Overview : Enceladus is the 6th largest moon of Saturn & is about 500 km in diameter. Despite many scientists referring to cryovolcanoes on Enceladus moon, no volcanic structure is seen on the surface. The gas & dust plumes originate mainly from 4 linear depressions called the "tiger stripes", on the moon's south pole region. Infra-red pics reveal some surface patterns (elephant skin wrinkles) to be identical to that of comet 67P. Enceladus plumes are visually, compositionally & behaviourally similar to comet plumes. Saturn's E-ring is supposedly formed by the plumes ejecting from Enceladus. Many of the papers below are relying on 2 key instruments of Cassini - Cassini Plasma Spectrometer (CAPS) explored plasma (highly ionised gas) within and near Saturn's magnetic field. Cosmic Dust Analyser (CDA) studied ice and dust grains in and near the Saturn system. There are multiple models, flyby geometries, data sets, interpretations & theories. Cassini was the 4th space probe to visit Saturn and the first to enter its orbit, where it stayed from 2004 to 2017. It did 23 flybys (E0-E22) of Enceladus from 2005-2015 & the closest it came to the moon's surface was 25 km.
Enceladus is the dark spot (right pic) inside the bright flare near the center of E-ring. Plumes of water ice grains, gas & dust can be seen ejecting off Enceladus into the E-ring. Credit : CICLOPS, JPL, ESA, NASA
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NASA article - Enceladus : Ocean Moon
Summary - Jets of icy particles from that ocean, laced with a brew of water and simple organic chemicals, gush out into space continuously from this fascinating ocean world. The material shoots out at about 400 m/s and forms a plume that extends hundreds of kilometers into space.
During a close flyby in 2008, Cassini's instruments sampled the plume directly and detected a surprising mix of volatile gases, water vapor, carbon dioxide and carbon monoxide, as well as organic materials. The density of organic materials was about 20 times denser than expected.
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NASA article - NASA's Cassini Images Reveal Spectacular Evidence of an Active Moon
Summary - "In some ways, Enceladus resembles a huge comet," said Dr. Torrence Johnson, imaging team member from NASA's Jet Propulsion Laboratory in Pasadena. "Only, in the case of Enceladus, the energy source for the geyser-like activity is believed to be due to internal heating by perhaps radioactivity and tides rather than the sunlight which causes cometary jets." My thread on similarity of plumes of Enceladus & comet 67P.
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Article - Plumes Of Enceladus Revealed In Incredible Cassini Flyby Images
Summary - The flyby, officially titled E21, is the deepest dive through Enceladus’ plumes ever completed by Cassini. Earlier on in its mission, the spacecraft flew closer to the moon’s surface, but never this low through a plume before. From both a science and engineering perspective, 49 kilometers was the ideal altitude for this pass, in order to conserve fuel and prolong the flyby — if only for a few tens of seconds — allowing for more science at the end of the mission in 2017.
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NASA Article - Electrical Circuit Between Saturn and Enceladus
Summary - This artist's concept shows a glowing patch of ultraviolet light near Saturn's north pole that occurs at the "footprint" of the magnetic connection between Saturn and its moon Enceladus. The footprint and magnetic field lines are not visible to the naked eye, but were detected by the ultraviolet imaging spectrograph and the fields and particles instruments on NASA's Cassini spacecraft. The footprint, newly discovered by Cassini, marks the presence of an electrical circuit that connects Saturn with Enceladus and accelerates electrons and ions along the magnetic field lines. In this image, the footprint is in the white box marked on Saturn, with the magnetic field lines in white and purple. NASA video.
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Title - Ion trapping by dust grains: Simulation applications to the Enceladus plume
By - WM Farrell et al, 2017
Summary - A plume of water ice that escapes Saturn’s moon Enceladus should be coursing with electrical currents, but data are mixed. Now simulations suggest that a sticky dust cloud may shield signals. Free ions get picked up and accelerated to speeds near 30 km/s. We conclude that the cause for the confusion in the interpretation of the observations is the presence of copious numbers of submicron particulates between 10 and 500 nm that, in equilibrium, are charged to many hundreds of elementary negative charges.
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Article - Where Are the Electrical Currents in the Enceladus Plume ?
