Saturn’s moon Enceladus harbours a global water ocean1, which lies under an ice crust and above a rocky core2. Through warm cracks in the crust3 a cryo-volcanic plume ejects ice grains and vapour into space4,5,6,7 that contain materials originating from the ocean8,9. Hydrothermal activity is suspected to occur deep inside the porous core10,11,12, powered by tidal dissipation13. So far, only simple organic compounds with molecular masses mostly below 50 atomic mass units have been observed in plume material6,14,15. Here we report observations of emitted ice grains containing concentrated and complex macromolecular organic material with molecular masses above 200 atomic mass units. The data constrain the macromolecular structure of organics detected in the ice grains and suggest the presence of a thin organic-rich film on top of the oceanic water table, where organic nucleation cores generated by the bursting of bubbles allow the probing of Enceladus’ organic inventory in enhanced concentrations.
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The research leading to these results received financial support from German Research Foundation (DFG) projects PO 1015/2-1, /3-1, /4-1 and ERC Consolidator Grant 724908—Habitat-OASIS (F.P., N.K, L.N., F.K. and R.R.), AB 63/9-1 (B.A. and F.S.), the Klaus Tschira Stiftung (M.T. and F.P.), NASA contract NAS703001TONMO71123, JPL subcontract 1405853 (J.H.W., C.R.G and B.M.), INMS science support grant NNX13AG63G (M.P.), NASA Habitable Worlds Program and JPL’s RTD funding (M.S.G. and B.L.H.) and Academy of Finland project 298571 (J.S.).Reviewer information
Nature thanks J. Lunine and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a, b, Identified organic mass lines of individual HMOC spectra. The distribution of resolved mass lines and flank peaks of 64 HMOC spectra with the most distinct HMOCs are shown (see Methods, ‘Selection of 64 high-quality spectra for Fig. 1 and Extended Data Figs. 1 and 3’). 19 more spectra with a high level of interference with water-cluster ions or low signal-to-noise ratio are not included here (see Extended Data Table 1). All peaks depicted here are also part of the data shown in Fig. 1b. The spectrum number (as defined in Extended Data Table 1) is indicated on the left as an identifier of the event. The extent is indicated by the horizontal length and the relative normalized amplitude of each spectral feature is given by the length in the vertical direction and the colour code (red being the highest and blue the lowest amplitude). The largest horizontal span of the symbol marks the peak maximum. In b, the amplitudes between 70 u and 85 u shown in grey indicate that they are not to scale with the symbols shown at higher masses (they would be much larger; see Fig. 1b for comparison). Spectra are sorted by their impact speed, as estimated from the orbital elements of the impacting grain, with the highest speed (~15 km s−1) at the top of the graphs and the lowest (~5 km s−1) at the bottom. Because the exact orbital elements are unknown, each impact speed has substantial intrinsic uncertainties, given in Extended Data Table 1. The 12 spectra for which a minimum impact speed could be derived from the presence of hydrogen mass lines (Extended Data Table 1; see Methods, ‘Selection of 64 high-quality spectra for Fig. 1 and Extended Data Figs. 1 and 3’) are placed at the top. The highest mass at which the recording of the CDA TOF spectrum ends varies between 174 u and 226 u (Methods, ‘Short description of CDA’s chemical analyser subsystem’), as indicated by the grey horizontal bars. As a consequence, the frequency of the HMOC peaks around 178 u and 191 u in Fig. 1b is reduced because not all individual spectra cover this mass range. The absolute masses in each individual spectrum have an intrinsic uncertainty (absolute value) of ±1 u at 80 u and ±2 u at 180 u owing to the limited calibration accuracy of the CDA in this high-mass regime. The mass intervals between peaks, however, are accurate to the integer level.
The figure was modified from figure 5 of ref. 19. The x-axis shows m (mass) over z (cation charge), with z = 1 for all major species. The impact ionization TOF mass spectrum of a polystyrene particle with a radius of ~1 µm was recorded at the Heidelberg dust accelerator facility36 with an impact speed of 5.2 km s−1. Above 100 u and below 70 u the spectrum shows cationic fragments, in good agreement with the CDA HMOC spectra and their characteristic spacing of 12.5 u. The inset shows the molecular structure of the polymer. See the main text and Methods section ‘Inferring the origin of HMOC peaks in CDA spectra’ for further discussion.
