Detection of Organic Compounds in Freshly Ejected Ice Grains from Enceladus’s Ocean

Saturn’s moon Enceladus ejects a plume of ice grains and gases originating from a subsurface ocean via fractures near its south pole. The chemical characterisation of organic material in such ice grains was previously conducted via the analysis of mass spectra obtained in Saturn’s E ring by Cassini’s Cosmic Dust Analyser (CDA) at impact speeds below 12 km/s. Here, we present a comprehensive chemical analysis of organic-bearing ice grains sampled directly from the plume during a Cassini flyby of Enceladus (E5) at an encounter speed of nearly 18 km/s. We again detect aryl and oxygen moieties in these fresh ice grains, as previously identified in older E ring grains. Furthermore, the unprecedented high encounter speed revealed previously unobserved molecular fragments in CDA spectra, allowing the identification of aliphatic, (hetero)cyclic ester/ alkenes, ethers/ethyl and, tentatively, N-O-bearing compounds. These freshly ejected species are derived from the Enceladus subsurface, hinting at a hydrothermal origin and involvement in geochemical pathways toward the synthesis and evolution of organics.

Main

The Saturnian moon Enceladus emits a plume of water ice grains and volatiles through surface fractures at its south pole. The Cassini-Huygens space mission conducted compositional analysis, both in situ with its mass spectrometers – the Cosmic Dust Analyser¹ (CDA) and the Ion and Neutral Mass Spectrometer² (INMS) – and with the Ultraviolet Imaging Spectrograph (UVIS)³ which acquired compositional data from plume observations, indicating an oceanic origin of this material^4-11. A global, salty subsurface ocean percolates through Enceladus’s rocky core, where hydrothermal activity is thought to occur^6,12-16. The vented material from Enceladus contains a variety of organic and inorganic species originating from the subsurface ocean^4-10,17,18. The recent identification of phosphates⁹ in the plume means that five of the six bioessential CHNOPS elements have been detected in material from Enceladus.

Cassini’s CDA recorded hundreds of thousands of in situ time-of-flight (TOF) mass spectra of ice grains in the E ring. After ejection from Enceladus’s interior, about 10 % of these grains¹⁹ settle across the E ring over days to decades at distances between about 2.5—20R_S (Saturn radii R_s = 60,330 km)^20-22. Mass spectral analysis revealed three distinct compositional types of Enceladean ice grains in the E ring^6,8,10,23: (i) Type I: almost pure water ice (ii) Type II: organic enriched and (iii) Type III: salt rich. In Type II E ring ice grains, Khawaja et al.¹⁰ found volatile, low mass (≤ 100 u), N- and O-bearing organic species as well as single-ringed aromatic compounds. In a particular Type II subtype, Postberg et al.⁸ discovered complex macromolecular fragments of refractory insoluble organic compounds with masses exceeding 200 u, with multiple aryl moieties connected to saturated and unsaturated hydrocarbons chains, alongside N- and O-bearing groups.

While the majority of previous results were inferred from relatively old E ring ice grains, flybys of Enceladus by Cassini provided a unique opportunity to sample freshly ejected grains. This offers compositional insights into ice grains immediately after ejection and ensures that the compounds detected arise from the Enceladean subsurface rather than space weathering in Saturn’s E ring²². The impact speed of ice grains significantly influences the spectral appearance of their mass spectra²⁴. While spectra of ice grains in Saturn’s E ring were mostly recorded between 4 – 12 km/s, Cassini’s E5 flyby occurred at the highest speed (17.7 km/s) of all Enceladus flybys, offering new diagnostics for the analysis of previously unseen highenergy-induced fragmentation pathways. Simultaneously, INMS recorded measurements during the E5 flyby, showing a similar correlation between flyby speed and extent of fragmentation²⁵. The absence of water cluster species – which are prevalent below 12 km/s and can mask signals arising from organic species – at such high impact velocities is advantageous. Here, we reanalyse the data from Cassini’s E5 flyby to identify specific organic species within Type II grains, from which Postberg et al.⁷ estimated relative proportions of ice grain types without detailed compositional analysis.

Results

In this E5 data set we identify certain groups of CDA mass spectra indicative of different organic compositions in freshly ejected ice grains: aromatics, O-bearing (probably carbonyl), esters/alkenes, and ethers/ethyls. Each of these groups will be discussed in the following sections using archetypal spectra as illustrative examples. CDA utilised impact ionisation, whereby (sub-)micron-sized ice grains collide with the instrument’s target at hypervelocities (3 km/s), resulting in the formation of ions (and other fragments) which generate time-offlight (TOF) mass spectra. In this study, the cation mass spectra obtained at an encounter speed of 17.7 km/s frequently exhibit features characteristic of this high impact velocity – namely [H_2,3]⁺, [C]⁺, and [Rh]⁺ ions. These rhodium and hydrogen cations originate from the material utilised for CDA’s impact target²⁶. 

