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Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes

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Science  14 Apr 2017:
Vol. 356, Issue 6334, pp. 155-159
DOI: 10.1126/science.aai8703
  • Fig. 1 Observations of H2, H2O, and CO2 by INMS during the E21 flyby.

    (A) Mass 2 (H2), mass 18 (H2O), and mass 44 (CO2) measurements made in CSN mode as counts per integration period (IP); INMS collects 31 ms of signal at each mass. (B) Interleaved OSNB measurements for the same three masses. (C) The velocity range sampled (black points), which corresponds to the field of view of the sampled region and affects both the speed and angle of the measured molecules. The altitude from Enceladus is also shown (black curve). The ordinate in (A) and (B) provides the number of detector ion counts in the indicated unitary mass-to-charge channel of the quadruple mass analyzer during the IP. The abscissa denotes the time of the observation relative to closest approach to Enceladus.

  • Fig. 2 Comparison of OSNB measurements of mass 2 (H2) with the estimated total mass 2 instrumental background.

    The detected count rates and estimated background rates are plotted as a function of time from closest approach to Enceladus. The bottom panel of the plot is on a linear scale showing all the data points at or below 10 counts; the top panel, plotted on the common log scale, shows the remaining data points that are above 10 counts. Data points are color-coded according to the statistical uncertainties and background estimation: open black circles, no distinguishable separation from the background signal; light blue circles, at least 1σ separation; dark blue diamonds, at least 2σ separation; purple triangles, at least 3σ separation.

  • Fig. 3 Distribution of the background-subtracted data.

    This plot shows background-subtracted H2 counts per IP (red) for a period of time from closest approach –1 s to +4 s that captures the portion of the H2 measurements with the highest signal-to-noise ratio. The empirical distribution is compared to a normal distribution (mean, 2.6 counts; SD, 3.8 counts) fit to background-subtracted data with counts ≤ 10. Red bars depict Wilson score confidence intervals of 95%.

  • Fig. 4 Apparent chemical affinity for hydrogenotrophic methanogenesis in the ocean of Enceladus (273 K, 1 bar).

    The orange lines bracket the observed range in the mixing ratio of H2 in the plume gas (Table 1). The dark blue lines are contours of constant ocean pH, a key model parameter. The cyan region indicates affinities for a pH range that may provide the greatest consistency between the results of (13, 15, 25). The dashed burgundy line designates chemical equilibrium, where no energy would be available from methanogenesis. These nominal model results are based on CH4/CO2 = 0.4 (Table 1), a chlorinity of 0.1 molal, and 0.03 molal total dissolved carbonate (25). Reported ranges in these parameters propagate to give an uncertainty in the computed affinities of ~10 kJ (mol CH4)−1.

  • Table 1 The major species composition of Enceladus’ plume gas.

    Volume mixing ratios are derived from Cassini INMS measurements [(20), sections 2.4 and 3.2].

    ConstituentMixing ratio (%)
    H2O96 to 99
    CO20.3 to 0.8
    CH40.1 to 0.3
    NH30.4 to 1.3
    H20.4 to 1.4

Supplementary Materials

  • Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes

    J. Hunter Waite, Christopher R. Glein, Rebecca S. Perryman, Ben D. Teolis, Brian A. Magee, Greg Miller, Jacob Grimes, Mark E. Perry, Kelly E. Miller, Alexis Bouquet, Jonathan I. Lunine, Tim Brockwell, Scott J. Bolton

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
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    • Tables S1 to S11
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