Observational data from the Cassini spacecraft are used to obtain a chemical model of ocean water on Enceladus. The model indicates that Enceladus' ocean is a Na-Cl-CO3 solution with an alkaline pH of ~11-12. The dominance of aqueous NaCl is a feature that Enceladus' ocean shares with terrestrial seawater, but the ubiquity of dissolved Na2CO3 suggests that soda lakes are more analogous to the Enceladus ocean. The high pH implies that the hydroxide ion should be relatively abundant, while divalent metals should be present at low concentrations owing to buffering by clays and carbonates on the ocean floor. The high pH is interpreted to be a key consequence of serpentinization of chondritic rock, as predicted by prior geochemical reaction path models; although degassing of CO2 from the ocean may also play a role depending on the efficiency of mixing processes in the ocean. Serpentinization leads to the generation of H2, a geochemical fuel that can support both abiotic and biological synthesis of organic molecules such as those that have been detected in Enceladus' plume. Serpentinization and H2 generation should have occurred on Enceladus, like on the parent bodies of aqueously altered meteorites; but it is unknown whether these critical processes are still taking place, or if Enceladus' rocky core has been completely altered by past hydrothermal activity. The high pH also suggests that the delivery of oxidants from the surface to the ocean has not been significant, and the rocky core did not experience partial melting and igneous differentiation. On the other hand, the pH is compatible with life as we know it; life on Earth may have begun under similar conditions, and serpentinites on Earth support microbial communities that are centered on H2 that is provided by water-rock reactions.
Fermentation heterotrophy can reproduce Enceladus's plume H2 and CH4 under limited serpentinization
University of Arizona（亚利桑那大学）；ETH Zurich（苏黎世联邦理工学院）；Queen Mary University of London（伦敦大学玛丽女王学院）；Natural History Museum, London（伦敦自然历史博物馆）；Aix Marseille Université（艾克斯-马赛大学）；Université de Toulon（土伦大学）；CNRS（法国国家科学研究中心）；IRD（法国国家科研中心）；ENS–PSL University（巴黎文理研究大学）；École Normale Supérieure（巴黎高等师范学院）；Muséum National d’Histoire Naturelle（法国国家自然历史博物馆）
The ice shell of Enceladus exhibits strong asymmetry between its hemispheres, with all known geysers concentrated over the south pole, even though its orbital configuration is almost perfectly symmetric. By exploring ocean circulation across a range of ocean salinities and core/shell heating partitions, we study the role of ice-ocean interaction in hemispheric symmetry breaking. We find that: (i) asymmetry is enhanced by cross-equatorial ocean heat transport when the ice shell is the major heat source and vice versa, (ii) the magnitude of ocean heat transport is comparable to the global heat p…
Saturn's moon Enceladus offers a unique opportunity in the search for life and habitable environments beyond Earth, a key theme of the National Research Council's 2013-2022 Decadal Survey. A plume of water vapor and ice spews from Enceladus's south polar region. Cassini data suggest that this plume, sourced by a liquid reservoir beneath the moon's icy crust, contain organics, salts, and water-rock interaction derivatives. Thus, the ingredients for life as we know it-- liquid water, chemistry, and energy sources-- are available in Enceladus's subsurface ocean. We have only to sample the plumes …
Several planetary satellites apparently have subsurface seas that are of great interest for, among other reasons, their possible habitability. The geologically diverse Saturnian satellite Enceladus vigorously vents liquid water and vapor from fractures within a south polar depression and thus must have a liquid reservoir or active melting. However, the extent and location of any subsurface liquid region is not directly observable. We use measurements of control points across the surface of Enceladus accumulated over seven years of spacecraft observations to determine the satellite's precise ro…
To test the potential of using amino acid abundances as a biosignature at icy ocean worlds, we investigate whether primordial amino acids (accreted or formed by early aqueous processes) could persist until the present time. By examining the decomposition kinetics of amino acids in aqueous solution based on existing laboratory rate data, we find that all fourteen proteinogenic amino acids considered in this study decompose to a very large extent (>99.9%) over relatively short lengths of time in hydrothermally active oceans. Therefore, as a rule of thumb, we suggest that if amino acids are detec…
Dissipation of ocean potential energy is proposed by Tyler (2020) as a new mechanism leading to possible high-power states for Enceladus. I show here that this process actually results from viscoelastic dissipation within the crust. For plausible values of Enceladus's ocean thickness, crustal dissipation can be computed with the standard approach of static deformations of solid layers by equilibrium tides.