In this study, laboratory electron ionisation (EI) mass spectra have been employed to facilitate spectral interpretation in accordance with Khawaja et al.²⁷ In comparison to the laser induced liquid beam ion desorption (LILBID)²⁴ that match CDA spectra at the lower spacecraft-ice grain encounter speeds obtained in the E ring, EI spectra match the observed ionic species particularly well at higher impact speeds. We select the EI mass spectra shown in this work based upon relative peak intensities of the characteristic fragments of a given species, as described in the Methods section (see also Extended Data Figure 1).  All EI spectra in this work are obtained from the National Institute of Standards and Technology (NIST), MassBank Europe, and MassBank of North America (MoNA) freely-available online databases. It should be noted that all EI spectra from these databases correspond to pure organic species. In contrast to the CDA spectra, they do not show any species from water ice nor traces of Na and K salts that are ubiquitous in Type II spectra. The proposed processes^8,10 of the formation of organic enriched ice grains suggest that each ice grain could contain more than one type of organic species. Whilst contributions from more than one type of organic may also complicate identification of chemical species, in general, the defining features of organics in ice grain mass spectra must belong to those compounds with a lower ionisation potential, or to those dominating the ice grain composition in the case of multiple compounds with similar ionisation potentials.

If present, mass spectral features at m/z 12 – 15 (C, CH⁺, CH₂⁺, CH₃⁺), 16 – 19 (O⁺, HO⁺, H₂O⁺, H₃O⁺), 23 (Na⁺), 27-29 (C₂H₃⁺, CH₄⁺, C₂H₅⁺/CHO⁺), and m/z 39 (K⁺/C₃H₃⁺) , which are common fragments of most organic compounds and target surface contaminants (in case of Na⁺, K⁺), are not considered diagnostic for specific species and are largely excluded from further interpretation. 

During the E5 flyby, the unique operational mode of CDA (see methods) resulted in lower quality type II spectra with only a few spectra showing signal-to-noise ratios high enough to reliably detect organic functional groups, making quantitative analysis impossible. It is possible that organics of the same classes as those identified here were not observed in such high-noise spectra in the data, or that they were present in quantities below the detection limit. 

Aryl Compounds

As illustrated in Figure 1a, the CDA spectrum exhibits spectral characteristics that are indicative of aryl group species. The peak at m/z ~ 77-79 represents either benzene or the phenyl cation [C₆H_5-7]⁺ – as observed in E ring ice grains^8,10. The phenyl cations coincide with a peak at m/z 90-91 indicative of the tropylium cation [C₇H₇]⁺. The spectral features at m/z 3840, 49-52, and 62-65 are characteristic fragment species of single-ringed aromatics. Some of these species with variable C/H ratios correspond to [C₃H₃]⁺, [C₄H_1-5]⁺, and [C₅H_1-5]⁺ and are directly produced via fragmentation of the aryl ring^8,10,28,29. The broad peak near m/z 27, which extends to approximately m/z 31 may represent the oxygen-bearing species [CH_1-3O]⁺, which is coincident with other O-bearing cations at m/z 43-45 – [C₂H_3,5O]⁺. The peaks corresponding to specific fragment ions that are present in the EI mass spectrum of benzyl methyl ether (C₈H₁₀O), shown in Figure 1b, correspond particularly well to the CDA spectrum.

Aliphatic O-bearing Compounds

Figure 2a shows a CDA spectrum that corresponds to an aliphatic O-bearing species, probably a carbonyl group attached to a C₂ organic (e.g. acetaldehyde or acetic acid). This spectrum exhibits a high degree of correlation with the EI spectrum of acetaldehyde (Figure 2b). The Obearing feature [C₂H₄O]⁺ at m/z 44 in the EI spectrum is consistent with the feature at m/z 45 [C₂H₅O]⁺ that is characteristic of O-bearing compounds in CDA spectra^10,27. The absence of organic peaks at masses greater than m/z 45 indicates that this may be a molecular peak. 

The presence of a peak at m/z ~ 31, which corresponds to [CH₃O]⁺, is a supporting feature of O-bearing species as discussed by Khawaja et al.¹⁰ Consequently, a few CDA spectra (see Extended Data Figure 2) that show a distinct feature at 30-31 without any peak at m/z 45 are classified as O-bearing spectra. 

Esters (and/or Alkene) compounds

Figure 3a depicts a CDA spectrum obtained from a single ice grain that contained esters and/or alkenes. Features from organics at approximately m/z 41 and 57 are observed in CDA data owing to the high impact speed. These features have the potential to interfere with water cluster species (Na⁺(H₂O)_1,2), which are commonly observed at lower impact velocities^8,10. The EI spectrum shown in Figure 3c of allyl propionate was found to be the best match to the observed CDA spectrum. The peaks at m/z 40-41 and 56-57, which correspond to fragments from allyl propionate [C₃H_3,5]⁺ and [C₃H₅O]⁺ respectively, appear in both the CDA and EI spectra. The fragment cations thus provide constraints on organic structure, for which the most plausible classes are ester and/or alkene species.

