P23B-1364
A Solar-Powered Enceladus Mission
We present the results of a concept design study for a New Frontiers or small Flagship-class mission to Enceladus, using solar power. By concentrating on the science objectives most critical for a Cassini follow- on, this mission maximizes science return while maintaining a power consumption level that can be provided by a practical practical solar power system. The optimized instrument payload is the product of a broad science community-based Science Definition Team study. The spacecraft and mission designs are the products of studies carried out by the GSFC Mission Design Lab and Ball Aerospace. In addition to the low insolation at Enceladus, its location deep in Saturn's gravity well makes it a challenging target to reach, meaning that careful consideration must be given to spacecraft mass and the potential mission types. This presentation summarizes the mission science objectives and payload, the dynamical work, and the notional operations plan of this mission.
P23B-1365
Liquid Water vs. Hydrogen Cyanide on Enceladus
The Ion and Neutral Mass Spectrometer onboard the Cassini spacecraft detected hydrogen cyanide (HCN) in plumes on Enceladus [1]. We explored computationally the thermodynamic stability and kinetic reactivity of HCN. Thermochemical equilibrium calculations show that HCN is unstable in liquid water, and should be overwhelmingly hydrolyzed to formic acid (HCOOH) and ammonia (NH3) under all conditions relevant to Enceladus. Even in an aqueous solution with an unreasonably high NH3 activity of 10, the HCOOH/HCN activity ratio is predicted to be greater than 107 at 0°C. This ratio decreases with increasing NH3 activity and with increasing temperature. Even at high temperatures (e.g., 200°C), it is at least 3-4 orders-of-magnitude above unity. It is notable that neither formic acid nor ammonia have been detected in any plumes [1]. We thus infer that the detected HCN is unlikely to have equilibrated with an aqueous solution below Enceladus' surface. Chemical kinetic considerations indicate that the hydrolysis lifetime of HCN has a complex dependence on pH, and decreases with increasing temperature. We find that, even at the pH value most conducive to the survival of HCN (~5), HCN can persist for only about 5 Ma in liquid water at 0°C. Thus, the HCN in the plumes could not have come from a long-lasting ocean or sea, and probably came from a recent aqueous melt or some icy phase. A liquid source for the HCN would be more restrictive than an ice source. For example, an aqueous solution must have a pH between 4-6 for HCN to be preserved in it for at least ~1 Ma at 0°C. However, this pH restriction would be lifted if the HCN source were a very young pool (i.e., centuries-millennia old). On the other hand, it is expected that HCN could survive indefinitely in an icy matrix. The proposed HCN sources may be less favorable abodes for life than a long-lived aqueous reservoir. Reference: [1] Waite J.H. et al. (2008) Eur. Planet. Sci. Congr. 3, EPSC2008–A–00593.
P23B-1366
Detecting a Subsurface Ocean From Periodic Orbits at Enceladus
Enceladus is a small icy satellite of Saturn which has been observed by the Cassini orbiter to eject plumes mainly consisting of water vapor from the "tiger stripes" located near its South pole. While tidal heating has been ruled out as an inadequate energy source to drive these eruptions, tidally induced shear stress both along and across the stripes appears to be sufficiently powerful. The internal constitution of Enceladus that fits this model is likely to entail a thin crust and a subcrustal water layer above an undifferentiated interior. Apart from the lack of a core/mantle boundary, the situation is similar to the current hypothetical models of Europa's interior. The determination of the existence of a subsurface fluid layer can therefore be pursued with similar methods, including the study of the gravitational perturbations of tidal origin on an Enceladus orbiter, and the use of altimeter measurements to the tidally deformed surface. The dynamical environment of an Enceladus orbiter is made very unstable by the overwhelming presence of nearby Saturn. The Enceladus sphere of influence is roughly twice its radius. This makes it considerably more difficult to orbit than Europa, whose sphere of influence is ~six times its radius. While low-altitude, near-polar Enceladus orbits suffer extreme instability, recent works have extended the inclination envelope for long-term stable orbits at Enceladus. Several independent methods suggest that ~65 degrees inclination is the maximum attainable for stable, perturbed Keplerian motion. These orbits are non-circular and exist with altitude variations from ~200 to ~300 km. We propose a nominal reference orbit that enjoys long term stability and is favorable for long-term mapping and other scientific experiments. A brief excursion to a lower altitude, slightly higher inclined, yet highly unstable orbit is proposed to improve gravity signatures and enable high resolution, nadir-pointing experiments on the geysers emanating from the tiger- stripes. Near-circular, low altitude highly inclined orbits with arbitrary initial conditions will impact Enceladus if uncontrolled in about 1 to 2 days. To reduce risk and station-keeping requirements we choose periodic orbits in the Hill's plus non-spherical Enceladus model. Despite the instability, the repeat ground track solutions represent equilibria in the dominant terms of the dynamics and therefore extend the uncontrolled lifetimes to ~7 to ~10 days. Round-trip transfers between the two orbital phases is expected to conservatively cost between ~50 and ~100 m/s. We use orbits of different altitudes and inclinations to simulate Earth-based ranging to the orbiter and altimeter measurements to the surface of Enceladus. The simulations are made assuming different tidal responses by adopting different values of the Love numbers. The synthetic measurements are then inverted and the tidal parameters h2 and k2 estimated. Results will be presented in terms of sensitivity of detection of Love numbers to the different orbital geometries. Indications will thus be provided for optimized orbit planning of future exploration missions aimed at investigating the internal structure of the satellite and the detection of its putative subcrustal ocean.
