Postberg Et Al. 2018

LETTER https://doi.org/10.1038/s41586-018-0246-4 Macromolecular organic compounds from the depths of Enceladus Frank Postberg1,2,3,13*, Nozair Khawaja1,13, Bernd Abel4, Gael Choblet5, Christopher R. Glein6, Murthy S. Gudipati7, Bryana L. Henderson7, Hsiang-Wen Hsu8, Sascha Kempf8, Fabian Klenner1, Georg Moragas-Klostermeyer9, Brian Magee6,8, Lenz Nölle1, Mark Perry10, René Reviol1, Jürgen Schmidt11, Ralf Srama9, Ferdinand Stolz4,12, Gabriel Tobie5, Mario Trieloff1,2 & J. Hunter Waite6 Saturn’s moon Enceladus harbours a global water ocean1, which indicate fragments from higher-mass parent molecules (outside the lies under an ice crust and above a rocky core2. Through warm CDA’s nominal mass range) and not a conglomerate of several mole- cracks in the crust3 a cryo-volcanic plume ejects ice grains and cules with masses below 200 u. This interpretation is consistent with the vapour into space4–7 that contain materials originating from the observation that the detection probability increases with impact speed ocean8,9. Hydrothermal activity is suspected to occur deep inside (see Methods, ‘Relative frequency of HMOC-type grains depends on the porous core10–12, powered by tidal dissipation13. So far, only impact speed and distance to Enceladus orbit’; Extended Data Table 1), simple organic compounds with molecular masses mostly below increasing the available energy for fragmentation and ionization. A 50 atomic mass units have been observed in plume material6,14,15. similar fragmentation pattern has been observed in impact experiments Here we report observations of emitted ice grains containing with polymers18,19 containing aromatic subunits (Extended Data Fig. 2; concentrated and complex macromolecular organic material with see Methods, ‘Inferring the origin of HMOC peaks in CDA spectra’). molecular masses above 200 atomic mass units. The data constrain In addition to the nominal mass spectrum (1 u to ∼200 u), the CDA the macromolecular structure of organics detected in the ice grains records extended TOF spectra of the same individual grains up to about and suggest the presence of a thin organic-rich film on top of the 8,000 u with resolution and sensitivity uniformly reduced by a factor of oceanic water table, where organic nucleation cores generated by ten. These extended spectra exhibit abundant macromolecular cations the bursting of bubbles allow the probing of Enceladus’ organic with masses in excess of 200 u in most HMOC-type spectra (Extended inventory in enhanced concentrations. Data Fig. 5). This further supports the interpretation that fragmentation Two mass spectrometers onboard the Cassini spacecraft, the Cosmic of macromolecular parent molecules or networks creates the HMOCs. Dust Analyzer (CDA) and the Ion and Neutral Mass Spectrometer The HMOC sequence always appears together with intense non-wa- (INMS), performed compositional in situ measurements of material ter signatures below 80 u (Fig. 1), offering further compositional and emerging from the subsurface of Enceladus. These measurements were structural constraints. Although there is some variation (Extended Data made inside both the plume and Saturn’s E ring, which is formed by ice Figs. 1, 4), non-water peaks are arranged in six groups at 15 u, 27–31 u, grains escaping Enceladus’ gravity16. 38–45 u, 51–58 u, 63–67 u, and 77–79 u and, like the HMOC pattern, The CDA records time-of-flight (TOF) mass spectra of cations gen- indicate a sequence of organic species with increasing number of car- erated by high-velocity impacts of individual grains onto a rhodium bon atoms. Organic mass lines below 45 u appear preferentially at odd target, with a mass resolution of m/Δm ≈ 20–5014,17. Previous CDA masses (Fig. 1b), indicating a typical cationic hydrocarbon fragmenta- measurements showed that about 25% of the ice grain spectra of the tion fingerprint. However, from stoichiometry, the mass lines at 30 u, E ring, the so-called type-2 spectra, exhibit the presence of organic 31 u, 44 u and 45 u cannot be pure hydrocarbon cations and indicate 8,9,14 + + material . A subgroup (about 3%) of type-2 spectra (see Methods O- or N-bearing cations from hydroxyl (CH2OH , CH3–CH-OH ), sections ‘Dataset’ and ‘Relative frequency of HMOC-type grains ethoxy or carbonyl functional groups or nitrogen-bearing ions (for + + depends on impact speed and distance to Enceladus orbit’; Extended example, CHN22H and CH–32CH–NH ) (Fig. 1b, Extended Data Data Table 1) is characterized by a sequence of repetitive peaks beyond Fig. 4). 