Figure 3(b,d) shows a CDA and a corresponding EI spectrum of cyclohexyl acetate (C₈H₁₄O₂; 142 u), a cyclic ester candidate that exhibits key CDA spectral characteristics. The peak at m/z 82-83 is attributed to the fragment ion [C₆H_10,11]⁺ potentially produced by the cleavage of the bond between the ester and cyclic parts of the molecule. Two pairs of peaks at m/z 67 & 43 and m/z 54 & 41 potentially derive from the fragmentation of the molecular ion, relating to the acetate and cyclic parts of the molecule, respectively. Other candidate compounds are given in Extended Data Table 1

Ether (and/or ethyl) compounds

Figure 4 (a,b) shows two CDA spectra of organic enriched plume ice grains, exhibiting a distinctive set of peaks at m/z 27, 31, 44-45 and 59. These features are well-matched by the EI spectrum of diethyl-ether (panel c) in addition to other potential candidate compounds (Extended Data Table 2). The two classes (panels c & d) of organic compounds exhibit significant major peaks at m/z 31 and 59 corresponding to [CH₃O]⁺ or [CH₃NH₂]⁺ and [C₃H₇O]⁺ or [C₂H₅(NH₂)]⁺ respectively. In both instances, the ethyl group is a common moiety. The peaks at 43-45 are characteristic spectral features corresponding to each class of organic compound.

N-,O-bearing moieties

Figure 5 shows a spectrum containing coincident cations at certain m/z values, which correspond to mass differences between other spectral features at higher masses. The base peak at m/z 53 is attributed to the mass difference between peaks 124-125 u and 72 u. The cleavage of an N-heterocyclic ring can produce two separate fragments that contribute to the feature at m/z 53 – [C₄H₅]⁺ and [C₃H₃N]⁺. [C₃HO]⁺ could also contribute to the base peak at 53 u generated by other routes of fragmentation from larger molecules (Extended Data Table 3). The oxygen-bearing group potentially generates a fragment cation at m/z 72-73 of [C₃H_4,5O₂]⁺, which can further fragment into formyl [CHO]⁺ and its derivative cations at m/z 31-33 [CH_3,5O]⁺. The molecule containing both N- and O-bearing moieties could be cleaved to produce a spectral feature at m/z 82-83 such as [C₄H_4,5NO]⁺ from the molecule. Species containing at least 5 carbon atoms (Figure 5), and a variety of various N- and O-bearing moieties, including derivatives of pyridine, pyrimidine, maleic acid, and nitriles (see Extended Data Table 3), are potential candidates for these spectral features. No compounds with exactly matching EI spectra could be identified for these CDA spectra. However, some potential candidate molecules (e.g. thymine, ethyl cyanoacrylate) with partially matching spectra are provided in Extended Data Figure 3. Similar spectral features were previously observed in E ring ice grain mass spectra obtained at lower velocities¹⁰. 

Discussion

This work examines the chemical composition of organic enriched ice grains that were ejected into the Enceladus plume mere minutes before sampling by the CDA mass spectrometer during the fastest flyby of Enceladus by Cassini (E5; ~17.7 km/s). The high impact speed provides new insights into the composition of these freshly ejected grains because of the formation of cationic species that either did not form at lower impact speeds or were obstructed by water cluster species that are not retained at these speeds²⁴. We rule out any influence of post impact chemistry on the interpretation of species detected in this work (see Supplementary Figures 1, 2, and 3). In this study, we present the identification of aliphatic and cyclic ester/alkene and ether/ethyl moieties in freshly emitted Enceladean ice grains. Additionally, some spectra show features indicative of mixed moieties, potentially N- and Obearing (Figure 5, see Extended Data Table 3). The detection of aryl and O-bearing compounds in the plume ice grains is confirmed, as these were previously identified in the E ring at lower impact velocities¹⁰.

Recent modelling work on the possibilities for organic synthesis on Enceladus within hydrothermal systems is consistent with our detections of aromatics, esters, alkenes, aldehydes, and low-mass N-bearing compounds³⁰, with the exception of ether compounds which they did not investigate. The presence of these compounds in fresh ice grains from Enceladus provides a number of implications for their formation and evolution in seafloor hydrothermal systems, as well as for interactions with previously identified organic compounds in the synthesis of more complex organics. 

        (i)       Aryls

It is currently unclear whether the aryl compounds (aromatics) identified in this study are primordial – akin to those observed in carbonaceous chondrites – and have been leached from accretional material, or formed endogenously through hydrothermal reactions on Enceladus. It is impossible to determine isotopic ratios using CDA mass spectra, which could shed light on their origin, although future instruments may provide this capability. In the endogenous case, redox chemistry has the potential to facilitate the synthesis of aromatic compounds from a range of volatiles that have already been detected on Enceladus (see Figure 6). The possibility that aromatic-like compounds could be synthesised in freeze-thaw cycles at the water-ice interface is unlikely due to the limited formation potential of carbon radicals, which are important for the formation of PAHs in terrestrial sea ice^58-60, and the fact that significant fractions of aromatic organics are excluded from the ice phase during freezing⁶¹.

