Cassini finds molecular hydrogen in the Enceladus plume Waite, J. Hunter; Glein, Christopher R.; Perryman, Rebecca S. ...
Science (American Association for the Advancement of Science),
04/2017, Letnik:
356, Številka:
6334
Journal Article
Recenzirano
Saturn’s moon Enceladus has an ice-covered ocean; a plume of material erupts from cracks in the ice. The plume contains chemical signatures of water-rock interaction between the ocean and a rocky ...core. We used the Ion Neutral Mass Spectrometer onboard the Cassini spacecraft to detect molecular hydrogen in the plume. By using the instrument’s open-source mode, background processes of hydrogen production in the instrument were minimized and quantified, enabling the identification of a statistically significant signal of hydrogen native to Enceladus. We find that the most plausible source of this hydrogen is ongoing hydrothermal reactions of rock containing reduced minerals and organic materials. The relatively high hydrogen abundance in the plume signals thermodynamic disequilibrium that favors the formation of methane from CO₂ in Enceladus’ ocean.
The Cassini spacecraft passed within 168.2 kilometers of the surface above the southern hemisphere at 19:55:22 universal time coordinated on 14 July 2005 during its closest approach to Enceladus. ...Before and after this time, a substantial atmospheric plume and coma were observed, detectable in the Ion and Neutral Mass Spectrometer (INMS) data set out to a distance of over 4000 kilometers from Enceladus. INMS data indicate that the atmospheric plume and coma are dominated by water, with significant amounts of carbon dioxide, an unidentified species with a mass-to-charge ratio of 28 daltons (either carbon monoxide or molecular nitrogen), and methane. Trace quantities (<1%) of acetylene and propane also appear to be present. Ammonia is present at a level that does not exceed 0.5%. The radial and angular distributions of the gas density near the closest approach, as well as other independent evidence, suggest a significant contribution to the plume from a source centered near the south polar cap, as distinct from a separately measured more uniform and possibly global source observed on the outbound leg of the flyby.
The plume composition at Enceladus contains clues about conditions and processes in the interior. We present new geochemical interpretations of Cassini mass spectrometry data from the plume gas and ...salt‐rich ice grains. It is found that self‐consistency between the data sets can be achieved with a derived range of 10−4.6 to 10−3.2 for the activity of CO2 in Enceladus' ocean. This range is compatible with long‐term buffering by reduced or oxidized seafloor rocks containing quartz, talc, and carbonate minerals in the MgO–FeO–SiO2–CO2–H2O system. Reaction path modeling shows that these types of rocks can be produced from accreted CO2‐rich fluids reacting with hydrous chondritic rocks over an intermediate regime of carbonation. These results, together with previous findings of silica and H2 at Enceladus, support the hypothesis of a heterogeneous structure for the rocky core (carbonated upper layer, serpentinized interior), which provides a geochemical gradient for habitability.
Plain Language Summary
Enceladus, an ocean‐harboring moon of Saturn, erupts a plume that contains gases and frozen sea spray into space. By understanding the composition of the plume, we can learn about what the ocean is like, how it got to be this way, and whether it provides environments where life as we know it could survive. This study presents a new perspective for analyzing the plume composition to estimate the concentration of dissolved carbon dioxide in the ocean. We find that the derived range based on two different data sets is intriguingly similar to what would be expected from the dissolution and formation of certain mixtures of silicon‐ and carbon‐bearing minerals at the seafloor. The deduced combination of minerals may be indicative of a fundamental process that has sequestered a large amount of Enceladus' initial inventory of carbon dioxide into the rocky core. This inference echoes an emerging vision of a complex interior that hosts geochemically diverse environments. The dynamic interface of such complexity is where energy sources for possible life may arise.