On icy worlds, the ice shell and subsurface ocean form a coupled system -- heat and salinity flux from the ice shell induced by the ice thickness gradient drives circulation in the ocean, and in turn, the heat transport by ocean circulation shapes the ice shell. Therefore, understanding the dependence of the efficiency of ocean heat transport (OHT) on orbital parameters may allow us to predict the ice shell geometry before direct observation is possible, providing useful information for mission design. Inspired by previous works on baroclinic eddies, I first derive scaling laws for the OHT on …
Raman Spectroscopy of Salt Deposits from the Simulated Subsurface Ocean of Enceladus
The University of Tokyo（东京大学）；Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA)（宇宙科学研究所 (ISAS)，日本宇宙航空研究开发机构 (JAXA)）
Enceladus is a saturnian satellite that should have liquid water inside of it (subsurface ocean). Measurements and experiments on water plume from Enceladus have revealed that Enceladus' ocean contains several salts such as NaCl. On the Earth, salt in soil has become a serious problem for agriculture, and importance of salt-tolerant plants are indicated. In order to test the effect of Enceladus' water to terrestrial plant, by hydroponic, we tried to grow three salt-tolerant plants (ice plant, swiss chard and salicornia) simulating Enceladus' water (Enceladus Farm project). Using water with 0.3…
Beneath the icy shell encasing Enceladus, a small icy moon of Saturn, a global ocean of liquid water ejects geyser-like plumes into space through fissures in the ice, making it an attractive place to investigate habitability and to search for extraterrestrial life. The existence of an ocean on Enceladus has been attributed to the heat generated in dissipative processes associated with deformation by tidal forcing. However, it remains unclear whether that heat is mostly generated in its ice shell or silicate core. Answering this question is crucial if we are to unravel patterns of ocean circula…
Enceladus is believed to have a saltwater global ocean with a mean depth of at least 30~km, heated from below at the ocean-core interface and cooled at the top, where the ocean loses heat to the icy lithosphere above. This scenario suggests an important role for vertical convection to influence the interior properties and circulation of Enceladus' ocean. Additionally, the ice shell that encompasses the ocean has dramatic meridional thickness variations that, in steady state, must be sustained against processes acting to remove these ice thickness anomalies. One mechanism that would maintain va…
Spacecraft observations suggest that the plumes of Saturn's moon Enceladus draw water from a subsurface ocean, but the sustainability of conduits linking ocean and surface is not understood. Observations show sustained (though tidally modulated) fissure eruptions throughout each orbit, and since the 2005 discovery of the plumes. Peak plume flux lags peak tidal extension by $\sim$1 radian, suggestive of resonance. Here we show that a model of the tiger stripes as tidally-flexed slots that puncture the ice shell can simultaneously explain the persistence of the eruptions through the tidal cycle,…
Over the south pole of Enceladus, an icy moon of Saturn, geysers eject water into space in a striped pattern, making Enceladus one of the most attractive destinations in the search for extraterrestrial life. We explore the ocean dynamics and tracer/heat transport associated with geysers as a function of the assumed salinity of the ocean and various core-shell heat partitions and bottom heating patterns. We find that, even if heating is concentrated into a narrow band on the seafloor directly beneath the south pole, the warm fluid becomes quickly mixed with its surroundings due to baroclinic in…
Enceladus is characterised by a south polar hot spot associated with a large outflow of heat, the source of which remains unclear. We compute the viscous dissipation resulting from tidal and libration forcing in the moon's subsurface ocean using the linearised Navier-Stokes equation in a 3-dimensional spherical model. We conclude that libration is the dominant cause of dissipation at the linear order, providing up to about 0.001 GW of heat to the ocean, which remains insufficient to explain the (about) 10 GW observed by Cassini. We also illustrate how resonances with inertial modes can signifi…
Under both engineering and natural conditions on Earth and in the Universe, some gas hydrates are found to be stabilised outside their window of thermodynamic stability by the formation of an ice layer-a phenomenon termed self-preservation. Low occupancy surface regions on type I CO2 clathrate structures together with the self-preserving ice layer lead to an effective buoyancy for these structures which restricts the size range of particles that float in the ocean on Enceladus, Pluto and similar oceanic worlds. Our goal here is to investigate the implications of Lifshitz forces and low occupan…