80 u (where u is the unified atomic mass unit), usually separated by A prominent peak preceding the HMOC sequence (Fig. 1b) indi- + mass intervals of 12 u–13 u (Fig. 1). In most cases, this sequence cates abundant cationic forms of a benzene ring, phenyl (C6H5 , 77 u) + extends to the maximum mass nominally covered by the CDA, about and benzenium (C6H7 , 79 u). The high abundance of these cations 200 u. The broad and irregular shape of the peaks indicates that they are is highly diagnostic because the energetically favourable cationic aro- 18–20 + composed of multiple unresolved overlapping mass lines (Fig. 1a and matic structure would be tropylium cations (C7H7 , 91 u) (Fig. 1a). Extended Data Fig. 1). The mass intervals between the peaks suggest Formation of tropylium cations must thus be inhibited, as it requires organic species with an increasing number of carbon atoms (C7 to C15), aromatic precursor molecules without hydrogenated C-atoms (for which we refer to as high-mass organic cations (HMOCs). While an example, alkyl groups) attached to the ring20,21 (see Fig. 2, Extended interval of 14 u would indicate the addition of a saturated CH2 group Data Fig. 7 and Methods section ‘Inferring the origin of HMOC peaks to an organic ‘backbone’, the actual average mass difference of 12.5 u in CDA spectra’). indicates the presence of predominately unsaturated carbon atoms. A high abundance of aromatic structures is further supported by + + In principle, each HMOC peak could be derived from a different mass lines at 63 u–65 u (C53H ,4,5), 51 u–53 u (C43H ,4,5) and 39 u (C3H3) parent molecule. However, a monotonic decrease without major inten- (Fig. 1b, Extended Data Fig. 1), which can be interpreted as coincident sity variations from 77 u to 191 u (Fig. 1a, Extended Data Fig. 1) rather unsaturated benzene fragments21 (Fig. 2, Extended Data Fig. 7). 1Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany. 2Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Heidelberg, Germany. 3Institut für Geologische Wissenschaften, Freie Universität Berlin, Berlin, Germany. 4Leibniz-Institute für Oberflächenmodifizierung (IOM), Leipzig, Germany. 5Laboratoire de Planétologie et Géodynamique, UMR-CNRS 6112, Université de Nantes, Nantes, France. 6Space Science and Engineering Division, Southwest Research Institute, San Antonio, TX, USA. 7Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. 8Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA. 9Institut für Raumfahrtsysteme, Universität Stuttgart, Stuttgart, Germany. 10Applied Physics Laboratory, Johns Hopkins University, Laurel, MD, USA. 11Astronomy Research Unit, University of Oulu, Oulu, Finland. 12Wilhelm-Ostwald-Institut für Physikalische und 13 Theoretische Chemie, Universität Leipzig, Leipzig, Germany. These authors contributed equally: Frank Postberg, Nozair Khawaja. *e-mail: [email protected] 564 | NATURE | VOL 558 | 28 JUNE 2018 © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. LETTER RESEARCH a Averaged CDA ice grain spectra with HMOC series 103 Water species 19.0 u Benzene cations Na species 37.5 u 40.8 u 55.4 u HMOC series Rh species 77.6 u 28.9 u 45.0 u 27.0 u 22.8 u 102 62.4 u 73.5 u 91.1 u 103.2 u 115.8 u 128.4 u 140.1 u 153.4 u 165.7 u 178.5 u 191.6 u 1.0 u 3.0 u 14.9 u 101 Amplitude (arbitrary units) 2.0 u 100 1 23456 Time of ight (μs) Line statistics of HMOC spectra b 60 Interference with above inorganic 50 77 species 40 30 20 10 01020304050607080 60 77 91 104 116 129 140 153 165 178 191 50 40 30 Line count (out of 64 mass spectra) 20 10 70 80 90 100 110 120 130 140 1500 160 170 180 190 200 Mass (u) Fig. 1 | Co-added CDA HMOC spectrum and mass line histogram. the histogram makes no distinction of the peak amplitude and shows only a, Time-of-flight spectrum representing the average of 64 high-quality the frequency of occurrence of resolved mass lines. Above 80 u the spectra (see Methods, ‘Selection of 64 high-quality spectra for Fig. 1 and characteristic HMOC mass lines appear, preceded by the peaks from Extended Data Figs. 1 and 3’). The amplitudes of individual spectra were benzene-derived cations at 77 u and 79 u. A peak that can be identified at + normalized and co-added. The corresponding masses of the peaks are around 95 u in some spectra is in agreement with phenyl cations (C6H5 , + labelled. The spectrum provides a representation of the average abundance 77 u) forming a water cluster (C6H5–H2O ), preferentially at lower-impact of cation species. All signatures that are not colour-shaded are exclusively speeds (Extended Data Figs. 1, 3). Organic mass lines below 45 u appear or mostly due to organic cations, as described in the text. Mass lines of preferentially at odd masses, at 15 u, 27 u, 29 u, 39 u, 41 u and 43 u, indicating + + exclusively inorganic origin are H3O at 19 u and Na at 23 u. Mass lines a preference for odd numbers of H atoms with the typical cationic + + + + + where interference with inorganic cations is likely are: H2O–H3O (37 u), hydrocarbon fragmentation fingerprint (CH3 , C23H , C25H , C33H , + + + + + H2O–Na (41 u), (H2O)2–H3O (55 u) and (H2O)3–H3O (73 u).

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