Ongoing hydrothermal activities within Enceladus

Journal name:
Nature
Volume:
519,
Pages:
207–210
Date published:
DOI:
doi:10.1038/nature14262
Received
Accepted
Published online

Detection of sodium-salt-rich ice grains emitted from the plume of the Saturnian moon Enceladus suggests that the grains formed as frozen droplets from a liquid water reservoir that is, or has been, in contact with rock1, 2. Gravitational field measurements suggest a regional south polar subsurface ocean of about 10 kilometres thickness located beneath an ice crust 30 to 40 kilometres thick3. These findings imply rock–water interactions in regions surrounding the core of Enceladus. The resulting chemical ‘footprints’ are expected to be preserved in the liquid and subsequently transported upwards to the near-surface plume sources, where they eventually would be ejected and could be measured by a spacecraft4. Here we report an analysis of silicon-rich, nanometre-sized dust particles5, 6, 7, 8 (so-called stream particles) that stand out from the water-ice-dominated objects characteristic of Saturn. We interpret these grains as nanometre-sized SiO2 (silica) particles, initially embedded in icy grains emitted from Enceladus’ subsurface waters and released by sputter erosion in Saturn’s E ring. The composition and the limited size range (2 to 8 nanometres in radius) of stream particles indicate ongoing high-temperature (>90 °C) hydrothermal reactions associated with global-scale geothermal activity that quickly transports hydrothermal products from the ocean floor at a depth of at least 40 kilometres up to the plume of Enceladus.

At a glance

Figures

  1. Identifying particle constituents.
    Figure 1: Identifying particle constituents.

    Shown is a co-added impact ionization mass spectrum from 32 selected Saturnian stream particle spectra with the strongest Si+ signals. As expected, the impacts produce more ions from the CDA’s target material (Rh+ and Rh2+; blue areas) and the target contaminants6, 10 (C+, H+; blue areas, H+ not shown) than from the nanoparticle itself. Ions O+ and Si+ are the most abundant potential particle mass lines. Na+/Mg+ (solidus indicates the two species can not be distinguished) form the only other potential particle mass line with a signal-to-noise ratio above 3σ (dashed line; σ, standard deviation). The particle composition agrees best with pure silica when the target impurities and the impact ionization process are taken into account (Methods).

  2. Minimum temperatures for formation of silica nanoparticles.
    Figure 2: Minimum temperatures for formation of silica nanoparticles.

    a, Solid lines show ΣSiO2 of a serpentine–talc/saponite buffer equilibrium as a function of temperature (x axis) and pH (line colour: see key above). This buffer system is consistent with the measured ΣSiO2 in fluid samples of the hydrothermal experiments using an orthopyroxene and olivine powder mixture at 400-bar pressure (filled black circles annotated with in situ pH values; Methods). Dashed lines show the 0 °C silica solubility at the respective pH. The difference between the solid and dashed lines determines the amount of ΣSiO2 available for silica nanoparticle formation at the respective pH. Insets, images of silica nanoparticles formed in cooled solutions. b, Relationships between minimum hydrothermal fluid temperatures and fluid pH for silica nanoparticle formation. Red and blue colours represent results with increasing and fixed pH, respectively, upon cooling and mixing with seawater. Data points show results for Na+ concentration 0.1 mol kg−1 and pressure 30 bar; shaded areas represent the uncertainties in Na+ concentrations (0.05–0.3 mol kg−1) and pressure (10–80 bar; ref. 3).

  3. A schematic of Enceladus/' interior.
    Figure 3: A schematic of Enceladus’ interior.

    The internal structure and conditions of Enceladus beneath its south polar region derived from this and previous work. The main components (core, subsurface ocean, ice crust and plume) are shown left to right; top row gives temperature and chemical properties of each component, middle row shows schematic structure, and bottom row gives physical properties. Distances labelling the grey line below the middle row are distances from the centre of Enceladus towards its south pole (not to scale).

  4. Concentration of silica nanoparticles in E-ring grains.
    Figure 4: Concentration of silica nanoparticles in E-ring grains.