Summary - A plume of water ice that escapes Saturn’s moon Enceladus should be coursing with electrical currents, but data are mixed. The geysers also create a wispy plume of ice that syphons material from the moon to Saturn, injecting Saturn’s magnetic field with plasma and sending electrical currents coursing through space. For more than a decade, scientists have struggled to understand conflicting measurements of those currents from Cassini. According to Cassini's Langmuir probe, which measures particle density, there were enough ions present to generate a current of 10 million amps. But the magnetometer indicated a current more than 20 times weaker. Now a new study from Farrell et al 2017 may solve this mystery. Their simulations suggest that a sticky dust cloud may shield signals.
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Title - The 3D Structure of Saturn Magnetospheric Neutral Tori Produced by the Enceladus Plumes
By - Howard Todd Smith & John Richardson, 2021
Summary - The Enceladus plumes provide a dominant source of heavy particles to Saturn's magnetosphere with an effective H2O source rate of 280 kg/s. The initial water gas source produces an OH torus (on the order of around 65 days) which then produces a O torus (on the order of around 40 days).
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Title - Characteristics of ice grains in the Enceladus plume from Cassini observations
By - Y Dong et al, 2014
Summary - This paper studies ice grains ranging from less than one nanometer to micrometers in size. The charged ice grains ejected southward from Enceladus' south pole are deflected toward Saturn by Lorentz force.
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Title - Fast and Slow Water Ion Populations in the Enceladus Plume
By - RP Haythornthwaite et al, 2020
Summary - .
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Title - Modification of the plasma in the near-vicinity of Enceladus by the enveloping dust
By - WM Farrell et al, 2010
Summary - .
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Title - Modeling the Enceladus plume–plasma interaction
By - BL Fleshman et al, 2010
Summary - .
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Article, 2011 - Beams of electrons link Saturn with its moon Enceladus
Summary - Enceladus is linked to Saturn by powerful electrical currents - beams of electrons that flow back and forth between the planet and moon. Research has found that jets of gas and icy grains emanate from the south pole of Enceladus, which become electrically charged and form an ionosphere. Scientists already knew that Jupiter is linked to 3 of its moons (Io, Europa and Ganymede) by charged current systems set up by the satellites orbiting inside its giant magnetic bubble, the magnetosphere, and that these current systems form glowing spots in the planet's upper atmosphere. The latest discovery at Enceladus shows that similar processes take place at the Saturnian system too.
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Title - The auroral footprint of Enceladus on Saturn
By - Wayne Pryor et al, 2011
Summary - Although there are substantial differences between the magnetospheres of Jupiter and Saturn, it has been suggested that cryovolcanic activity at Enceladus could lead to electrodynamic coupling between Enceladus and Saturn, like that which links Jupiter with Io, Europa and Ganymede. Powerful field-aligned electron beams associated with the Io–Jupiter coupling, for example, create an auroral footprint in Jupiter’s ionosphere. Auroral ultraviolet emission associated with Enceladus–Saturn coupling is anticipated to be just a few tenths of a kilorayleigh, about an order of magnitude dimmer than Io’s footprint and below the observable threshold, consistent with its non-detection. Here we report the detection of magnetic-field-aligned ion and electron beams (offset several moon radii downstream from Enceladus) with sufficient power to stimulate detectable aurora, and the subsequent discovery of Enceladus-associated aurora in a few per cent of the scans of the moon’s footprint. The footprint varies in emission magnitude more than can plausibly be explained by changes in magnetospheric parameters—and as such is probably indicative of variable plume activity.
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Title - Auroral hiss, electron beams and standing Alfvén wave currents near Saturn's moon Enceladus
By - DA Gurnett et al, 2011
Summary - Enceladus has a strong electrodynamic interaction with Saturn's magnetosphere, involving acceleration of electrons and emission of auroral hiss. Most likely the electron acceleration is caused by a standing Alfven wave. However, other possibilities such as electric fields due to electrostatic charging of the moon's surface or of particles in the water vapor plume should be considered.
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Title - The Cassini Enceladus encounters 2005–2010 in the view of energetic electron measurements
By - N Krupp et al, 2012
Summary - A plume of water ice molecules and dust released through geysers on the south polar region provides enough material to feed the E-ring and also the neutral torus of Saturn and the entire magnetosphere. In the time period 2005–2010 the Cassini spacecraft flew close by the moon 14 times, sometimes as low as 25 km above the surface and directly through the plume. Energetic electrons were measured in the energy range 27 keV to 21 MeV. Features in the data can be interpreted as that the spacecraft was connected to the plume material along field lines well before entering the high density region of the plume. Sharp absorption signatures as the result of losses of energetic electrons bouncing along those field lines, through the emitted gas and dust clouds, clearly depend on flyby geometry as well as on measured pitch angle/look direction of the instrument. During at least 2 of the flybys, presence of dust in energetic electron data was detected when Cassini flew directly through the south polar plume. In addition, we found gradients in the magnetic field components, indicating that complex electron drifts in the vicinity of Enceladus could form forbidden regions for electrons, which may appear as intensity drop-outs.