White and grey spectra represent the average of all spectra from impacts below and above ~10 km s−1, respectively (see Methods, ‘Selection of 64 high-quality spectra for Fig. 1 and Extended Data Figs. 1 and 3’). All signatures with possible major contributions from inorganic species are colour-shaded as in Fig. 1. Signatures not marked are exclusively or mostly due to organic cations. The abundance and position of the HMOC species is relatively independent of the impact speed of the ice grain (see also Extended Data Fig. 1). By contrast, fast impacts induce stronger organic fragmentation signatures at masses below 70 u and HMOCs form more distinct, evenly spaced groups, characteristic of impact-induced dissociation processes. In turn, slow impacts show more abundant intact benzene-like cations. There seems to be a tendency of some organic cations to carry fewer H atoms at fast impacts (27 u, 51 u and 62 u), which is indicative of ‘softer’ ionization from the slower impact. In fast spectra, interference with water-cluster ions is less frequent than at lower speeds. In contrast to fast spectra, fragmentation below (15 u) is usually not observed in slow spectra. Spectra from slow impacts are prone to abundant water clustering, creating mass lines of the form H+(H20)n, with n = 1–4, at 19 u, 37 u, 55 u and 73 u (blue). In fast spectra, clustering is limited and only the mass lines at 19 u and 37 u are generally present; occasionally, formation of the smaller water ions OH+ (17 u) and H2O+ (18 u) is observed. Similarly, Rh+ (103 u) forms from excavation of the impact target only at fast impacts and interferes with HMOC species there. To a lesser extent, this is also true for the rhodium–water cluster Rh+(H2O) at 121 u. See the individual CDA spectra in Extended Data Fig. 4 for comparison. a.u., arbitrary units; ToF, time of flight; lg, log.
In these individual spectra, the peak definition is naturally higher than in the co-added spectra shown in Fig. 1 and Extended Data Fig. 3, and therefore some of the spectral features collected in Fig. 1b become more apparent. a, HMOC spectrum from one of the fastest recorded impacts (12–18 km s−1). The appearance of hydrogen cations (H+, and ) at 1 u, 2 u and 3 u, as well as the disintegration of the ion into , CH+ and C+ (12 u–15 u) and the formation of H2O+ (18 u), are evidence of the high-speed impact. The abundance of unsaturated small cations below 70 u, probably fragments from aromatic structures, is increased compared to slower spectra. The frequently occurring mass line at 45 u (Fig. 1b and Extended Data Fig. 1) is noticeable; it cannot originate from pure hydrocarbons and requires heteroatoms, probably oxygen in this case. While a 45 u feature is quite common in our HMOC dataset, the peak at 86 u is only apparent in this spectrum. b, HMOC spectrum from a grain detected at intermediate speed (5–8 km s−1). High-mass fragments and benzene species are abundant whereas further fragmentation of the benzene ring into C5 and C4 species is less apparent compared to high-velocity impacts (a). We note that organic cations with 2, 3, 4 and 5 C atoms show a tendency to carry more H atoms compared with the high-speed impact, which is indicative of ‘softer’ ionization from the slower impact. Organic fragmentation below is usually not observed in this speed regime.
Extended Data Fig. 5 Example HMOC CDA mass spectrum with extended mass range and statistics for all features.
a, Ice grain spectrum showing the HMOC event with the strongest extended mass range signal of the dataset. The dashed line at 6.4 μs divides the spectrum into the high-resolution part (10 ns sampling) and the low-resolution part (100 ns sampling) (see Methods, ‘Short description of CDA’s chemical analyser subsystem’). There are several relatively narrow peaks between 250 u and 500 u and two much more extended features peaking at about 1,000 u and 1,800 u. In this case, the cations with mass in excess of 200 u are more than twice as abundant (defined by the area under the curve) as those below 200 u. We note the logarithmically scaled TOF axis in this case. These features are usually less frequent and less pronounced than in the extreme case shown here. The extended spectrum frequently shows an instrument-artefact peak at 6.8 µs, which was not considered in our analysis. b, Histogram showing the frequency of occurrences of the features observed in the extended mass range. The definition, and thus significance, of peaks in the extended spectrum is generally lower than in the nominal spectrum. In particular, features above 500 u are sometimes ambiguous and their interpretation should be taken with great caution. However, the statistics shows three preferred mass regions: 200 u–500 u with decreasing frequency, around 1,000 u and around 1,700 u. Even if no sizeable peaks are present, the cation signal in the HMOC spectra is generally higher than the noise level when the low-resolution recording starts and typically only decays to noise level at around 500 u or later.
During Cassini’s E17 flyby of Enceladus’ south pole at 75 km altitude, where the CDA recorded about 40 plume spectra with its full mass range, one spectrum was of the HMOC type. This is in agreement with the proportion of this particle type being a few per cent in the plume and in the E ring close to Enceladus (see Methods, ‘Relative frequency of HMOC-type grains depends on impact speed and distance to Enceladus orbit.’). The flyby speed determined the impact speed of 8.6 km s−1 and the particle had a radius of about 2 µm. To operate the CDA in the dense dust environment of the plume, the instrument settings had to be modified in a way that compromised the spectrum quality (lower sensitivity and lower mass resolution; see Extended Data Fig. 4 for comparison). The spectrum is baseline-corrected. On the only occasion when the CDA recorded a large number of spectra with high cadence directly in the plume during Cassini's E5 flyby9, the spectral range was truncated below about 100 u to allow for a higher data rate. This unfortunately did not allow the identification of the defining HMOC signatures.
