Surface Of Enceladus Research Articles

Electrostatic Effects and Formation of Dusty Plasma above the Surface of Enceladus

Electrostatic Effects and Formation of Dusty Plasma above the Surface of Enceladus

Low effective ultraviolet exposure ages for organics at the surface of Enceladus

The saturnian moon Enceladus presents a remarkable opportunity in our solar system for searching for evidence of life, given its habitable ocean and plume that deposits organic-bearing ocean material onto the surface. Organic ocean material could be sampled by a lander mission at Enceladus. It is of interest to understand the amount of relatively pristine, unaltered organics present on the surface, given the ultraviolet (UV) and plasma environment. Here, we investigate UV penetration into Enceladus’s surface and the resultant effective exposure ages for various regions, using the UV reflectance spectrum of Enceladus as measured by the Hubble Space Telescope and considering the rate of resurfacing by plume fallout. In high plume fallout regions near the south pole, plume grains are buried by fresher grains within years, resulting in low levels of exposure to solar UV, which penetrates only ~100 micrometers. Regions at latitudes south of ~40°S can have exposure ages <100 years, translating to relatively high abundances of pristine organic material preserved in the regolith.

Cohesive properties of ice powders analogous to fresh plume deposits on Enceladus and Europa

Active jets of water vapour and ice grains discovered by the Cassini spacecraft at the South Pole of Enceladus result in the deposit of freshly erupted materials on the icy surface of the moon, potentially coming directly from its subsurface water ocean. As suggested by sporadic detection of plume activity, similar processes may be at work on Europa as well. Such fresh ice deposit areas are primary targets for future landing and sampling missions for the search of potential bio-signatures. Determining the mechanical properties of such fresh icy powder-like materials is essential to identify safe landing sites and appropriate sampling techniques. Ice surface energy or cohesion is a key parameter driving the porosity of the material, its strength, stickiness and flowability, which determine the stability of fresh surface deposits. In this work we performed an experimental study on laboratory analogues of fresh icy regolith, consisting in investigating the flowability of ice powders through a rotating drum device at several temperatures. Our experimental results show that the flowability of fresh icy deposits significantly decreases with increasing temperatures over the range of 90 to 150 K. Using a simple scaling analysis, we derive the surface energy of ice grains from the observed angle of the flowing surface inside the drum and we show that the surface energy increase by a factor of 5 to 10 between 90 K to 150 K, a behaviour not observed in any other powder material. This result indicates that the flowability of fresh icy deposits at the surface of Enceladus and Europa should significantly change depending on the local surface temperature, which can vary spatially and temporally on the same range of temperature explored here. Due to lower gravity, ice powder will behave as a more cohesive powder on Enceladus than on Europa. Higher cohesion on Enceladus may result in the formation of a very loose regolith with high porosity, making stable landing more challenging. On Europa, the reduction of ice cohesiveness with decreasing temperature should help the flow of freshly eruptive granular ice materials, potentially explaining numerous flow features observed throughout the surface.

New global topography of Enceladus: Hypsometry, basins, spherical harmonics, shell thickness, and true polar wander revisited

A new global high-resolution stereogrammetric and photogrammetric topographic map and updated 3-color global mosaic of Enceladus reveal that the shape of this active icy ocean world is distorted from a simple triaxial ellipsoid. Although the topographic range (high-to-low) of Enceladus, when referenced to its global triaxial figure, is the narrowest among icy bodies mapped globally thus far, the global DEM indicates a pronounced zonal latitudinal zoning of topography, with the north polar region and mid-southern latitudes being ∼300–400 m higher and south polar and mid-northern latitudes being ∼400 m lower on average than the triaxial figure. Dynamically, both north and south poles are low, but the south remarkably so, by >1 km. Spherical harmonic analysis also indicates the tidal axis of the body is located ∼4°W of the current IAU-defined prime meridian. Superposed on this global pattern are topographic features associated with geologic units within each terrain, as well as several prominent regional 100-km-scale basins, 1-to-1.8 km deep, previously reported. The shapes of these large basins depend on the terrains they formed in; those in older cratered terrains are relatively simple (i.e., ovate) in shape, but those in resurfaced terrains are more complex in shape, often correlating with geologic features and units within those terrains. Thus, while these basins exhibit no explicit local-scale geologic deformation or disruption of the icy shell in association with their elevations/formation, they are partially controlled by the physical characteristics of the terrains they formed in, suggesting that the icy shell can be heterogenous in character. Most of these basins occur on the trailing hemisphere and although a few of these are aligned, we find that they do not form a global great circle pattern, weakening previous arguments for true polar wander. Our preferred hypothesis is that these basins are due to isostatic adjustment of the ice shell in response to thermal anomalies from below and suggest that they may be surface manifestations of diffused hydrothermal plumes originating at the surface of Enceladus' rocky core. If so, similar patterns in core activity might be revealed once global topographic mapping is complete on Europa.