Key Points
New estimates of the thermodynamic activity of CO2 in Enceladus' ocean based on the carbonate chemistry of the plume
Formation of CO2‐buffering assemblages of quartz‐talc‐carbonate from partial carbonation of chondritic compositions
Distinct sources of observed CO2, silica, and H2 imply mineralogically and thermally diverse environments in the rocky core
The internal ocean of Enceladus can be expected to present conditions favorable to the trapping of volatiles in clathrates. This process could influence the eventual composition of the ocean and ...therefore of the plumes emitted by the south polar region. Here we used a statistical thermodynamic model to assess which species detected in the plumes by the Cassini‐Ion and Neutral Mass Spectrometer experiment are trapped in clathrates. We treated Enceladus' internal ocean as a terrestrial subglacial lake with a mixture of dissolved volatiles indicated by plume gas measurements. We find that the conditions for clathrate formation are met in this ocean, except above 20 km or in hypothetical hot spots. The formation of multiple guest clathrates depletes methane below plume levels, suggesting that clathrates eventually dissociate (releasing methane) in the fissure that connects the ocean to the surface or that another mechanism (such as hydrothermal reactions) is compensating by adding methane into the ocean.
Key Points
Enceladus' ocean is modeled as a subglacial lake
The conditions in Enceladus' internal ocean are met for production of clathrates
Efficient trapping of methane reduces its abundance below plume levels
The Cassini Ion Neutral Mass Spectrometer (INMS) has obtained the first in situ composition measurements of the neutral densities of molecular nitrogen, methane, molecular hydrogen, argon, and a host ...of stable carbon-nitrile compounds in Titan's upper atmosphere. INMS in situ mass spectrometry has also provided evidence for atmospheric waves in the upper atmosphere and the first direct measurements of isotopes of nitrogen, carbon, and argon, which reveal interesting clues about the evolution of the atmosphere. The bulk composition and thermal structure of the moon's upper atmosphere do not appear to have changed considerably since the Voyager 1 flyby.
Saturn's moon Enceladus harbours a global water ocean
, which lies under an ice crust and above a rocky core
. Through warm cracks in the crust
a cryo-volcanic plume ejects ice grains and vapour into ...space
that contain materials originating from the ocean
. Hydrothermal activity is suspected to occur deep inside the porous core
, powered by tidal dissipation
. So far, only simple organic compounds with molecular masses mostly below 50 atomic mass units have been observed in plume material
. 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.
The Cassini Ion Neutral Mass Spectrometer (INMS) has measured both neutral and ion species in Titan's upper atmosphere and ionosphere and the Enceladus plumes. Ion densities derived from INMS ...measurements are essential data for constraining photochemical models of Titan's ionosphere. The objective of this paper is to present an optimized method for converting raw data measured by INMS to ion densities. To do this, we conduct a detailed analysis of ground and in‐flight calibration to constrain the instrument response to ion energy, the critical parameter on which the calibration is based. Data taken by the Cassini Radio Plasma Wave Science Langmuir Probe and the Cassini Plasma Spectrometer Ion Beam Spectrometer are used as independent measurement constraints in this analysis. Total ion densities derived with this method show good agreement with these data sets in the altitude region (∼1100–1400 km) where ion drift velocities are low and the mass of the ions is within the measurement range of the INMS (1–99 Daltons). Although ion densities calculated by the method presented here differ slightly from those presented in previous INMS publications, we find that the implications for the science presented in previous publications is mostly negligible. We demonstrate the role of the INMS ion densities in constraining photochemical models and find that (1) cross sections having high resolution as a function of wavelength are necessary for calculating the initial photoionization products and (2) there are disagreements between the measured ion densities representative of the initial steps in Titan photochemistry that require further investigation.