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Title - Charged nanograins in the Enceladus plume
By - TW Hill et al, 2012
Summary - Nanograins leave Enceladus largely uncharged, and become increasingly negatively charged as they move away from Enceladus. The implied electric current is evidently closed by the simultaneous escape of a dense plume plasma, which has the opposite sign of (+) net charge density.
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Title - Modeling of electron fluxes in the Enceladus plume
By - N Ozak et al, 2012
Summary - In the south polar region of Enceladus, a set of linear depressions known as the “tiger stripes” emit plumes of gas and dust, releasing it into the region around the orbit of Enceladus. The plume extends out beyond 4,000 km from the surface, eventually blending in with the Enceladus torus. It is composed mainly of H2O (90%), CO2 (5%) and other minor neutrals (mainly organics, some argon, methane, and ammonia). Cassini observed changes in absorption signatures of energetic electrons, large perturbations in the magnetic field and changes in ion composition.
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Title - Resolving the Mass Production and Surface Structure of the Enceladus Dust Plume
By - BS Southworth et al, 2017
Summary - There are ongoing arguments with regards to the Enceldaus plume, both on the total mass of ice particles produced by the plume (ranges from 5 kg/s to 51 kg/s), as well as the structure of plume ejection along the tiger stripes, produced by ice volcanoes. More recently, plasma models and data from Cassini suggest a more modest 12 kg/s, however, this result is based solely on nanograins, which make up a small percentage of plume mass. While a 2014 paper found over 100 jet locations for Enceladus plumes, a 2015 paper suggested continuous emission along the tiger stripes, demonstrating that some of the jets published in the 2014 paper, could in fact result from viewing multiple continuous emissions in an overlapping manner. Simulations demonstrate that it is likely that both emissions exist and are active, with surface deposition dominated by the continuous, “curtain” emissions, while temporally variable jetting activity likely contributes primarily to the E-ring.
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Title - Modeling the total dust production of Enceladus from stochastic charge equilibrium and simulations
By - Patrick Meier et al, 2015
Summary - The output of heat and the cryovolcanic activity are concentrated along Enceladus׳ “tiger stripe” fissures. 8 prominent jet sources were first identified in images taken by Cassini׳, while a later analysis of a larger set of high resolution images identified more than 100 jets along the tiger stripes. Among these, various jets were found to be intermittent, turning on and off between images taken at different times. Ejection occurs more uniformly along larger segments of the tiger stripes. Negatively and positively charged nano-sized ice grains were detected in the Enceladus plume. Dust production rate is estimated to be about 12 kg/s, including larger dust grains up to a few microns in size. In Enceladus׳ plume, ion densities are about 2 orders of magnitude higher than expected. Water vapor emanates from the tiger stripes, forming Enceladus׳ gas plume, partly at supersonic speeds, replenishing Saturn׳s neutral gas torus. A compositional analysis of E-ring grains, showed that it is dominated by grains with a low (but significant) salt content. Majority of dust grains are smaller than 10 nm.
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Title - The composition and structure of the Enceladus plume
By - CJ Hansen et al, 2011
Summary - The inferred rate of water vapor injection into Saturn's magnetosphere is 200 kg/s. Collimated gas jets are detected in the plume with Mach numbers of 5–8, implying vertical gas velocities that exceed 1 km/s. Our results support the subsurface liquid model, with gas escaping and being accelerated through nozzle-like channels to the surface, and are consistent with recent particle composition results from the Cassini Cosmic Dust Analyzer.
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Title - Plume and Surface Composition of Enceladus
By - F Postberg et al, 2018
Summary - .
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Aritcle - Enceladus’ plumes might not come from an underground ocean
By - Science News, 2022
Summary - Saltwater spray could come from pockets of watery mush in the moon’s icy shell. See entry 22.