Predicting the Effect of Surface Properties on Enceladus for Landing

The prospect of landing on the surface of Enceladus comes with the question of whether the surface conditions permit selection and certification of one or more safe landing sites in an area of high science value. On Enceladus, the search for biosignatures in plume materials is a high science value objective that correlates with proximity to the south polar terrain, where the plume deposition rate is highest; however, such areas may be unsafe if unsintered particles make the landing site unstable. To investigate this, the surface of Enceladus was modeled using the level set discrete element method. This method models the kinetics and kinematics of large groups of individual ice particles both in contact and sintered together. Using the model, a rigid footpad was initialized at a 1 m s−1 descent just above the ice surface under Enceladus gravity. Parameters studied were the sintering amount, particle size distribution, footpad geometry, and surface slope. The model predicted that some sintering is required for the surface to support a lander; however, too much sintering can cause a lander to bounce. For tests on sloped surfaces, landing could be possible on slopes as steep as 20° for certain conditions, but it is safest to land in areas with a slope angle of 15° or less. While slope angle and sintering level were much more important than footpad geometry, the hemisphere footpad had the best performance (lowest slipping) in most cases compared to the cone or disk.

Sampling Plume Deposits on Enceladus’ Surface to Explore Ocean Materials and Search for Traces of Life or Biosignatures

Abstract Enceladus is unique as an astrobiology target in that it hosts an active plume sourced directly from its habitable subsurface ocean. Ice particles from the plume contain geochemical constituents that are diagnostic of the ocean conditions, and may hold traces of life and/or biosignatures, if they exist. Up to 93% of the plume particles fall back onto the surface of Enceladus. The low radiation environment and present-day activity are favorable to the preservation of any complex organics and putative biosignatures contained within these particles. Laboratory experiments and modeling suggest that plume deposits would likely be weakly consolidated and relatively easy to sample. Sampling systems like a dual rasp, under development to achieve technology readiness level (TRL) 5 in 2021, would enable a landed mission on Enceladus’ surface to acquire large amounts of surface materials, a requirement for analysis of trace constituents. A landed mission on Enceladus could greatly enhance our understanding of the chemical makeup of plume particles and the subsurface ocean, and seek traces of life and/or biosignatures.

Adsorption of organic compounds at the surface of Enceladus’ ice grains. A grand canonical Monte Carlo simulation study

ABSTRACT In this paper, we characterise the adsorption of ethylene, propanol and hexanal molecules on crystalline ice by grand canonical Monte Carlo simulations performed at 236 K, a temperature which is typical of some Enceladus’ environments. We show that at low coverage of the ice surface, the adsorption of propanol and hexanal is driven by the interaction of these molecules with the ice phase and, as a consequence, the adsorbed molecules lie more or less parallel to the ice surface. On the other hand, upon saturation, the adsorbate–adsorbate interactions become more and more important and the molecules tend to become tilted with respect to the surface, the aliphatic chain pointing towards the gas phase, while the polar head of propanol and hexanal molecules always stays attached to the ice surface, irrespective of the coverage. By contrast, the ethylene molecules do not show any strong affinity for the ice surface because of the corresponding weak binding energy. These results are in good agreement with the major features provided by the available experiments on similar systems.