The presence of an aryl group linked with alkyl or O-bearing moieties may offer avenues for the synthesis of biologically pertinent organic compounds^10,33. Aryls with certain moieties are reactive under hydrothermal conditions, facilitating their transformation into more stable species such as benzene and phenol^62,63. Single ring aromatics play a pivotal role in the organic chemistry of hydrothermal systems on Earth’s seafloor, serving as primary sources of further organic compounds^64-67. The presence of aryl groups in the fresh plume suggests that these compounds retain their aromatic structure in hydrothermal sites prior to transport through the ocean and incorporation into ice grains at the water surface.  The detection of organics directly in the plume rules out space weathering as the sole production pathway, and Liu et al.³⁰ show that cold aqueous chemistry is also ruled out, so therefore the grains are fresh, unaltered, and proof of survival through ocean transit and plume emission of compounds indicative of warm hydrothermal chemistry. We discount an aliphatic origin of these mass spectra, as aromatic molecules generally create high yields of low mass ions only in highspeed impacts, whereas aliphatics experience extensive fragmentation⁶⁸. 

        (ii)      O-bearing organics

Among the aliphatic O-bearing moieties (Figure 2), we posit that aldehydes represent a moiety present in freshly ejected ice grains from Enceladus. Aldehydes may have been accreted in the building materials of Enceladus since they are relatively abundant in comets^69,70. Aldehydes represent intermediates in the redox pathway from simple hydrocarbons toward carboxylic and amino acids⁷¹. The presence of acetaldehyde on Enceladus would offer possibilities for synthetic routes toward more complex organics essential for life^72,73. Acetaldehyde is also linked to acetylene – detected in the plume by INMS^4,17 – in chemical cycles within prebiotic hydrothermal systems^53,57. Ethylene oxide (C₂H₄O) could also be a potential candidate for this type of spectral features, which could catalyse the formation of polymers with multiple end groups (e.g. -CH₂, -OH). For example, ethylene oxide can assist alkylation of simple aryl compounds (e.g. benzene) through FriedelCrafts-like reactions, resulting in the formation of aryl-substituted alcohols under hydrothermal conditions³⁰. This process could subsequently lead to the synthesis of more complex organics such as polycyclic aromatic hydrocarbons (PAH)s or the macromolecular species detected in Enceladean ice grains⁸.

        (iii)      Ester/alkene and Ether/ethyl

This detection of aliphatic and/or cyclic ether, ester and alkene moieties in plume ice grains complements previous identifications of these molecules in other planetary bodies across the solar system. Ethers and esters are rarely found in comets⁷⁴, but they occur as bridges between aromatic moieties of insoluble organic matter (IOM) in carbonaceous chondrites⁷⁵. This suggests that ether and ester moieties can be formed via aqueous/hydrothermal reactions in carbonaceous chondritic bodies. The presence of these compounds, and the possibility for their synthesis in the hydrothermal systems of Enceladus is relevant for planetary habitability, given their role in terrestrial biological contexts, as shown in Figure 6. The isotopic analysis of diether lipids from the Lost City Hydrothermal Field in the mid-Atlantic Ocean provides insight into the abiotic or biotic origins of carbon in hydrothermal systems⁷⁶, emphasizing the significance of such analysis for future missions targeting ocean world sampling. It has been demonstrated that ester compounds can form under reductive hydrothermal conditions from lipid precursors in the presence of ammonium ions, a realistic scenario for the water-rock interface of Enceladus⁷⁷. Such esters are stable under hydrothermal conditions and retain a distinct abiotic signature, namely no even carbon number predominance. The detection of both phosphates⁹ and esters on Enceladus is of significance for astrobiology, offering potential pathways towards important biomolecules⁷⁸. Alkenes are intermediates in a variety of reactions between more abundant classes of organics in submarine hydrothermal systems⁷⁹. Such compounds are involved in hydration, oxidation, and dimerisation reactions under these conditions. The presence of alkenes on Enceladus likely diversifies the chemical reactions accessible at hydrothermal sites. 

        (iv)      Mixed N-,O-bearing Molecules

For compounds with multiple moieties tentatively identified in this work (see Extended Data Table 3), possible fragmentation pathways are presented in Figure 5. Candidate compounds include derivatives of pyrimidine, pyridine, acetonitrile, and maleic acid, among others. It is noteworthy that acetonitrile and various amine derivatives have a particularly affinity for synthesis under Enceladean conditions³⁰. The cyanate ion was also detected in INMS data by Peter et al.¹⁷ and could react with acetylene, providing pathways towards larger, morecomplex N-bearing species. It is also possible that the cyanate species detected by INMS were produced by the fragmentation of acetonitrile, a molecule that could be related to the candidate species for this type of spectrum.