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Title - A mushy source for the geysers of Enceladus
By - Colin Meyer et al, 2022
Summary - Sourcing the plumes via a direct connection from the ocean to the surface, requires a fracture through the entire ice shell. Diurnal tidal stresses (1 bar), however, are insufficient to fracture the shell, especially given the warmer, more ductile ice at depth. Here we explore an alternative: the ocean chemistry is frozen into the shell and shear heating within the tiger stripe fractures produces partial melting in the ice shell, allowing the interstitial fluid to be ejected as geysers. Using observational constraints on the surface heat flux from the south pole of Enceladus, we predict the likely thickness, permeability, and salt content of a mushy zone within the ice shell surrounding a fracture.
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Aritcle - Not Just Water. Enceladus is Also Blasting Silica Into Space
By - Universe Today, Feb 2023
Summary - Nanometer-scale silica particles were found in the plumes.
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Title - Particle entrainment and rotating convection in Enceladus’ ocean
By - Ashley Schoenfeld et al, 2023
Summary - .
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Title - Fast and Slow Water Ion Populations in the Enceladus Plume
By - RP Haythornthwaite et al, 2020
Summary - 2 velocities for the positive ions are found, with the fast ions (2.3–5.8 km/s) originating from the high-speed neutral gas emission and slow ions (0.2–2.2 km/s) associated with the low-speed thermal gas emission from Enceladus. Negative ions were found to be near stationary or northerly traveling, implying a deceleration mechanism within the plume, such as an ambipolar electric field. (More extracts).
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Title - A model of the spatial and size distribution of Enceladus׳ dust plume
By - Patrick Meier et al, 2014
Summary - Locations of "Tiger stripe” fissures coincide with particularly warm areas and they represent the main sources of the material. More, finer jet structures along the tiger stripes were recently announced. The vapor leaves Enceladus׳ surface with supersonic velocity and drags the dust particles from the ruptures. The dust grains show diverse chemical composition : on one hand, salt-poor particles are seen by Cassini׳s Cosmic Dust Analyzer (CDA), with mixing ratio, dominating far from the south pole and in the E ring. On the other hand, salt rich particles with salinities on the percent level dominate closer to the sources. The dust ejection velocity drops with grain size. It is generally lower than the gas velocity which was attributed to decelerating collisions with the rift walls (More extracts).
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Title - Spatial variations in the dust-to-gas ratio of Enceladus’ plume
By - MM Hedman et al, 2018
Summary - The total particle mass output from Enceladus has been estimated to be around 50 kg/s but recent work suggests that this mass could be a factor of several times lower if the particles are loose aggregates instead of solid grains. These data confirm that both the dust and vapor components of the plumes can be highly structured, and also suggest that the mass density of solids in the plume is 20% of the vapor density (dust is 1/5th of vapor). The occultation data also suggests that the dust-to-gas mass ratio of the material emerging from certain sources is close to one at altitudes around 25 km (equal parts of dust & vapor).
Material emerging from 2 of the 4 main fissures are more particle-rich than that those emerging from the other 2 fissures. Across Enceladus’ south polar terrain, there are systematic spatial & spectral variations in plume properties. There are also variations in grain size & composition of the grains. This likely reflects trends in the source material for the plumes and/or the plumbing connecting these reservoirs to the surface.
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By - Susan Kieffer et al, 2009
Summary - The discovery of plumes of water vapor and ice particles erupting from the south pole of Enceladus, sparked controversy over whether these plumes are produced by boiling, or by sublimation with subsequent recondensation of the sublimated vapor. Porco et al.’s analysis that the masses of ice (I) and vapor (V) in the plume were comparable was taken to argue against the occurrence of sublimation and recondensation, leading to the hypothesis that the reservoir was boiling water, possibly as close as 7 m to the surface. Thus, it has been advocated that Enceladus should be a target for astrobiology exploration.
But here we show ... that the mass of ice in the column is significantly less than the mass of water vapor, and that by considering three additional effects, ice to vapor ratio I/V is likely to be less than 0.2 to 0.1. This means that the plume is dominated by vapor that the thermodynamics permits to be easily produced by sublimation with recondensation. The low I/V ratio provides no compelling criterion for consideration of a liquid water reservoir. Although the I/V ratio is sensitive to particle sizes and size distributions, the masses of ice (I) and vapor (V) are not comparable in any scenario constrained by available observations. We thus discuss the implications of sublimation from a thermodynamic point of view in a context that has not been presented previously.
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Title - Enceladus' Water Vapor Plume
By - Candice Hansen et al, 2006
Summary - Cassini's UltraViolet Imaging Spectrograph observed stellar occultations on two flybys and confirmed the existence, composition, and regionally confined nature of a water vapor plume in the south polar region of Enceladus. This plume provides an adequate amount of water to resupply losses from Saturn's E ring and to be the dominant source of the neutral OH and atomic oxygen that fill the Saturnian system.