    The mass fraction of silica nanoparticles in E-ring ice grains is estimated by comparing the production rates derived from the dynamical model (sloping red lines) and CDA measurements (blue horizontal line and shaded region). We assume that the stream particle release rate is directly proportional to the E-ring sputtering erosion rate. The steeper the power-law size distribution slope (μ), the larger the total surface area of E-ring grains and thus the higher the production rate of silica nanoparticles. The lower limit for the nanosilica mass fraction is ~150 p.p.m. (equivalent to 2.5 mM shown in the lower x axis) with μ = 5.4 (yellow dashed line)24.

  5. Maps of grain density, sputtering erosion rate, and stream particle production rate in the E-ring region.
    Extended Data Fig. 1: Maps of grain density, sputtering erosion rate, and stream particle production rate in the E-ring region.

    a, The total E-ring ice grain surface area map in the ρz frame, where ρ and z are distance to Saturn’s rotation axis and to the ring plane, respectively. Note that each bin integrates azimuthally over the entire torus, meaning that the outer bins contain a much larger volume than do the inner ones. b, Plasma sputtering erosion rate of E-ring ice grains in torus segments. The total sputtering rate is 8.6 × 1024 H2O molecules per second, lower but still comparable to the 4.5 × 1025 H2O molecules per second derived in ref. 32. c, Normalized nanoparticle production rate in particles per second. RS, Saturn radius.

  6. Ejection probability of 5-nm particles from the E ring.
    Extended Data Fig. 2: Ejection probability of 5-nm particles from the E ring.

    a, For silica nanoparticles, the ejection probability is mostly close to unity (except within 4.5RS). The higher local plasma density there leads to negative dust potential and thus reduces the ejection probability7. The typical timescale for silica nanoparticles to acquire sufficient kinetic energy to escape is of the order of a day7. b, Water ice nanoparticles have lower secondary emission and are charged less positively and thus are less likely to be ejected. This ‘forbidden region’ (the black region) extends further outward to ~5.5 RS, consistent with the CDA measurements53.

  7. Stream particle emission patterns.
    Extended Data Fig. 3: Stream particle emission patterns.

    a, Ejection region (ER) profiles, derived from the nanodust and solar wind measurements (blue)7 and the ejection model (red), both peak at 7–9RS. The uncertainty of both profiles stems from the adopted co-rotation fraction of Saturn’s magnetosphere (80–100%), which determines the electromagnetic acceleration amplitude. The location of the outer rim of Saturn’s A ring and the orbits of icy satellites are marked by grey dashed lines. b, Latitudinal-dependent ejection pattern. Scatter and binned stream particle rates (normalized to 25RS distance) are shown in blue squares and crosses, respectively. The vertical length of the crosses represents the standard deviation of the stream particle rate in the corresponding bin. Our model (red) reproduces the measured trend. c, d, Modelled patterns assuming direct ejection from Enceladus. While the ER profile is similar, these particles are only ejected along the ring plane.

  8. Energy dispersive spectrum of clustered silica nanoparticles formed from the fluid sample.
    Extended Data Fig. 4: Energy dispersive spectrum of clustered silica nanoparticles formed from the fluid sample.

    See Methods for details.

Tables

  1. Stream particle flux measurements
    Extended Data Table 1: Stream particle flux measurements

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Author information

  1. These authors contributed equally to this work.

    • Hsiang-Wen Hsu,
    • Frank Postberg &
    • Yasuhito Sekine

Affiliations

  1. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA

    • Hsiang-Wen Hsu,
    • Sascha Kempf,
    • Mihály Horányi &
    • Antal Juhász
  2. Institut für Geowissenschaften, Universität Heidelberg, 69120 Heidelberg, Germany

    • Frank Postberg
  3. Institut für Raumfahrtsysteme, Universität Stuttgart, 70569 Stuttgart, Germany

    • Frank Postberg,
    • Georg Moragas-Klostermeyer &
    • Ralf Srama
  4. Department of Complexity Science and Engineering, University of Tokyo, Kashiwa 277-8561, Japan

    • Yasuhito Sekine
  5. Laboratory of Ocean–Earth Life Evolution Research, JAMSTEC, Yokosuka 237-0061, Japan

