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.
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We acknowledge support from the CDA team, the Cassini project, and NASA. This research was partly supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan, by the Japan Society for the Promotion of Science, and by the Astrobiology Program of the National Institutes of Natural Sciences, Japan. This work was partly supported by the DLR grant 50 OH1103. We thank J. Schmidt, M. Y. Zolotov and D. J. Tobler for discussions, and E. S. Guralnick, J. K. Hillier, A. Rasca and T. Munsat for advice on writing this Letter. Y.S. thanks A. Okubo for her technical help in taking FE-SEM images.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 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 ρ–z 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.
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.
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.
Extended Data Figure 4 Energy dispersive spectrum of clustered silica nanoparticles formed from the fluid sample.
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Hsu, H., Postberg, F., Sekine, Y. et al. Ongoing hydrothermal activities within Enceladus. Nature 519, 207–210 (2015) doi:10.1038/nature14262
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