) compounds — for example, fragments characteristic of benzene and phenyl rings.
  These stable structures are likely formed in hot hydrothermal systems on the ocean floor.
• Oxygen-containing aliphatic compounds, including aldehydes (e.g., acetaldehyde).
  These may be intermediate products in reactions leading to the formation of amino acids and fatty acids.
• Esters and alkenes — organic groups often involved in the synthesis of lipids and other biomolecules.
• Ethers and ethyl groups — compounds capable of serving as "bridges" between larger organic structures.
• Nitrogen and oxygen-containing compounds — possible derivatives of pyridine, pyrimidine, cyano, and carboxyl compounds.
  These fragments are particularly interesting as similar structures are part of the nucleotide bases in DNA and RNA.

Significance

Previously, Cassini had already found water, salts, carbon dioxide, methane, ammonia, and phosphates on Enceladus — five of the six elements vital for biochemistry (CHNOPS).
Now the presence of a wide range of organic molecules has been confirmed, including more complex ones than simple hydrocarbons.

The totality of this data points to hydrothermal processes in the moon's interior — similar to those occurring in hot springs at the bottom of Earth's oceans. There, as is known, organic synthesis is concentrated and primitive life forms can emerge.

Conclusions

• For the first time, esters, alkenes, ethers, and nitrogen-containing compounds have been detected in fresh ice particles ejected directly from Enceladus's subsurface ocean.
• These substances form inside the moon, not on its surface or within Saturn's ring.
• Their chemical composition suggests rich organic chemistry and active geochemistry, including possible hydrothermal reactions.
• These findings strengthen the argument that Enceladus's ocean is one of the most promising targets for the search for extraterrestrial life in the Solar System.

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