    • Takazo Shibuya
  6. Institute for Particle and Nuclear Physics, Wigner RCP, 1121 Budapest, Hungary

    • Antal Juhász
  7. European Space Agency, ESAC, E-28691 Madrid, Spain

    • Nicolas Altobelli
  8. Research and Development Center for Submarine Resources, JAMSTEC, Yokosuka 237-0061, Japan

    • Katsuhiko Suzuki &
    • Yuka Masaki
  9. Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan

    • Tatsu Kuwatani
  10. Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan

    • Shogo Tachibana
  11. Graduate School of Environmental Sciences, Nagoya University, Nagoya 464-8601, Japan

    • Sin-iti Sirono

Contributions

H.-W.H., F.P. and Y.S. outlined the study and wrote the Letter. H.-W.H. performed the CDA dynamic analyses with assistance from A.J., M.H. and S.K.; F.P. and S.K. performed the CDA composition analyses; S.K., G.M.-K. and R.S. performed the CDA measurements and initial data processing; F.P. and N.A. performed the CDA mass spectra data acquisition and data reduction; Y.S. performed the experiments and calculations simulating Enceladus’ ocean conditions; T.S. designed the hydrothermal experiments and the analysis system; S.T. synthesized starting minerals for the experiments; K.S., Y.M. and T.K. contributed to performing fluid and solid analyses in the experiments; and S.-i.S. estimated the lifetime of silica nanoparticles in Enceladus’ ocean. All authors discussed the results and commented on the manuscript.

Competing financial interests

The authors declare no competing financial interests.

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Author details

Extended data figures and tables

Extended Data Figures

  1. Extended Data Figure 1: Maps of grain density, sputtering erosion rate, and stream particle production rate in the E-ring region. (237 KB)

    a, The total E-ring ice grain surface area map in the ρz frame, where ρ and z are distance to Saturn’s rotation axis and to the ring plane, respectively. Note that each bin integrates azimuthally over the entire torus, meaning that the outer bins contain a much larger volume than do the inner ones. b, Plasma sputtering erosion rate of E-ring ice grains in torus segments. The total sputtering rate is 8.6 × 1024 H2O molecules per second, lower but still comparable to the 4.5 × 1025 H2O molecules per second derived in ref. 32. c, Normalized nanoparticle production rate in particles per second. RS, Saturn radius.

  2. Extended Data Figure 2: Ejection probability of 5-nm particles from the E ring. (130 KB)

    a, For silica nanoparticles, the ejection probability is mostly close to unity (except within 4.5RS). The higher local plasma density there leads to negative dust potential and thus reduces the ejection probability7. The typical timescale for silica nanoparticles to acquire sufficient kinetic energy to escape is of the order of a day7. b, Water ice nanoparticles have lower secondary emission and are charged less positively and thus are less likely to be ejected. This ‘forbidden region’ (the black region) extends further outward to ~5.5 RS, consistent with the CDA measurements53.

  3. Extended Data Figure 3: Stream particle emission patterns. (327 KB)

    a, Ejection region (ER) profiles, derived from the nanodust and solar wind measurements (blue)7 and the ejection model (red), both peak at 7–9RS. The uncertainty of both profiles stems from the adopted co-rotation fraction of Saturn’s magnetosphere (80–100%), which determines the electromagnetic acceleration amplitude. The location of the outer rim of Saturn’s A ring and the orbits of icy satellites are marked by grey dashed lines. b, Latitudinal-dependent ejection pattern. Scatter and binned stream particle rates (normalized to 25RS distance) are shown in blue squares and crosses, respectively. The vertical length of the crosses represents the standard deviation of the stream particle rate in the corresponding bin. Our model (red) reproduces the measured trend. c, d, Modelled patterns assuming direct ejection from Enceladus. While the ER profile is similar, these particles are only ejected along the ring plane.

  4. Extended Data Figure 4: Energy dispersive spectrum of clustered silica nanoparticles formed from the fluid sample. (157 KB)

    See Methods for details.

Extended Data Tables

  1. Extended Data Table 1: Stream particle flux measurements (702 KB)

Additional data