P23B-1367
Comparing Enceladus to Comets: Implications for Enceladus' Activity
Despite obvious physical differences in size, shape, and surface albedo between Enceladus and cometary nuclei, the compositional similarities between the vented gases from Enceladus and the gases in cometary comae are striking. The approximately 1028 molecules/s of material emitted from Enceladus, composed mostly of water with some hypervolatile species, flows out in jets similar to those seen for moderately active comets. Using recent data from Cassini instruments, including the Ion and Neutral Mass Spectrometer (INMS), the Visual and Infrared Mapping Spectrometer (VIMS) and the Imaging Science Subsystem (ISS), we investigate the compositional similarities between Enceladus' plumes and cometary comae and we compare the physical properties (densities, speeds, collimation) of the plumes and cometary jets. We will discuss the implications our findings have for various aspects of the origin and evolution of Enceladus.
P23B-1368
Is Enceladus a Comet? A Cometary Perspective on Enceladus
The discovery of icy plumes emanating from Saturn's moon Enceladus by the Cassini spacecraft has raised questions about the cometary nature of this small satellite. The release of gas and dust from cometary nuclei is restricted to "jets" or plumes also and this activity has been observed in comets at distances much further than Saturn's orbit, including comet-like activity and a resolved coma of the Centaur Chiron. Enceladus and Chiron have sizes that are much larger than cometary nuclei but their atmospheres are still largely unbound, similar to the exospheres of comets. With Chiron, Enceladus may represent a transitional object in this respect, intermediate to the tightly bound, thin atmospheres typical of planets and large satellites and the greatly extended atmospheres in free expansion typical of cometary comae. Measurements of the neutral and ion composition of the plumes reveal the presence of water group species, nitrogen-bearing molecules, and other species that have been found in comets. The nature of the volatile materials in Enceladus may also bear similarities with ideas of cometary ices. In other respects, the large size of Enceladus relative to comets and the presence of Saturn and its magnetosphere nearby, brings into question the validity of applying scaling laws to cometary results in order to understand the environment surrounding Enceladus. In addition, release mechanisms for the icy grains and gases at Enceladus, including liquid water mixtures below the cold, icy surface, are not thought to be applicable to comets. These issues and others are discussed as we offer a cometary perspective on our current understanding of Enceladus. Acknowledgements. We acknowledge funding and support from the NASA Cassini Mission, contract NAS71407NMO710023.
P23B-1369
Saturn Magnetospheric Impact on Surface Molecular Chemistry and Astrobiological Potential of Enceladus
The active south polar surface of Enceladus is exposed to strong chemical processing by direct interaction with charged plasma and energetic particles in the local magnetospheric environment of this icy moon. Chemical oxidation activity is suggested by detection of H2O2 at the surface in this region and less directly by substantial presence of CO2, CO, and N2 in the plume gases. Molecular composition of the uppermost surface, including ejecta from plume activity, is radiolytically transformed mostly by penetrating energetic electrons with lesser effects from more depleted populations of energetic protons. The main sources of molecular plasma ions and E-ring dust grains in the magnetospheric environment are the cryovolcanic plume emissions from Enceladus. These molecular ions and the dust grains are chemically processed by magnetospheric interactions that further impact surface chemistry on return to Enceladus. For example, H2O neutrals dominating the emitted plume gas return to the surface mostly as H3O+ ions after magnetospheric processing. Surface oxidant loading is further increased by return of radiolytically processed ice grains from the E-ring. Plume frost deposition and micrometeoroid gardening protect some fraction of newly produced molecular species from destruction by further irradiation. The evident horizontal and vertical mobility of surface ices in the south polar region drive mixing of these processed materials into the moon interior with potential impacts on deep ice molecular chemistry and plume gas production. Similarly as suggested previously for Europa, the externally driven source of radiolytic oxidants could affect evolution of life in any subsurface liquid water environments of Enceladus.