Processing of 72-K water-rich ices by keV and MeV oxygen ions: implications for the Saturnian moon Enceladus

ABSTRACT The processing of H2O:CO2:CH4 (10:1:1) and H2O:CO2:CH4:NH3 (10:1:1:1) ices at 72 K by oxygen ions was studied in an attempt to simulate the physicochemical effects induced by energetic ions on the surface of Enceladus (or similar cold surfaces in the outer Solar System). The experiments were carried out at the Grand Accélérateur National d’Ions Lourds (GANIL) in Caen/France. The samples were irradiated with 15.7-MeV O5+ at the IRRadiation SUD (IRRSUD) beamline and with 108-keV O6+ at the Accélérateurs pour les Recherches avec les Ions de Basses Energies (ARIBE) beamline. The frozen samples were monitored by Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy (4000–650 cm−1, 2.5–15.4 μm, at 1 cm−1 resolution). The identified molecular species formed during irradiation were CO, OCN−, CH3OH, HCN, CN−, H2CO3, HNCO, HCO and CO3. The effective formation cross-sections for the synthesis of new species and the effective molecular destruction cross-sections of the parent species in the experiments with MeV ions were found to be of the order of 10−14–10−13 cm2. For the keV ion experiments, these values were of the order of 10−16–10−15 cm2. The fluence at which the sample reaches chemical equilibrium and the molecular abundances in this region (equilibrium branching ratios) were estimated. These experiments suggest that the chemical inventory on the surface of Enceladus can be influenced by the incidence of charged particles on to the moon’s surface.

Nature, distribution and origin of CO2 on Enceladus

We present the first map of CO2 at the surface of Enceladus using data obtained by the Cassini Visible-Infrared Mapping Spectrometer (VIMS). In order to measure the weak and narrow CO2 absorption band depths around 4.26 µm, we improved: (1) the calibration of VIMS spectra; (2) the calculation of geographic coordinates; and (3) the projection techniques. We averaged multiple observations of a given area to obtain a signal to noise ratio high enough to map the CO2 abundances. CO2 is reliably detected mostly in the South Polar Region. This region includes active faults (Tiger Stripes), the highest observed surface temperatures, and locations of active plume eruptions. The occurrence here of CO2 is consistent with an endogenic origin controlled by tectonics. Both pure CO2 ice and complexed CO2 are detected from the positions of absorption bands between 4.27 and 4.24 µm. The highest concentrations of CO2 are between the main active faults of the South Polar Region, where the surface temperature is low. Pure CO2 ice deposits at the surface of Enceladus are best modeled by the formation of gas pockets below the icy crust and above the surface of the internal ocean. These pockets eventually release cold CO2 gas (∼70 to ∼119 K) at low-velocity (seeping) between the Tiger Stripes [Matson et al., 2018, Icarus, 302, 18–26]. CO2 clathrate hydrates may form in the ocean and may be subsequently released when a CO2 gas pocket blows out and erupts. Other mechanisms may contribute to reinforcing the anti-correlation of the CO2 distribution (of any type) with respect to the location of the Tiger Stripes, such as successive sublimation of CO2 and condensation on colder areas, and partial frost cover by H2O releases from plume eruptions.

The impact of a pressurized regional sea or global ocean on stresses on Enceladus

AbstractLiquid water is likely present in the interior of Enceladus, but it is still debated whether this water forms a global ocean or a regional sea and whether the present‐day situation is stable. As the heat flux of Enceladus exceeds most heat source estimates, the liquid water is likely cooling and crystallizing, which results in expansion and pressurization of the sea or ocean. We determine, using an axisymmetric Finite Element Model, the tectonic patterns that pressurization of a regional sea or global ocean might produce at the surface of Enceladus. Tension is always predicted above where the ice is thinnest and generates cracks that might be at the origin of the Tiger Stripes. Tectonic activity is also expected in an annulus around the sea if the ice shell is in contact with but slips freely along the rocky core of the satellite. Cracks at the north pole are expected if the shell slips along the core or if there is a global ocean with thin ice at the pole. Water is likely injected along the base of the ice when the shell is grounded, which may lead to cycles of tectonic activity with the shell alternating between floating and grounded states and midlatitude faulting occurring at the transition from a grounded to a floating state.