Of profound astrobiological interest, Enceladus appears to have a global subsurface ocean that is salty, indicating water-rock reaction at present or in the past, important for its habitability. Here, we investigate how salinity and the partition of heat production between the silicate core and the ice shell affect ocean dynamics and the associated heat transport -- a key factor that determines the equilibrium ice shell geometry. Assuming steady state conditions, we show that the meridional overturning circulation of the ocean, driven by heat and salt exchange with the ice, has opposing signs …
Enceladus's gravity and shape have been explained in terms of a thick isostatic ice shell floating on a global ocean, in contradiction of the thin shell implied by librations. Here we propose a new isostatic model minimizing crustal deviatoric stress, and demonstrate that gravity and shape data predict a $\rm{38\pm4\,km}$-thick ocean beneath a $\rm{23\pm4\,km}$-thick shell agreeing with -- but independent of -- libration data. Isostatic and tidal stresses are comparable in magnitude. South polar crust is only $7\pm4\rm\,km$ thick, facilitating the opening of water conduits and enhancing tidal …
The Cassini mission to the Saturn system discovered a plume of ice grains and water vapor erupting from cracks on the icy surface of the satellite Enceladus. This moon has a global ocean in contact with a rocky core beneath its icy exterior, making it a promising location to search for evidence of extraterrestrial life in the solar system. The previous detection of H$_2$ in the plume indicates that there is free energy available for methanogenesis, the metabolic reaction of H$_2$ with CO$_2$ to form methane and water. Additional metabolic pathways could provide sources of energy in Enceladus' …
A framework for evaluating biosignature potential against the abiotic baseline on ocean worlds
Harvard University（哈佛大学）；University of Toronto（多伦多大学）；University of Ottawa（渥太华大学）；University of Washington（华盛顿大学）；Massachusetts Institute of Technology（麻省理工学院）；UK Centre for Astrobiology, University of Edinburgh（爱丁堡大学英国天体生物学中心）；Institut de Physique du Globe de Paris (IPGP), Université Paris Cité（巴黎地球物理研究所（IPGP），巴黎西岱大学）
Enceladus, an icy moon of Saturn, possesses an internal water ocean and jets expelling ocean material into space. Cassini investigations indicated that the subsurface ocean could be a habitable environment having a complex interaction with the rocky core. Further investigation of the composition of the plume formed by the jets is necessary to fully understand the ocean, its potential habitability, and what it tells us about Enceladus' origin. Moonraker has been proposed as an ESA M-class mission designed to orbit Saturn and perform multiple flybys of Enceladus, focusing on traversals of the pl…
Water vapor geysers on Europa have been inferred from observations made by the Galileo spacecraft, the Hubble Space Telescope, and the Keck Observatory. Unlike the water-rich geysers observed on Enceladus, Europa's geysers appear to be an intermittent phenomenon, and the dynamical mechanism permitting water to sporadically erupt through a kilometers-thick ice sheet is not well understood. Here we outline and explore the hypothesis that the Europan geysers are driven by CO$_2$ gas released by dissociation and depressurization of CO$_2$ clathrate hydrates initially sourced from the subsurface oc…
Methanol is a potentially important impurity in subsurface oceans on Titan and Enceladus. We report measurements of the freezing of methanol-water samples at pressures up to 350~MPa using a volumetric cell with sapphire windows. For low concentrations of methanol, the liquidus temperature is typically a few degrees below the corresponding ice freezing point, while at high concentrations it follows the pure methanol trend. In the Ice-III regime, we observe several long-lived metastable states. The results suggest that methanol is a more effective antifreeze than previously estimated, and might …
Geophysical measurements can reveal the structure of icy ocean worlds and cycling of volatiles. The associated density, temperature, sound speed, and electrical conductivity of such worlds thus characterizes their habitability. To explore the variability and correlation of these parameters, and to provide tools for planning and data analyses, we develop 1-D calculations of internal structure, which use available constraints on the thermodynamics of aqueous MgSO$_4$, NaCl (as seawater), and NH$_3$, water ices, and silicate content. Limits in available thermodynamic data narrow the parameter spa…
