Tracing Hydrothermal Alteration and Chemical Evolution of Aromatic and Heterocyclic Compounds in Enceladus Ice Grains

Saturn’s geologically-active, icy moon Enceladus possesses a subsurface liquid water ocean beneath its icy crust, which is in contact with an unconsolidated rocky core. The ocean water percolates through the core where hydrothermal reactions potentially produce a multitude of organic compounds. Originating deep within the core, chemical species may rise from the ocean floor to the surface and are ejected into space in a plume of gas and ice grains through vents in the icy crust. Thus, markers of chemical or even biological processes occurring in the ocean or at larger depth could be mirrored in the plume composition and detected via spacecraft performing flybys through the plume. In this way, the Cosmic Dust Analyser (CDA; [1]) and the Ion and Neutral Mass Spectrometer (INMS, [2]) onboard the Cassini spacecraft sampled Enceladean material. A variety of organic and inorganic compounds, including salts and silica, and most of the  bioessential CHNOP(S) elements have been found in the erupted plume material [3,4,5,6,7,8,9]. The organic fraction of the compounds detected by CDA ranges from simple to complex, and aromatic rings represent a significant component of their molecular structures.

The analysis of ice grains from Enceladus [3,4], and returned samples from the carbonaceous asteroids Bennu and Ryugu [10,11,12], have revealed the presence of diverse organic compounds, including aromatic species, amines, and heterocyclic molecules. Aromatic compounds are amongst the most abundant and stable organic compounds in the universe and are thought to be involved in subsurface reaction chemistry on Enceladus [3]. Additionally, the polycyclic aromatic hydrocarbon (PAH) world hypothesis, aromatic groups may have acted as stabilising compounds within early pre-cellular container elements before the evolution of modern lipid membranes [13].  The identification of nitrogen-bearing heterocyclic compounds (i.e. nucleobases) in chondritic material from Bennu and Ryugu [10,11,12], with a more tentative detection of N-heterocycles in Enceladus plume ice grains [3], further highlights the importance of investigating the origin and chemical evolution of aromatic and heterocyclic compounds in extraterrestrial aqueous environments.

As part of the ERC-CoG Analogue Icy Moon Simulations (AIMS) project, we simulate high-temperature and -pressure processes under conditions though to be found at Enceladus’ hydrothermal seafloor, and investigate the alteration of organic compounds in such conditions [14]. This project will advance our understanding of the synthesis, degradation, alteration, and evolution of organic, inorganic and biogenic material in the ocean before ejection into the plume, which is vital for missions aiming to detect biosignatures. In this work, we present the latest updates on the organic composition of Enceladean ice grains detected by Cassini [3], as well as results from our new hydrothermal simulations using aromatic and heterocyclic compounds. This allows not only further constraint of Cassini data interpretation, but will also provide feedback for future missions visiting Enceladus, e.g. the future ESA’s L4 mission alongside the ongoing  ESA’s JUICE and NASA’s Europa-Clipper missions to Europa.

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