True polar wander of Enceladus from topographic data

Many objects in the solar system are suspected to have experienced reorientation of their spin axes. As their rotation rates are slow and their shapes are nearly spherical, the formation of mass anomalies, by either endogenic or exogenic processes, can change objects’ moments of inertia. Therefore, the objects reorient to align their largest moment of inertia with their spin axis. Such a phenomenon is called True Polar Wander (TPW).Here we report the discovery of a global series of topographic lows on Saturn's satellite Enceladus that we interpret to show that this synchronously locked moon has undergone TPW by ∼55° about the tidal axis. We use improved topographic data from the spherical harmonic expansion of Cassini limb and stereogrammetric measurements to characterize regional topography over the surface of Enceladus. We identify a group of nearly antipodal basins orthogonal to a basin chain tracing a non-equatorial circumglobal belt across Enceladus' surface. We argue that the belt and the antipodal regions are fossil remnants of earlier equator and poles, respectively. We argue that these lows arise from isostasic compensation and that their pattern reflects spatial variations in internal dynamics of the ice shell. Our hypothesis is consistent with a variety of geological features visible in Cassini images.

The obliquity of Enceladus

The extraordinary activity at Enceladus’ warm south pole indicates the presence of an internal global or local reservoir of liquid water beneath the surface. While Tyler (Tyler, R.H. [2009]. Geophys. Res. Lett. 36(15), L15205; Tyler, R.H. [2011]. Icarus 211(1), 770–779) has suggested that the geological activity and the large heat flow of Enceladus could result from tidal heating triggered by a large obliquity of at least 0.05–0.1°, theoretical models of the Cassini state predict the obliquity to be two to three orders of magnitude smaller for an entirely solid and rigid Enceladus. We investigate the influence of an internal subsurface ocean and of tidal deformations of the solid layers on the obliquity of Enceladus. Our Cassini state model takes into account the external torque exerted by Saturn on each layer of the satellite and the internal gravitational and pressure torques induced by the presence of the liquid layer. As a new feature, our model also includes additional torques that arise because of the periodic tides experienced by the satellite. We find that the upper limit for the obliquity of a solid Enceladus is 4.5×10-4 degrees and is negligibly affected by elastic deformations. The presence of an internal ocean decreases this upper limit by 13.1%, elasticity attenuating this decrease by only 0.5%. For larger satellites, such as Titan, elastic effects could be more significant because of their larger tidal deformations. As a consequence, it appears that it is easier to reconcile the theoretical estimates of Titan’s obliquity with the measured obliquity than reported in previous studies wherein the solid layers or the entire satellite were assumed to be rigid. Since the obliquity of Enceladus cannot reach Tyler’s requirement, obliquity tides are unlikely to be the source of the large heat flow of Enceladus. More likely, the geological activity at Enceladus’ south pole results from eccentricity tides. Even in the most favorable case, the upper limit for the obliquity of Enceladus corresponds to about two meters at most at the surface of Enceladus. This is well below the resolution of Cassini images. Control point calculations cannot be used to detect the obliquity of Enceladus, let alone to constrain its interior from an obliquity measurement.

A unified nomenclature for tectonic structures on the surface of Enceladus

Enceladus has experienced widespread and diverse tectonic deformation. The diversity and nonuniform distribution of structures visible on the surface attests to a complicated and perhaps long-lived tectonic history. Currently, no fundamental classification scheme based on morphology exists for the tectonic structures on Enceladus, which limits analysis and discussion of tectonic structures and regional and global tectonic histories. Characterization and classification of structures on Enceladus is an important first step in understanding its tectonic history. Here, we propose a global morphologic (i.e., nongenetic) classification scheme for tectonic structures on Enceladus. Five classes of tectonic structures on Enceladus have been identified based on images and limited topographic data: troughs, scarps, chasmata, ridges, and bands. Morphological variation of structures exists within these classes, reinforcing the complicated tectonic history of this small body.

Cryoclastic origin of particles on the surface of Enceladus

Analogous to volcanic deposits on Earth, we can infer eruption characteristics on Enceladus from the relationship between particle size and distance from the vent. We develop a model in which ice particles feeding plumes are accelerated by the gas. We consider two cases: drag-limited and collision-limited acceleration, which link particle size to exit velocity. After being ejected at the vent, particles follow ballistic trajectories. We fit the model to observations of particle size on the surface inferred from modeled VIMS data collected by the Cassini spacecraft. We obtain a relationship between gas temperature and characteristic acceleration length, whereby lower gas temperatures require longer acceleration lengths. The model shows that the large size of particles on the surface is consistent with the size of particles observed with the CDA and VIMS instruments at heights of Cassini flybys, and the size of particles that reach escape velocity and are found in Saturn's E-ring.

