Abstract

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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References

  1. 1.

    Thomas, P. C. et al. Enceladus’s measured physical libration requires a global subsurface ocean. Icarus 264, 37–47 (2016).

  2. 2.

    Iess, L. et al. The gravity field and interior structure of Enceladus. Science 344, 78–80 (2014).

  3. 3.

    Spencer, J. R. et al. Cassini encounters Enceladus: background and the discovery of a south polar hot spot. Science 311, 1401–1405 (2006).

  4. 4.

    Spahn, F. et al. Cassini dust measurements at Enceladus and implications for the origin of the E ring. Science 311, 1416–1418 (2006).

  5. 5.

    Porco, C. C. et al. Cassini observes the active south pole of Enceladus. Science 311, 1393–1401 (2006).

  6. 6.

    Waite, J. H. Jr et al. Cassini ion and neutral mass spectrometer: Enceladus plume composition and structure. Science 311, 1419–1422 (2006).

  7. 7.

    Hansen, C. J. et al. Enceladus water vapor plume. Science 311, 1422–1425 (2006).

  8. 8.

    Postberg, F. et al. Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus. Nature 459, 1098–1101 (2009).

  9. 9.

    Postberg, F., Schmidt, J., Hillier, J., Kempf, S. & Srama, R. A salt-water reservoir as the source of a compositionally stratified plume on Enceladus. Nature 474, 620–622 (2011).

  10. 10.

    Hsu, S. et al. Ongoing hydrothermal activities within Enceladus. Nature 519, 207–210 (2015).

  11. 11.

    Sekine, Y. et al. High-temperature water-rock interactions and hydrothermal environments in the chondrite-like core of Enceladus. Nat. Commun. 6, 8604 (2015).

  12. 12.

    Waite, J. H., Jr et al. Cassini finds molecular hydrogen in the Enceladus plume: evidence for hydrothermal processes. Science 356, 155–159 (2017).

  13. 13.

    Choblet, G. et al. Powering prolonged hydrothermal activity inside Enceladus. Nat. Astronomy 1, 841–847 (2017).

  14. 14.

    Postberg, F. et al. The E ring in the vicinity of Enceladus. II. Probing the moon’s interior—the composition of E-ring particles. Icarus 193, 438–454 (2008).

  15. 15.

    Waite, J. H. Jr et al. Liquid water on Enceladus from observations of ammonia and 40Ar in the plume. Nature 460, 487–490 (2009); corrigendum 460, 1164 (2009).

  16. 16.

    Kempf, S., Beckmann, U. & Schmidt, J. How the Enceladus dust plume feeds Saturn’s E ring. Icarus 206, 446–457 (2010).

  17. 17.

    Postberg, F. et al. Discriminating contamination from particle components in spectra of Cassini’s dust detector CDA. Planet. Space Sci. 57, 1359–1374 (2009).

  18. 18.

    Goldsworthy, B. J. et al. Time of flight mass spectra of ions in plasmas produced by hypervelocity impacts of organic and mineralogical microparticles on a cosmic dust analyser. Astron. Astrophys. 409, 1151–1167 (2003).

  19. 19.

    Srama, R. et al. Mass spectrometry of hyper-velocity impacts of organic micro grains. Rapid Commun. Mass Spectrom. 23, 3895–3906 (2009).

  20. 20.

    Silverstein, R. M., Webster, F. X. & Kiemle, D. J. Spectrometric Identification of Organic Compounds 7th edn, 1–70 (John Wiley and Sons, Hoboken, 2005).

  21. 21.

    Dass, C. Fundamentals of Contemporary Mass Spectrometry 1st edn 210–238 (John Wiley and Sons, Hoboken, 2007).

  22. 22.

    Schmidt, J., Brilliantov, N., Spahn, F. & Kempf, S. Slow dust in Enceladus’ plume from condensation and wall collisions in tiger stripe fractures. Nature 451, 685–688 (2008).

  23. 23.

    Teolis, B. D., Perry, M. E., Magee, B. A., Westlake, J. & Waite, J. H. Detection and measurement of ice grains and gas distribution in the Enceladus plume by Cassini’s ion neutral mass spectrometer. J. Geophys. Res. 115, A09222 (2010).

  24. 24.

    Yeoh, S. K., Chapman, T. A., Goldstein, D. B., Varghese, P. & Trafton, L. M. On understanding the physics of the Enceladus south polar plume via numerical simulation. Icarus 253, 205–222 (2015).

  25. 25.

    Matson, D. L., Castillo-Rogez, J. C., Davies, A. G. & Johnson, T. V. Enceladus: a hypothesis for bringing both heat and chemicals to the surface. Icarus 221, 53–62 (2012).