Key Points
Importance of environmental conditions for analysis of Cassini INMS ion data
Role of INMS ion densities in validating Titan photochemical model simulations
Production and loss of primary photoionization products is not fully understood
A surprisingly strong influx of organic‐rich material into Saturn's upper atmosphere from its rings was observed during the proximal obits of the Grand Finale of the Cassini mission. Measurements by ...the Ion and Neutral Mass Spectrometer (INMS) gave insights into the composition of the material, but it remains to be resolved what fraction of the inferred heavy volatiles should be attributed as originating from the fragmentation of dust particles in the instrument versus natural ablation of grains in the atmosphere. In the present study, we utilize measured light ion and neutral densities to further constrain the abundances of heavy volatiles in Saturn's ionosphere through a steady‐state model focusing on helium ion chemistry. We first show that the principal loss mechanism of He+ in Saturn's equatorial ionosphere is through reactions with species other than H2. Based on the assumption of photochemical equilibrium at altitudes below 2,500 km, we then proceed by estimating the mixing ratio of heavier volatiles down to the closest approaches for Cassini's proximal orbits 288 and 292. Our derived mixing ratios for the inbound part of both orbits fall below those reported from direct measurements by the INMS, with values of ∼2 × 10−4 at closest approaches and order‐of‐magnitude variations in either direction over the orbits. This aligns with previous suggestions that a large fraction of the neutrals measured by the INMS stems from the fragmentation of infalling dust particles that do not significantly ablate in the considered part of Saturn's atmosphere and are thus unavailable for reactions.
Plain Language Summary
During the final orbits of the Cassini mission, the spacecraft flew between Saturn's rings and the planets upper atmosphere. The onboard plasma instruments detected a large amount of ring particles falling toward the planet, but direct measurements of the composition of these grains are complicated due to the high spacecraft speed and instrumental effects. In this study, we present an independent method to estimate the abundance of heavier neutral species entering the atmosphere from infalling ring material. This method relies on helium ion chemistry and the measured light ion and neutral densities. Our results generally fall below those inferred from direct measurements. Together with comparisons to other studies, this potentially suggests that a large fraction of the infalling neutral species do not significantly ablate in the considered part of Saturn's atmosphere (and remain bound to the dust grains instead) and are thus unavailable for reactions.
Key Points
The dominant loss mechanism for He+ ions in Saturn's equatorial ionosphere are reactions with heavier neutral species, not H2
The mixing ratio of volatiles in Saturn's ionosphere can be estimated from light ion and neutral measurements, using helium ion chemistry
Comparing with other studies potentially suggests that only the most volatile species (CO, N2 and CH4) enter the atmosphere as vapor
Since its discovery in the first half of the 20th century, scientists have puzzled over the origins of Titan's atmosphere. Current models suggest that atmospheric N2 on Titan may have originated from ...NH3-bearing ice with N-isotopic ratios similar to those observed in NH2 in cometary comae (14N/15N ∼ 136). In contrast, N2 ice appears to be too 15N poor to explain Titan's atmosphere (14N/15N ∼ 168). Additionally, data from the Rosetta mission to comet 67P/Churyumov-Gerasimenko suggest that the Ar/N2 ratio of outer solar system planetesimals may be too high for a comet-like N2 source on Titan. The Rosetta mission also revealed an astonishing abundance of N-bearing complex organic material. While thermal fractionation of cometary sources during Titan accretion may explain the loss of N2- and Ar-rich ices, more refractory materials such as complex organics would be retained. Later heating in the interior may lead to volatilization of accreted organics, consistent with Cassini-Huygens measurements of 40Ar that suggest outgassing from the interior may have played a role in atmosphere formation. Here, we develop a three endmember mixing model for N isotopes and the 36Ar/14N ratio of Titan's atmosphere, and consider the implications for the source of atmospheric methane. Our model suggests that Titan's interior is likely warm, and that N from accreted organics may contribute on the order of 50% of Titan's present-day nitrogen atmosphere.
We use a thermodynamic statistical model to evaluate how the composition of Europa's internal ocean may have been affected by clathrate hydrate formation. Assuming an input of the observed O2 and CO2 ...from the surface into a mildly acidic ocean (pH < 6), and considering the possibility of contributions by reduced (with CH4 and H2S) or oxidized (CO2-bearing) hydrothermal fluids, we calculate the fractional occupancies in clathrate and deduce the effect on the ocean's composition. The structure of the clathrate formed, and therefore its density and composition is influenced by the relative amount of O2 compared to the other compounds present. We also include a mixture of noble gases-argon, krypton, and xenon-based on cometary abundances measured at comet 67P and find that the Ar/Kr ratio can be affected by almost two orders of magnitude. In most cases, the formed clathrate is likely to become part of the icy crust, with guest molecules possibly accessible to future in situ measurements by the Europa Clipper and JUICE missions.
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