Extended Data Fig. 7 Laser ionization mass spectrum of benzoic acid and benzyl alcohol dissolved in water.
Analogue TOF mass spectrum recorded with the liquid microbeam ionization setup (see Methods, ‘Laser dispersion analogue experiments for icy dust impacts’ and Extended Data Fig. 9) to simulate the formation of tropylium and benzene cations and their fragmentation ions at impact speeds78 of the order of 10 km s−1. The concentrations of benzoic acid and benzyl alcohol are 3 g l−1 and 0.2 g l−1, respectively. Water ions are marked in blue, aromatic ions and ions from aromatic fragmentation are marked in orange and mixed organic–water species are yellow. To yield both benzene cations (77 u–79 u) and tropylium ions (91 u), two different aromatic structures are required (Fig. 2). The predominant aromatic fragments of benzoic acid are at 77 u and 79 u, whereas benzyl alcohol almost exclusively forms tropylium ions at 91 u. The peak at 95 u is a water cluster of the phenyl cation, which is much more pronounced than in the HMOC spectra. Although the strong phenyl–water cluster signature here illustrates the intimate mixing of organics with water, the much lower 95 u signature in HMOC spectra argues for less efficient mixing of organics with water there, probably due to a core–shell structure that physically separates organics from ice in the grain. Cations from the fragmented ring can be seen at 39 u, 51 u–53 u and 63 u–65 u and agree with the CDA observations (Fig. 1b). In contrast to the CDA spectra, however, saturated C3 fragments (41 u–43 u) are depleted, and C2 (27 u–29 u) and C1 (15 u) fragment cations are entirely absent, confirming the presence of an abundance of aliphatic cations in HMOC grains. The ratios of benzene and tropylium ions and the water ions match the HMOC spectra well. The total concentration of organic species used here (~0.32% by weight) can be used to estimate a lower limit for the concentration of organics in CDA HMOC grains for two reasons. First, in the analogue experiment we selected substances that most efficiently yield the desired aromatic species and other, less efficient precursors would yield even lower signals at 77 u and 91 u. Second, to account for both the low- and high-mass fragments between 100 u and 2,000 u, which are absent in the laboratory spectrum, additional organic substances or larger molecules would be needed to further increase the organic concentration. Therefore, the concentration in Enceladean HMOC ice grains in many cases can be estimated to be near or even above the per cent level.
Cationic TOF mass spectrum recorded with the liquid microbeam ionization setup (see Methods, ‘Laser dispersion analogue experiments for icy dust impacts’, and Extended Data Fig. 9) containing 0.1% pyrene dissolved in a mixture of water and acetic acid. The laser pulse simulates an ice grain impact with a speed78 of about 10 km s−1. Features marked as ‘pyrene fragment’ do not appear in the blank experiment with just the solvent mixture and are either direct pyrene fragments or cations formed from pyrene fragments clustering with the solvent (for example, at 159 u). The molecular mass lines of pyrene are about 10 times more abundant than those of any fragments, indicating the stability of the PAH molecule. In contrast to CDA HMOC spectra, no isolated benzene ring fragment (77 u or 91 u) forms.
Extended Data Fig. 9 Laboratory setup used to simulate ice grain impacts onto space-borne impact ionization detectors.
See Methods, ‘Laser dispersion analogue experiments for icy dust impacts’, for a detailed description of the setup.
See Methods section ‘INMS ice grain spectrum’ for details on how the spectrum was composed and analysed. Error bars (1 s.d.) are derived from the dispersion of the count rate from the three separate measurements of the individual encounters. The spectrum suggests the presence of CO fragments (blue circles) as an oxygen-bearing species. N2 has very low abundance and contributes less than ~10% of the 28 u signal. CO2 and C2H4 collectively contribute less than ~10% of the 28 u signal. CO (blue circles) is required to fit the rest of the 28 u signal and the entire 29 u signal and matches its other dissociative peaks (C at 12 u and CO++ at 14 u) well. The spectrum also indicates the presence of nitrogen-bearing species: the ‘stair step’ pattern around 41 u matches best to the C2H3N spectrum (red circles).
a, b, The individual spectra for fast (E5; a) and slow (b) flybys are shown. c, The spectra of a and b are plotted with the E5 (black) spectrum, which is normalized to match the 15 u signal of the slow spectrum (grey). The residual (difference) between the two spectra is plotted in Fig. 3.
Extended Data Fig. 12 Schematic on the formation of organic condensation cores from a refractory organic film.
a, Ascending gas bubbles in the ocean25 efficiently transport organic material30 into water-filled cracks in the south polar ice crust. b, Organics ultimately concentrate in a thin organic layer (orange) on top of the water table, located inside the icy vents. When gas bubbles burst, they form aerosols made of insoluble organic material that later serve as efficient condensation cores for the production of an icy crust from water vapour, thereby forming HMOC-type particles. In parallel, larger, pure salt-water droplets form (blue), which freeze and are later detected by the CDA as salt-rich type-3 ice particles in the plume8,9.