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By - F Postberg et al, 2008
Summary - Here we present a statistical evaluation of more than 2000 impact ionisation mass spectra of Saturn's E-ring particles, with sizes predominantly below 1 μm, detected by the Cosmic Dust Analyser onboard the Cassini spacecraft. We focus on the identification of non-water features in spectra otherwise dominated by water ice signatures. Here we specify the categorisation of two different spectrum types, which probably represent two particle populations. Type I spectra imply pure water ice particles, whereas in Type II spectra organic compounds and/or silicate minerals are identified as impurities within the icy particles. This finding supports the hypothesis of a dynamic interaction of Enceladus' rocky core with liquid water.
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Title - Cassini Observes the Active South Pole of Enceladus
By - Carolyn Porco et al, 2006
Summary - .
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Title - Cassini Encounters Enceladus: Background and the Discovery of a South Polar Hot Spot
By - Spencer et al, 2006
Summary - The polar region is the source of a plume of gas and dust, which probably emanates from prominent warm troughs seen on the surface. Cassini's Composite Infrared Spectrometer (CIRS) detected 3 to 7 gigawatts of thermal emission from the south polar troughs at temperatures up to 145 K (-128.15 C) or higher, making Enceladus only the third known solid planetary body-after Earth and Io-that is sufficiently geologically active for its internal heat to be detected by remote sensing. If the plume is generated by the sublimation of water ice and if the sublimation source is visible to CIRS, then sublimation temperatures of at least 180 K (-93.15 C) are required.
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Title - Understanding the escape of water from Enceladus
By - MH Burger et al, 2006
Summary - The small icy moon emits 300 kg/s of water from its south pole, most of which is distributed around Saturn either as neutral H2O gas, ice grains in the E-ring, or as plasma trapped in Saturn’s magnetic field. We have shown that the gas plume observed by the instruments on Cassini is consistent with a two-component source from Enceladus, with more than 99% of the material associated with the south polar plume. The remaining water is consistent with a global source of a 2 – 3 kg/s.
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Title - 18 Enceladus papers from LPSC 2023
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Title - JWST molecular mapping and characterization of Enceladus' water plume feeding its torus
By - Villanueva, Heidi Hammel et al, May 2023
Summary - JWST revealed an extraordinarily extensive plume of water vapor, up to 10,000 km (20 times the size of Enceladus moon), at cryogenic temperatures (25 K / -248 C). Observed outgassing rate of 300 kg/s is similar to what Cassini found 15 years ago (& also roughly similar to comet 67P). At this rate, you could fill an Olympic-sized swimming pool in just a couple of hours. In comparison, doing so with a garden hose on Earth would take more than 2 weeks. Vigor of gas eruption from Enceladus has been relatively stable over decadal timescales. This level of activity establishes Enceladus as the prime source of water across the Saturnian system. Several non-water gases (CO2, CO, CH4, C2H6, CH3OH) were missing in the spectra. About 30% of the ejected water vapor plumes feed the donut-shaped E ring.
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Title - Gas loss from enceladus’ ocean due to long-term plume eruption
By - Lucas Fifer et al, 2023
Summary - The possibility that Enceladus is continuously losing primordial volatiles without replenishment has yet to be explored in the literature, motivating our current work. While Enceladus has similar D/H ratios and high volatile abundances to comets, Enceladus seems to have lower concentrations of CO2 and CH4 than expected for a comet-like composition. Even the recent high estimates for Enceladus’ oceanic CH4 concentrations appear to be much lower (>2 orders of magnitude) than cometary abundances. This begs the question – where did the methane go ?
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Title - The Science Case for a Return to Enceladus
By - Morgan L Cable et al, 2021 (includes Carolyn Porco)
Summary - This paper summarizes the evidence for Enceladus' ocean, its habitability and identifies constraints & questions on the detectability of life within its ocean. The plume of Enceladus provides direct access to fresh material from an extraterrestrial subsurface ocean. Evidence gathered from multiple instruments points to a global, subsurface liquid water ocean rich in salts and organic compounds, with water-rock interactions occurring presumably in hydrothermal systems at or below the moon's sea floor. The ocean of Enceladus is considered to be habitable.
Video - YouTube interview by AAS Journal
Comet 67P & Enceladus : 20 common ingredients
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