Conclusions

We report the presence of organic moieties, including the detections of esters/alkenes, ethers/ethyl, and N-O-bearing species by Cassini’s CDA, in ice grains freshly ejected from Enceladus and sampled at the highest flyby speed (17.7 km/s). These new organic functional groups enable further avenues for hydrothermal chemistry, in addition to those pathways previously postulated^5,8,10,13,14 (Figure 6). We confirm also the presence of previously identified¹⁰ aromatic and O-bearing species, and impose new constraints on their origin. This work demonstrates that such moieties in these freshly ejected ice grains are probably derived from within Enceladus rather than from space weathering²² during their lifetime in the E ring. Although the possibility of post-impact plasma chemistry cannot be ruled out, the detection of species identified in this work are solely independent of such process at these velocities (see Supplementary Section 1). The data obtained by INMS during the E5, E17, E18, and E21 flybys^5,17 are in good agreement with the results reported here (see Extended Data Table 4). The detected moieties in the plume further hint at an organic-enriched subsurface, with a diverse range of reaction pathways expanding both the known and potential chemical space of the Enceladus ocean.

Methods:

        (i)       E5 Flyby and Data Collection

Cassini’s CDA recorded 1519 distinct TOF mass spectra of ice grains in the close vicinity of Enceladus over approximately six minutes during its E5 flyby. The flyby was performed at 17.7 km/s and the closest approach (CA) occurred at 2008-283T19:06:40 UTC (Coordinated Universal Time) at a distance of 21 km from the fringe of the tiger stripes near Alexandria. The flyby saw Cassini traverse the part of the plume with the highest number density of ice grains.

The mass spectra obtained during the E5 flyby of Enceladus were recorded in a distinctive operational mode of CDA. This flyby was the only close approach to Enceladus during which a specially-modified flight software (FSW 12.0) was employed by the instrument, enabling a spectrum recording rate of up to 5/second rather than the 1/second in the nominal configuration. Under these conditions, however, the recorded mass range spanned 2 to 110 u (or in a few cases up to 125 u), a reduction from the standard 1 – 200 u. Additionally, the sampling rate was also reduced (one data point every 20 ns instead of 10 ns), further limiting the mass resolution⁷.

In most cases, the spectrum recording of the incident water ice grain was triggered by the detection of high amplitude mass lines corresponding to the short flight times of fast hydrogen ions H⁺, H₂⁺,or H₃⁺. In addition, there are some instances when the spectra recording started upon the impact of the ice grain at the target even before the first H⁺ ions could have arrived at the CDA’s multiplier, potentially due to a higher particle impact rate that increased the noise level, which prematurely triggered the instrument. The average value of the stretch parameter (for detail, see ref.⁷ and Supplementary Figure 4) of the instrument is approximately 506 ns. This mechanism of spectrum recording is described in detail by previous works^6-8,10,23. The mass range of the recorded spectra generally extends to an upper limit of 103 u, with some exceptions where it reaches 125 u. The water cluster features [H₃O]⁺(H₂O)_1,2,3… that appear in lower speed spectra^8,10 are not observed due to the extremely high impact speeds encountered during the E5 flyby relative to E ring traversals of the Cassini spacecraft. In all CDA spectra shown in this work, the impact of ice grains at such velocities generates rhodium ions [Rh]⁺ from the Chemical Analyser target material.

        (ii)       Classification of Type II Spectra

The CDA spectra shown in this work exhibit a peak at m/z 19 corresponding to the hydronium cation [H₃O]⁺ without any additional water cluster features, as are typically detected in E ring ice grain spectra recorded at much lower velocities. Postberg et al.⁷ previously classified these spectra into three categories, corresponding to different populations of ice grains originating from Enceladus: (i) Type I: almost pure water ice with only trace amounts of organics. The hydronium peak (H₃O⁺) at m/z 19 dominates these spectra, with no pronounced peaks between 28-29 u; (ii) Type II: organic enriched ice grains, dominated by the hydronium peak (H₃O⁺), but also containing pronounced features between 28-29 u; and (iii) Type III: salt rich grains. Type III spectra are dominated by Na⁺ and NaOH and/or NaCl and/or Na₂CO₃⁺ cluster peaks, exhibiting neither a significant hydronium feature nor peaks at 28-29 u.

In this work, we focus only on Type II mass spectra, with a slight modification in the criterion set by Postberg et al.⁷ In our reinvestigation, we include spectra with peaks between m/z 2533 and/or m/z 39-46 corresponding to hydrocarbons, oxygen-, and nitrogen-bearing species. In total, 409 spectra are classified as Type II, and these are selected for in-depth analysis. Five compositional subgroups of organic enriched Type II grains (86 spectra) are identified with a unique set of peaks in their spectra (see Supplementary Table 1). A list of possible fragment ions corresponding to the mass lines observed in these spectra  (see Supplementary Table 2).

In this work, we have analysed the full data set of the E5 flyby in detail. The type II spectra recorded during this particular flyby are generally of lower quality due to the higher spectral recording rate (5 s^-1), lower mass range (125 u) and reduced sampling rate (one data point every 20 ns instead of 10 ns) in this unique operational mode of CDA. We find only a small number of spectra with signal-to-noise ratios large enough to identify features characteristic of certain organic functional groups. The data quality was not only generally poor, but also varied across the plume traversal. It is possible that organics of the same classes as those identified here were not observed in such high-noise spectra, or that they were present in quantities below the detection limit. Quantitative analysis is not possible under these nonoptimal conditions and therefore this paper aims solely to identify organic compounds wherever possible.