  26. 26.

    de Leeuw, G. et al. Production flux of sea spray aerosol. Rev. Geophys. 49, RG2001 (2011).

  27. 27.

    Wilson, T. W. et al. A marine biogenic source of atmospheric ice-nucleation particles. Nature 525, 234–238 (2015).

  28. 28.

    Leck, C. & Bigg, E. K. Comparison of sources and nature of the tropical aerosol with the summer high arctic aerosol. Tellus B 60, 118–126 (2008).

  29. 29.

    Gantt, B. & Meskhidze, N. The physical and chemical characteristics of marine primary organic aerosol: a review. Atmos. Chem. Phys. 13, 3979–3996 (2013).

  30. 30.

    Porco, C. C., Dones, L. & Mitchell, C. Could it be snowing microbes on Enceladus? Assessing conditions in its plume and implications for future missions. Astrobiology 17, 876–901 (2017).

  31. 31.

    Srama, R. et al. The Cassini cosmic dust analyzer. Space Sci. Rev. 114, 465–518 (2004).

  32. 32.

    Hillier, J. K., McBride, N., Green, S. F., Kempf, S. & Srama, R. Modelling CDA mass spectra. Planet. Space Sci. 54, 1007–1013 (2006).

  33. 33.

    Postberg, F. et al. Composition of jovian dust stream particles. Icarus 183, 122–134 (2006).

  34. 34.

    Srama, R. et al. The cosmic dust analyzer onboard Cassini: ten years of discoveries. CEAS Space Jour. 2, 3–16 (2011).

  35. 35.

    Fiege, K. et al. Calibration of relative sensitivity factors for impact ionization detectors with high-velocity silicate microparticles. Icarus 241, 336–345 (2014).

  36. 36.

    Mocker, A. et al. A 2 MV Van de Graaff accelerator as a tool for planetary and impact physics research. Rev. Sci. Instrum. 82, 095111 (2011).

  37. 37.

    Delcorte, A., Segda, B. G. & Bertrand, P. ToF-SIMS analyses of polystyrene and dibenzanthracene: evidence for fragmentation and metastable decay processes in molecular secondary ion emission. Surf. Sci. 381, 18–32 (1997).

  38. 38.

    Stephan, T., Jessberger, E. K., Heidd, C. H. & Rost, D. TOF-SIMS analysis of polycyclic aromatic hydrocarbons in Allan Hills 84001. Meteorit. Planet. Sci. 38, 109–116 (2003).

  39. 39.

    Wiederschein, F., Vöhringer-Martinez, E. & Postberg, F. Charge separation and isolation in strong water droplet impacts. Phys. Chem. Chem. Phys. 17, 6858–6864 (2015).

  40. 40.

    Le Roy, L. et al. COSIMA calibration for the detection and characterization of the cometary solid organic matter. Planet. Space Sci. 105, 1–25 (2015).

  41. 41.

    Waite, J. H. Jr et al. The Cassini ion and neutral mass spectrometer (INMS) investigation. Space Sci. Rev. 114, 113–231 (2004).

  42. 42.

    Perry, M. E. et al. Cassini INMS measurements of Enceladus plume density. Icarus 257, 139–162 (2015).

  43. 43.

    Kieffer, S. W. et al. A clathrate reservoir hypothesis for Enceladus’ south polar plume. Science 314, 1764–1766 (2006).

  44. 44.

    Ingersoll, A. P. & Pankine, A. A. Subsurface heat transfer on Enceladus: conditions under which melting occurs. Icarus 206, 594–607 (2010).

  45. 45.

    Gaston, C. J. et al. Unique ocean-derived particles serve as a proxy for changes in ocean chemistry. J. Geophys. Res. 116, D18310 (2011).

  46. 46.

    Keene, W. C. et al. Chemical and physical characteristics of nascent aerosols produced by bursting bubbles at a model air–sea interface. J. Geophys. Res. 112, D21202 (2007).

  47. 47.

    Facchini, M. C. et al. Primary submicron marine aerosol dominated by insoluble organic colloids and aggregates. Geophys. Res. Lett. 35, L17814 (2008).

  48. 48.

    Russell, L. M. et al. Carbohydrate-like composition of submicron atmospheric particles and their production from ocean bubble bursting. Proc. Natl Acad. Sci. USA 107, 6652–6657 (2010).

  49. 49.

    Burrows, S. M. et al. A physically based framework for modeling the organic fractionation of sea spray aerosol from bubble film Langmuir equilibria. Atmos. Chem. Phys. 14, 13601–13629 (2014).

  50. 50.