Interaction of Saturn's magnetosphere and its moons: 1. Interaction between corotating plasma and standard obstacles

The interaction of Saturn's inner magnetosphere with its moons ranges from the addition of significant quantities of gas, dust, and plasma, causing significant consequences for the dynamics and energetics of the entire Saturnian magnetosphere, to the simple absorption of plasma and energetic particles by the icy moons with non‐electrically conducting interiors. The interaction with these moons is complex with the contribution of many physical processes, depending on the geometry of any plume, the structure of the atmosphere, and its interaction with the surface and interior of the moon, the latter by induced fields. Our ultimate goal is to understand the complexities of this interaction and its temporal variations, especially at Enceladus. In this paper we use magnetohydrodynamics (MHD) code for addressing the flow around obstacles that are simpler than the Enceladus interaction. These simulations both help us understand the interaction with other icy moons and prepare us for the simulation of the flow around Enceladus. The processes involved include ordinary collisions, impact ionization, photoionization, and charge exchange. We examine a series of simple canonical interactions before we later apply our simulation where the multiple processes are occurring simultaneously with asymmetric geometries. We apply our 3‐D MHD model to simulate the interaction between the Saturnian corotational plasma flow for the following cases: an absorbing body having an insulating surface; ion pickup via photo and impact ionization from a spherically symmetric neutral cloud; charge exchange with such a neutral cloud; and ion pickup at an insulating, absorbing body with an atmosphere acted upon by the sum of the three ionization processes. In addition to validating the model and obtaining a deeper understanding of the consequences of each interaction, we can immediately make some conclusions about the Enceladus interaction. We find that the magnetometer data are most consistent with the surface of Enceladus being absorbing and insulating, rather than the surface being reflecting and electrically conducting. For the conditions in the corotating flow at Enceladus, the perturbation to the plasma flow produced by photo/impact ionization is an order of magnitude smaller than that produced by charge exchange. Moreover, the perturbation to the magnetic field Bz component by a spherically symmetric mass loading source alone is an order of magnitude smaller than that observed in the neighborhood of the plume. Thus, the perturbation observed in the magnetometer data is primarily due to the mass loading in the plume, which is primarily ion‐neutral charge exchange. The geometry and source strength of the plume are investigated in a following paper.

Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus

Saturn's moon Enceladus emits plumes of water vapour and ice particles from fractures near its south pole, suggesting the possibility of a subsurface ocean. These plume particles are the dominant source of Saturn's E ring. A previous in situ analysis of these particles concluded that the minor organic or siliceous components, identified in many ice grains, could be evidence for interaction between Enceladus' rocky core and liquid water. It was not clear, however, whether the liquid is still present today or whether it has frozen. Here we report the identification of a population of E-ring grains that are rich in sodium salts ( approximately 0.5-2% by mass), which can arise only if the plumes originate from liquid water. The abundance of various salt components in these particles, as well as the inferred basic pH, exhibit a compelling similarity to the predicted composition of a subsurface Enceladus ocean in contact with its rock core. The plume vapour is expected to be free of atomic sodium. Thus, the absence of sodium from optical spectra is in good agreement with our results. In the E ring the upper limit for spectroscopy is insufficiently sensitive to detect the concentrations we found.

Photometric and spectral analysis of the distribution of crystalline and amorphous ices on Enceladus as seen by Cassini

Photometric and spectral analysis of data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS) has yielded significant results regarding the properties and composition of the surface of Saturn's satellite Enceladus. We have obtained spectral cubes of this satellite, containing both spatial and spectral information, with a wavelength distribution in the infrared far more extensive than from any previous observations and at much higher spatial resolution. Using a composite mosaic of the satellite, we map the distribution of crystalline and amorphous ices on the surface of Enceladus according to a “crystallinity factor” and also the depth of the temperature- and structure-dependent 1.65 micron water-ice band. These maps show the surface of Enceladus to be mostly crystalline, with a higher degree of crystallinity at the “tiger-stripe” cracks and a larger amorphous signature between these stripes. These results suggest recent geological activity at the “tiger stripe” cracks and an intriguing atmospheric environment over the south pole where amorphous ice is produced either through intense radiative bombardment, flash-freezing of cryovolcanic liquid, or rapid condensation of water vapor particles on icy microspherules or on the surface of Enceladus.