P23B-1370
Photochemistry of Methane-water Ices
We report a study on the broadband ultraviolet photolysis of methane-water ice mixtures, at low concentrations and temperatures relevant to the icy satellites of the outer Solar System. Although the results are applicable to a variety of bodies in the Solar System, we concentrate on Enceladus, where methane mixed with water is expected to occur on the surface. The photochemistry of these mixtures is dominated by the action of hydroxyl radicals on methane and the resulting products, yielding methanol, formaldehyde, carbon monoxide and carbon dioxide. Near infrared spectra of the surface of Enceladus returned by Cassini show the presence of an absorption feature at 3.53 microns, ascribed to short chain organics. We assign this feature specifically to methanol, and suggest a photochemical origin. The photochemical oxidation observed in our experiments implies that, given sufficient exposure time, methane (or other organics) will eventually be completely oxidized to carbon dioxide. Thus the observation of organics on the surface of Enceladus suggests that the organics are likely of recent origin.
P23B-1371
On the formation of sodium bearing E ring ice grains on Enceladus.
Small but significant concentrations of sodium have been detected by the Cassini CDA in mass spectra of E ring ice grains. Since Enceladus' plume is the dominant source of the E ring, this sodium must come from the interior of Enceladus. This is consistent with a plume originating from a liquid reservoir that has been, or is, in contact with the rocky core of the satellite, since in this case the liquid is expected to be enriched in minerals (Zolotov, GRL, 2007). However, the physical processes how these minerals find their way into the ice grains of Enceladus' plume are less clear. In this talk we discuss possible scenarios and relate them to the concentrations found in CDA data.
P23B-1372
High Spatial Resolution Observations of Thermal Emission from Enceladus' Active South Pole
Cassini's four close flybys of Enceladus in 2008 are providing unprecedented views of the active "tiger stripe" fractures in the moon's south polar region. The Composite Infrared Spectrometer (CIRS) instrument is obtaining spectral maps of the endogenic thermal emission from the tiger stripes at wavelengths from 7 to 1000 microns. Here we concentrate on observations of the short-wavelength radiation made with CIRS's high spatial resolution focal planes "FP3" (9 -- 16 microns) and "FP4" (7 -- 9 microns). FP3 and FP4 observations during the March 12 2008 flyby mapped most of the active region at a spatial resolution of 4 -- 10 km, revealing large variations in emission along and between the tiger stripes. FP4 data revealed temperatures in excess of 180 K along the brightest parts of the tiger stripe fracture named Damascus Sulcus. The August 11th flyby provided some scattered observations with spatial resolution as high as 400 meters, a very high signal-to-noise spectrum of 7 -- 9 micron emission from Damascus Sulcus with 1.5 x 3.0 km resolution, and high-resolution mapping of emission along parts of Baghdad Sulcus. Lower spatial resolution observations helped to complete the mapping of the entire active south polar region, and provided constraints on the time variability of the emission on month-to-year timescales. Additional high- resolution observations are planned during the two upcoming flybys on October 9 and October 31 2008.
P23B-1373
Improved High-Resolution Enceladus Coverage after the Flybys in 2008
The Cassini Imaging Science Sub-system (ISS) acquired many high-resolution im-ages (< 1 km/pixel) during two close targeted flybys of Enceladus in March and August 2008. More close flybys are planned this fall. These images gave us the possibility to improve the high-resolution global mosaic of Enceladus. We also could improve some map sheets of the Enceladus atlas. Crater Salih which defines the longitude system of Enceladus was observed with high-resolution during the March flyby. This gave us the possibility to determine the shift of our mosaic com-pared to the definition of the International Astronomical Union (IAU). The global mosaic was shifted 3.5° to the West to be consistent with the IAU definition. The global mosaic and the atlas will be made available to the public through CICLOPS (http://ciclops.org).