    Jayarathne, T. et al. Enrichment of saccharides and divalent cations in sea spray aerosol during two phytoplankton blooms. Environ. Sci. Technol. 50, 11511–11520 (2016).

  51. 51.

    Schmitt-Kopplin, P. et al. Dissolved organic matter in sea spray: a transfer study from marine surface water to aerosols. Biogeosciences 9, 1571–1582 (2012).

  52. 52.

    McCollom, T. M. et al. The influence of carbon source on abiotic organic synthesis and carbon isotope fractionation under hydrothermal conditions. Geochim. Cosmochim. Acta 74, 2717–2740 (2010).

  53. 53.

    Milesi, V. et al. Thermodynamic constraints on the formation of condensed carbon from serpentinization fluids. Geochim. Cosmochim. Acta 189, 391–403 (2016).

  54. 54.

    Williams, L. B. et al. Organic molecules formed in a “primordial womb”. Geology 33, 913–916 (2005).

  55. 55.

    Cody, G. D. et al. Establishing a molecular relationship between chondritic and cometary organic solids. Proc. Natl Acad. Sci. USA 108, 19171–19176 (2011).

  56. 56.

    Stevenson, F. J. Humus Chemistry: Genesis, Composition, Reactions (Wiley-Interscience, New York, 1982).

  57. 57.

    Hänninen, K. I., Klöcking, R. & Helbig, B. Synthesis and characterization of humic acid-like polymers. Sci. Total Environ. 62, 201–210 (1987).

  58. 58.

    Alexander, C. M. O’D. et al. The nature, origin and modification of insoluble organic matter in chondrites, the major source of Earth’s C and N. Chem. Erde 77, 227–256 (2017).

  59. 59.

    Alexander, C. M. O’D., Fogel, M., Yabuta, H. & Cody, G. D. The origin and evolution of chondrites recorded in the elemental and isotopic compositions of their macromolecular organic matter. Geochim. Cosmochim. Acta 71, 4380–4403 (2007).

  60. 60.

    Derenne, S. & Robert, F. Model of molecular structure of the insoluble organic matter isolated from Murchison meteorite. Meteorit. Planet. Sci. 45, 1461–1475 (2010).

  61. 61.

    Remusat, L. Organic material in meteorites and the link to the origin of life. BIO Web Conf. 2, 03001 (2014).

  62. 62.

    Sephton, M. Organic compounds in carbonaceous meteorites. Nat. Prod. Rep. 19, 292–311 (2002).

  63. 63.

    Pizzarello, S., Cooper, G. W. & Flynn, G. J. in Meteorites and the Early Solar System II (eds Lauretta D. & McSween H. Y.) 625–651 (Univ. of Arizona Press, Tucson, 2006).

  64. 64.

    Bardyn, A. et al. Carbon-rich dust in comet 67P/Churyumov–Gerasimenko measured by COSIMA/Rosetta. Mon. Not. R. Astron. Soc. 469, S712–S722 (2017).

  65. 65.

    Altwegg, K. et al. Organics in comet 67p – a first comparative analysis of mass spectra from ROSINA–DFMS, COSAC and Ptolemy. Mon. Not. R. Astron. Soc. 469, S130–S141 (2017).

  66. 66.

    Tissot, B. P. & Welte, D. H. Petroleum Formation and Occurrence 2nd edn (Springer, Berlin, 1984).

  67. 67.

    Didyk, B. M. & Simoneit, B. R. T. Hydrothermal oil of Guaymas Basin and implications for petroleum formation mechanisms. Nature 342, 65–69 (1989).

  68. 68.

    McKinnon, W. B., Simonelli, D. P. & Schubert, G. in Pluto and Charon (eds Stern, S. A. & Tholen, D. J.) 295–343 (Univ. of Arizona Press, Tucson, 1997).

  69. 69.

    Sephton, M. A. et al. Hydropyrolysis: a new technique for the analysis of macromolecular material in meteorites. Planet. Space Sci. 53, 1280–1286 (2005).

  70. 70.

    Kissel, J. & Krueger, F. R. The organic component in dust from comet Halley as measured by the PUMA mass spectrometer on board Vega 1. Nature 326, 755–760 (1987).

  71. 71.

    Schoell, M. Multiple origins of methane in the Earth. Chem. Geol. 71, 1–10 (1988).

  72. 72.

    Von Damm, K. L. et al. The Escanaba Trough, Gorda Ridge hydrothermal system: temporal stability and subseafloor complexity. Geochim. Cosmochim. Acta 69, 4971–4984 (2005).

  73. 73.

    Horányi, M., Juhász, A. & Morfill, G. E. Large-scale structure of Saturn’s E-ring. Geophys. Res. Lett. 35, L04203 (2008).

  74. 74.