One interesting aspect that should be mentioned is the case of aromatic species. Aromatics seem to be enhanced in type II ice grains relative to the other organic subtypes detected at 18 km/s. This could be due to the fact they are more stable across all impact speeds, or form more abundant characteristic fragmentation products relative to other organics. As described prior, however, we cannot reliably draw conclusions about the relative proportions of aromatics in the larger dataset. 

        (iii)      Electron Ionisation Mass Spectra

In this work, electron ionisation (EI) mass spectra (see Extended Data Tables 1, 2, 3), extracted from Massbank Europe, NIST and MoNA, have been used to aid spectral interpretation following the methodology of Khawaja et al.²⁷. High velocity (i.e. 18 km/s) impacts provide such energy that water cluster formation is inhibited in the plasma cloud post-impact. With the exception of the hydronium [H₃O]⁺ ion at m/z 19, the standard patterns of water clustering²⁴ are not observed in these CDA spectra. As EI spectra are not recorded in the presence of a matrix, unlike the analogue experiment laser-induced liquid beam ion desorption (LILBID), they offer a strong match to CDA spectra in these high-velocity cases. Even at the highest laser power densities, water clusters still appear in LILBID and complicate the identification of organic features in the spectra.

Both EI and impact ionisation are considered “hard” ionisation methods, in which a large number of fragment ions are formed, reducing the intensity of the molecular ion peak, particularly in the case of higher impact velocities⁸⁰. We note that Mikula et al.⁸⁰ observed a number of quantitative discrepancies between EI and impact ionisation mass spectra obtained from dust accelerator experiments with polypyrrole-coated anthracene. This work, however, used dust particles with little to no water content, whereas CDA detects ice grains dominated by water. Similarly, their experiments investigate only PAHs with fused aromatic rings; here, we consider single-ringed aromatic compounds which, even if part of a larger molecular structure, have a number of substituted groups on the ring. Furthermore, our findings illustrate that aryl and O-bearing moieties remain stable even at the highest impact speeds of ~18 km/s, probably due to the protective nature of the ice matrix. The abundance of characteristic low-mass fragment ions from polypyrrole-coated anthracene has been shown to rapidly decline above 15 km/s in experiments with dust accelerators⁸⁰, indicating that the ice matrix plays a pivotal role in retaining spectral characteristics associated with aromatics. Whilst fragmentation is influenced by the size and structure of the ice matrix at low velocities, such a dependence is insignificant at high impact speeds. The shielding role of the ice matrix is largely uniform above a certain threshold velocity, which is unique to the embedded organic molecule^81-83.

Species with high ionisation energies appear in both EI and impact ionisation mass spectra, owing to the excess energy available in each ionisation method^84,85. In both cases, fragment ions convey crucial structural information in the mass spectra. In EI, minor peaks adjacent to m/z values of major fragment ions can also be observed, which assists in the identification of broad peaks in CDA mass spectra that occur due to its higher noise level than laboratory mass spectrometers. CDA and INMS often detect the same organic species, as INMS can also be triggered by the impact of incident ice grains, suggesting that similar fragmentation pathways are accessible by both methods⁸. INMS is sensitive to neutral fragments, offering a useful mode of comparison.

Note that LILBID is a crucial method for interpretating mass spectra of ice grains emitted by icy ocean worlds in the outer solar system at lower impact velocities such as those expected for Europa Clipper and its SUrface Dust Analyzer⁸⁶. See Supplementary Figure 3 for further examples of the behaviour of water clustering in both CDA and LILBID mass spectra. The high impact velocities experienced in the E5 flyby can guide the interpretation of mass spectra from other fast flyby and interplanetary missions such as JAXA’s Demonstration and Experiment of Space Technology for INterplanetary voYage with Phaethon fLyby and dUst Science (DESTINY+), which will also carry an impact ionisation mass spectrometer – the DESTINY+ Dust Analyser (DDA)⁸⁷ – for the compositional analysis of interplanetary and interstellar dust particles.

        (iv)      Assignment of Species to CDA Spectra

The highest impact speed CDA spectra (Figures 1, 2, 3, 4, 5) of freshly ejected ice grains provide new constraints on the structure of different embedded species. We constrain the structure of aryl species by the presence or absence of a peak at m/z 91 which correspond to tropylium or benzyl cations respectively (Figure 1a). The presence of this peak indicates a more benzyl-like nature of aromatics in ice grains, whereas its absence implies phenyl-type species^8,10. Certain O-bearing species can produce the methenium cation at m/z 15 coincident with a formyl cation at m/z 29-31. It is clear that the methyl groups must exist in some form and survive impact at such high velocities. The combined spectral features of Extended Data Figure 2, with peaks at m/z 15, 29-31, and 45, are likely to correspond to [CH₃]⁺, [CH_1-3O]⁺, and [C₂H₅O]⁺. The best EI spectral match to the observed cationic distribution is the acetaldehyde (C₂H₄O) spectrum, which also correlates strongly to earlier predictions of the Enceladus chemical inventory^4,10. 