Dynamics of dust ejected from Enceladus: Application to the Cassini dust detector

The satellite Enceladus is obviously the main source of Saturn's E ring. Up to now, different mechanisms of how particles are delivered from this satellite have been suggested. In this paper, we try to answer the question of whether these different launch processes can be distinguished by the cosmic dust analyzer (CDA) aboard the Cassini spacecraft. To this aim, the dynamics of dust particles just launched from the surface of Enceladus is studied numerically. We have integrated the equations of motion for a wide range of initial conditions including ejecta from interplanetary and E ring impactors onto Enceladus. According to our simulations, Cassini will encounter a significant dust stream about the time of closest approach to Enceladus. The duration and intensity of this expected enhanced impact rate onto the CDA depends on the way the particles are ejected from the satellite. The counting rate yields information about the distribution of ejecta sources on the surface of Enceladus. For instance, an anisotropy of the ejecta between the leading and the trailing hemispheres of Enceladus should be detectable, and impactors of different origin should be distinguishable. Furthermore, the vertical component of the ejecta velocities can explain the vertical extent of the E ring.

Structure and particle properties of Saturn's E Ring

We have systematically reanalyzed a large subset of the available photometric data on Saturn's E Ring, including Voyager images and star tracker data, Earth-based CCD images, photometry, and spectrophotometry. In order to compare data sets with vastly different vi viewing geometries on this broad, vertically extended ring, we have calibrated every observation based on a common three-dimensional model. The ring shows a density peak at a location indistinguishable from the Enceladus orbit, with a radial offset of <3000 km. We have found that a very simple power-law model describes the ring's normal optical depth profile with orbital radius. The ring shows a general increase in vertical thickness with distance from the planet, ranging from 6000 km FWHM at its inner bound to nearly 40, 000 km at its outer. However, this trend is violated near the density peak, where a localized decrease in thickness by ∼30% is observed. The ring is also displaced northward from the planet's equatorial plane by 310 ± 25 km in one set of Voyager images, although the overall nature of the ring's vertical asymmetry is unclear. No power-law size distribution is compatible with the available photometry. A narrow distribution of slightly nonspherical particles of radius 1.0 ± 0.3 μm provides the best fit to the data. This highly peculiar size distribution clearly indicates that the ring does not originate from collisional or disruptive processes, and is therefore unlike any other known ring. Hence, we can give some credence to the possibility that the E Ring originates in “geyser-like” eruptions from the surface of Enceladus. Depending on the precise particle size chosen, the ring's peak normal optical depth is found to be 1.5(±0.4) × 10 −5, corresponding to a geometric cross-section per unit area of 5.3(±1.3) × 10 −5. Macroscopic bodies comprise less than 1% of the ring's optical depth.

On the heterogeneous appearance of Saturn's rings at high optical resolution

Shoemaker and Terrile have shown that several small satellites can be the parents of Saturn's rings and have suggested these as the origin of the broad systematic differences in the photometric properties of the rings. Dramatic variations on the smallest scales appear in the images obtained from Voyager. The variations of the surface of Enceladus suggest the importance of differentiation of satellites heated by tidal action maintained by resonances, as pointed out by C.M. Yoder. Subsequent break-up of such satellites, either under a large impact or under increasing tidal action of Saturn as its mass increased, would have yielded a mixture of different crustal and interior materials with different spectra of sizes and masses. We shall discuss the possible role of radial differentiation of particles in terms of their masses initiated by the combined activity of instabilities discussed long ago by Maxwell and by Cook and Franklin with those proposed recently by Ward and by Lin and Bodenheimer. In this way, we may reach an explanation of variations on the smallest scales. Comparison will be made with the expected behaviour of a primary accretion disk composed of a uniform distribution of material.