    Hsu, H.-W. et al. Understanding the E-ring puzzle. AGU Fall General Assembly, abstr. P33E-01 (2016).

  75. 75.

    Waite, J. H. et al. The process of tholin formation in Titan’s upper atmosphere. Science 316, 870–875 (2007).

  76. 76.

    Lavvas, P. et al. Aerosol growth in Titan’s ionosphere. Proc. Natl Acad. Sci. USA 110, 2729–2734 (2013).

  77. 77.

    Charvat, A. & Abel, B. How to make big molecules fly out of liquid water: applications, features and physics of laser assisted liquid phase dispersion mass spectrometry. Phys. Chem. Chem. Phys. 9, 3335–3360 (2007).

  78. 78.

    Srama, R. Cassini–Huygens and Beyond—Tools for Dust Astronomy. Habil. Thesis, Univ. of Stuttgart (2009).

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Acknowledgements

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.

Author information

Author notes

  1. These authors contributed equally: Frank Postberg, Nozair Khawaja.

Affiliations

  1. Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany

    • Frank Postberg
    • , Nozair Khawaja
    • , Fabian Klenner
    • , Lenz Nölle
    • , René Reviol
    •  & Mario Trieloff
  2. Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Heidelberg, Germany

    • Frank Postberg
    •  & Mario Trieloff
  3. Institut für Geologische Wissenschaften, Freie Universität Berlin, Berlin, Germany

    • Frank Postberg
  4. Leibniz-Institute für Oberflächenmodifizierung (IOM), Leipzig, Germany

    • Bernd Abel
    •  & Ferdinand Stolz
  5. Laboratoire de Planétologie et Géodynamique, UMR-CNRS 6112, Université de Nantes, Nantes, France

    • Gael Choblet
    • , Gabriel Tobie
    •  & J. Hunter Waite
  6. Space Science and Engineering Division, Southwest Research Institute, San Antonio, TX, USA

    • Christopher R. Glein
    •  & Brian Magee
  7. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

    • Murthy S. Gudipati
    •  & Bryana L. Henderson
  8. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA

    • Hsiang-Wen Hsu
    • , Sascha Kempf
    •  & Brian Magee
  9. Institut für Raumfahrtsysteme, Universität Stuttgart, Stuttgart, Germany

    • Georg Moragas-Klostermeyer
    •  & Ralf Srama
  10. Applied Physics Laboratory, Johns Hopkins University, Laurel, MD, USA

    • Mark Perry
  11. Astronomy Research Unit, University of Oulu, Oulu, Finland

    • Jürgen Schmidt
  12. Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Leipzig, Germany

    • Ferdinand Stolz

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Contributions

F.P. led the writing of the manuscript; N.K. and F.P. led the CDA data analysis with support from L.N.; G.M.-K. and R.S. designed the CDA observation; N.K., H-W.H., S.K., L.N. and F.P. did the programming and CDA data reduction; F.K., R.R., F.S., M.S.G. and B.L.H. conducted laboratory experiments; J.H.W. led the INMS observation; B.M. and M.P. conducted the INMS data reduction and analysis; F.P., J.S., C.R.G., M.T., G.T., G.C., M.S.G. and B.A. were responsible for the geophysical and geochemical interpretation of the data. All authors contributed to the discussion and commented on the manuscript.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Frank Postberg.

Extended data figures and tables

  1. Extended Data Fig. 1 Semi-quantitative display of CDA HMOC spectra.

    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.

  2. Extended Data Fig. 2 Impact ionization laboratory spectrum of a polystyrene bead.

    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.

  3. Extended Data Fig. 3 Comparison of CDA HMOC spectra from fast and slow impacts.

    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 CH 3 + (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.

  4. Extended Data Fig. 4 Example CDA spectra from individual HMOC-type ice grains.

    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+, H 2 + and H 3 + ) at 1 u, 2 u and 3 u, as well as the disintegration of the CH 3 + ion into CH 2 + , 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 CH 3 + is usually not observed in this speed regime.

  5. 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.

  6. Extended Data Fig. 6 CDA HMOC spectrum recorded in the Enceladean plume.

    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.

  7. 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.

  8. Extended Data Fig. 8 Laser ionization analogue spectrum of pyrene in water–acetic acid mixture.

    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.

  9. 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.

  10. Extended Data Fig. 10 Co-added INMS ice grain spike spectrum from three plume encounters.

    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).

  11. Extended Data Fig. 11 INMS spectra used to produce the differenced spectrum in Fig. 3.

    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.

  12. 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.

  13. Extended Data Table 1 List of 83 CDA mass spectra identified as of HMOC type

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https://doi.org/10.1038/s41586-018-0246-4

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