In addition to the characteristic identifiers of aromatic and O-bearing species observed in these mass spectra of plume ice grains, we also detect characteristic spectral features of esters, alkenes, ethers, ethyl and NO-bearing compounds. The CDA mass spectral features of esters/alkenes and ethers/ethyl group are compared with EI spectra using online open-source mass spectral databases. Two different sets of peaks correspond to ester/alkenes (Figure 3 & see Extended Data Table 1): (i) in this case compounds are included where the peaks at m/z 41 and 56 should have an intensity within 30 % of each other in EI spectra. In addition, if present, the intensity of the 43 u feature should be 20 % less than the peak at 41 u, (ii) in this case, compounds are included whose EI spectra show a set of peaks at m/z 41-43, 54, 67, 82. The relative intensities of these peaks should lie within 20 % of each other. In addition, no other peaks should appear in their spectra with a relative intensity greater than 20 %. This logic is shown graphically in Extended Data Figure 1.

In the ether/ethyl group, candidate compounds are included if their EI spectra show two different sets of peaks (Figure 4, and see Extended Data Table 2): (i) in this case, peaks at m/z 31 and 59 are present with a relative intensity greater than 30 %. In addition, a peak at m/z 43-45 can be present only if any peak at m/z 41 u occurs with less than half its intensity. (ii) in the second case, compounds are included which show peaks at m/z 43-45, 58-59, 71-72, and 88-89 in their EI mass spectra. In both groups (esters/alkene & ether/ethyl), a few additional candidates are given related to other, typically more exotic and unlikely, moieties – e.g. halides. There is another group of CDA spectra corresponding to N+O-bearing compounds, which show a set of peaks at m/z 124-125, 82-83, 71-72, 31-33, 26-27 in addition to a significant peak at m/z 52-54 (Figure 5). For these mass spectral features, no representative EI mass spectra could be found in freely-available online databases. However, the observed fragment ions can be traced back to possible parent compounds²⁸, shown in Figure 5 and Extended Data Table 3.

        (v)      Sampling and Composition of Ice Grains

The spectral features observed in this work are attributed to compounds with masses below ~ 125 u, but we cannot completely rule out that the ion species analysed here are moieties of much larger molecules⁸ that are fragmented during high-speed impacts. This could mean that some fragmentation pathways of large molecules may require energies only accessible at higher impact speeds, leading to new observed fragment ions in this work. Alternatively, some species (ether/alkene, ester/ethyl and NO-bearing) detected in this work may become unstable due to space weathering effects, explaining their non-detection in E ring mass spectra by Postberg et al.⁸ The lack of detection in the E ring can be due to one of, or a combination of, the following factors:

1. Spectral features in E ring spectra were obscured by water cluster species that do not form at the high impact velocities we consider in this work.
2. Space weathering effects lead to the dissociation of organic compounds and the loss of volatile components in E ring ice grains.
3. Some fragment ions identified in this work can only be produced at the high impact velocities encountered in the E5 flyby.

In this distinct operational mode of CDA, the mass range of the instrument does not extend to masses > 120 u, where indicators of macromolecular species were previously observed. For these reasons, we report the detection only of specific functional groups, and suggest low- or intermediate mass candidate compounds (mostly less than 150 molecular weight species) that provide useful constraints on molecular structure. 

In these datasets, we can infer structural constraints on the organic compounds responsible for the CDA mass spectra, but the complexity of high-speed impacts means that the absolute identification of molecules is challenging. In such high-speed (>17.7 km/s) collisions associated with impact ionisation, the distribution of energy across the organic species within the impact cloud is non-uniform compared to the better-characterised EI technique – meaning that, whilst fragments produced are similar between the two ionisation methods, they do not necessarily occur in the same abundances. Similarly, due to limitations of the CDA software – which was not originally designed for the high fluxes of ice grains encountered during plume flybys, the high impact velocities, the quality of the mass spectra, and the discrepancies between peak amplitudes in the CDA spectra and example EI spectra also means that concentrations of candidate compounds cannot be inferred in the bulk ocean. The CDA spectra detailed in each figure (Figures 1-5) were generated by the impact of a single ice grain, rather than averaged and co-added as were often used in previous works^6,8,9,13. The recent work of Klenner et al.⁸⁸ demonstrated that the compositional analysis of single ice grains is an important mode of sampling at ocean worlds. Not only does this yield relatively high-quality spectra, with distinct peaks generally more observable than in co-added spectra, but also acknowledges the inhomogeneity of ice grains, where bulk ocean constituent compounds can vary significantly in concentration between different ice grains. Co-added spectra are not conducive to the detection of compounds that are generally present in low quantities in the bulk ocean, but incorporated into some ice grains with elevated concentrations. Such scenarios would not be detected by analysis of co-added spectra alone. Each individual ice grain thus provides a unique window into the potential habitability of the Enceladus subsurface.

Data availability

All CDA data used for this work are listed in Supplementary Table 1 and are archived on PDS– SBN, at https://sbn.psi.edu/pds/resource/cocda.html. Electron Ionisation data are obtained from the National Institute of Standards and Technology (NIST: https://chemdata.nist.gov/), MassBank Europe (https://massbank.eu/MassBank/), and MassBank of North America (MoNA: https://mona.fiehnlab.ucdavis.edu/) freely-available online databases. The source data used to compile Supplementary Figures 1a and 1b are provided as Supplementary Datasets 1 and 2, respectively.

Acknowledgements

N.K., J.S. and R.S. were supported by the project DESTINY+/DDA funded by the German Space Agency (DLR) through the grant No. 50OO2101. F.P., J.H. and also N.K. were supported by the European Research Council under the European Union’s Horizon 2020 research and innovation programme (Consolidator grant no. 724908 Habitat-OASIS). N.K. and L.H.S. were also supported by the European Research Council under the European Union’s Horizon 2020 research and innovation programme (Consolidator grant no. 101171589-AIMS). T.R.O. acknowledges support from the state of Berlin via the Elsa-Neumann Stipendium des Landes Berlin. F.K. acknowledges support from NASA Habitable Worlds Program Grant No. 80NSSC19K0311.

Author Contributions Statement

N.K. led the study and prepared the manuscript, with support from F.P. and T.R.O. N.K. led the CDA and EI data analysis and interpretation with support from F.P. and T.R.O. J.H. and S.K. assisted with the interpretation and contributed to writing and editing the manuscript. R.S. designed CDA observations. M.N. performed experiments and assisted in calibrating the data. All authors contributed to the discussion and commented on the manuscript.

Competing Interests Statement

The authors declare no competing interests.

Tables Figure Legends and Captions

Figure 1. Archetypal CDA and EI mass spectra of an aromatic organic compound(s). (a) CDA mass spectrum of a single plume ice grain characteristic of monocyclic aromatic compounds. (b) An EI spectrum of benzyl methyl ether, which exhibits similar spectral features. Note that, for all figures, mass values are assigned to organic-related, and other notable, peaks to account for differences in the x-axis parameters between CDA and standard EI mass spectra.

Figure 2. Archetypal CDA and EI mass spectra of an aliphatic O-bearing organic compound(s). (a) CDA mass spectrum of a single ice grain indicative of an aliphatic O-bearing compound. (b) The EI spectrum of acetaldehyde, which offers a good match to some spectral features observed here. 

Figure 3. Archetypal CDA spectra of two individual plume ice grains containing an organic compound(s) bearing an ester/alkene group and corresponding EI spectra. CDA spectrum of potential (a) aliphatic and (b) cyclic ester species and/or alkene compounds in Enceladus individual plume ice grains. (c) an EI spectrum of allyl propionate and (d) EI spectrum of cyclohexyl acetate corresponding to aliphatic and cyclic ester/alkene species.

Figure 4. Archetypal CDA spectra of two individual plume ice grains containing an ether/ethyl group and corresponding EI spectra. (a,b) CDA mass spectracorresponding to ether/ethyl compounds in ice grains (c,d) EI spectra of diethyl ether and triethylene glycol monoethyl ether, potential candidate compounds for the CDA spectra. 

Figure 5. Archetypal CDA mass spectrum for N-, O-bearing species, and their corresponding molecular species and fragments. CDA spectrum of a plume ice grain showing coincident cations that correspond to a species potentially containing nitrogen and oxygen. Examples of candidate species responsible for molecular ions include C₆H₅NO₂; C_5,6H_4,8N₂O_1,2 and  C_6,7H_7,11NO₂. Their potential fragment species include C₄H₆N₂;C₄H₅NO;C4H9N; C3H5NO; C3H3N; C4H5; C3H3; CH4O; H2N2; HCN; C2H3; CHO.

Figure 6. Potential chemical pathways between organic compounds on Enceladus.

Potential chemical pathways between compounds detected in this and earlier works on Enceladus^{4,5,8,10,17,31,32}, including both aqeuous and ice-phase reactions, as well as those compounds that have not been detected to date, but would have astrobiological significance or provide pathways between other classes of compound. Legend: Blue boxes correspond to a CDA detection of the given functional group. Yellow boxes correspond to an INMS detection. Dashed boxes relate to those compounds potentially detected by CDA in this work, including both reliable detections and tentative suggestions. Solid boxes represent compounds that have potentially been detected in prior works, regardless of the confidence interval. Black boxes refer to compounds that have not yet been detected on Enceladus, but would be significant either in the context of astrobiology (e.g. amino acids) or as intermediates between other detected compounds (e.g. cyanoalkynes). The arrows between compound classes describe putative reaction pathways, with red arrows referring to abiotic pathways and green arrows representing biotic pathways. Note that some pathways would depend on ice-ocean exchange processes. The lowercase letters indicating pathways between compound classes denote the corresponding citation printed on the

figure. a) ref.^10,33, b) ref.^34-37 c) ref.³⁸, d) Oxidation, e) ref.^39-41, f) ref.⁴², g) ref.^43,44, h) ref.^45,46, i) Esterification, j) ref.⁴⁷, k) ref.^48,49, l) ref.^37,50-52, m) ref.^52-54, n) ref.⁵⁵, o) ref.^56,57.

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