P23B-1374
Cataloging of Craters on Enceladus
The surface of Saturn's satellite Enceladus is unique in terms of the amount of geologic activity that is taking place on what many had once assumed would be a cold and dead icy moon. Instead of a cold, cratered surface we have found a surface scarred with signs of tectonic activity in the form of numerous long rifts and fractures and we have seen cryovolcanic activity emanating from the south polar region. Using mostly Cassini images (a few of the map images are from Voyager), we are currently in the process of creating a comprehensive catalog of craters that, we believe, will be an invaluable tool in aiding our understanding of this enigmatic moon. The catalog will give the location of all craters measuring at least one-half degree (~2.2 km) in diameter. In addition to location and size, the catalog will also note deformation of the craters, both in terms of rifting and ellipticity. The deformations can give us insight to the tectonic history (i.e. many of the craters show post impact rifting) as well as giving us a further tool to study tectonic stresses across the surface. Areas of differing resolution are highlighted as they are an important limiting factor in determining crater densities. It is for this reason that crater sizes of one-half degree were chosen as they are more identifiable in lower resolution areas than craters that are much smaller. We intend to study crater distribution and have so far noted high crater densities between 216° W and 144° W and between 10° S and 10° N approximately centered around 180° longitude (the antipode to the sub-Saturnian point). In addition to our study of crater distribution we believe this catalog, upon completion, will be useful in the study of surface processes and surface heating of Enceladus.
P23B-1375
Examining Enceladus' plume through observations and simulations
The discovery of Enceladus" plume raised important questions pertaining to the interaction of Saturn's magnetosphere with this small moon. Traditional MHD as well as several other plasma dynamic simulations assume a collisionless plasma, and generally exclude the possibility for charge exchange and momentum transfer between neutrals and plasma. The interaction of Saturn's magnetosphere with Enceladus and its plume provides an exciting natural laboratory for expanding our understanding of change-neutral interactions and their impact on mass and momentum loading of the system and the associated magnetic perturbations. Here we present a simulation that treats several ion and neutral species in a multi-fluid simulation approach that incorporates both the plasma dynamics and neutral-ion interactions responsible for energy and momentum transfer between the neutral and ion fluids. These simulations are compared in detail to and benchmarked against the observations made from the recent Enceladus flyby on March 12, 2008, and can potentially lead to interpretation of features observed in the ion population by CAPS.
P23B-1376
Evidence for temporal variability of Enceladus' gas jets: Modeling of Cassini observations
Time variability of Enceladus' gas plume is deduced from a joint investigation of Cassini spacecraft magnetic field data obtained during the first three flybys E0, E1 and E2 and neutral density measurements during the E2 flyby with a model that describes Enceladus' plasma interaction with individual jets. We infer a total plume content of ~7 × 1032 H2O molecules corresponding to a mass loss rate of ~1600 kg/s for the E0 flyby and ~9 × 1031 H2O molecules corresponding to a mass loss of ~200 kg/s for the E1 and E2 flybys. The magnetic field measurements also indicate upstream/downstream changes of the plume activity locations. On the first three Enceladus flybys Cassini did not fly through the densest parts of the plumes, which were a factor of 10-100 times higher at the same altitudes.
P23B-1377
Interaction of Enceladus's Water Plume with Saturnian Magnetosphere via Hybrid Numerical Simulations
Several close Cassini flybys of the Santurnian moon Enceladus provided direct in situ measurements of neutral water molecules escaping from the surface showing their interaction with the ambient plasma environment. Cassini measurements indicate Enceladus to act as an obstacle to the magnetized Saturnian plasma flow resulting in an effect of field line draping. Ionization of escaping neutrals by way of charge exchange with the ambient plasma produces fresh ions which are picked up by the Saturnian magnetosphere. The Saturnian co-rotating plasma flow therefore slows down and the ambient magnetic field is affected. We study these local plasma interaction of Enceladus and its neutral water plume with the Saturnian magnetosphere by using a full 3D hybrid code numerical simulation. The results of our model are subsequently compared with Cassini observations. Since a complete and accurate description of Enceladus surroundings is still missing, the initialialization of our simulations is based on currently published estimations. However, by use the hybrid code we are able to recover very similar magnetic field signatures as some of those realy observed by Cassini spacecraft.
P23B-1378
Neutral Composition of Enceladus' Plume
In July of 2005, the Cassini spacecraft passed within 170 km of the Saturnian moon Enceladus, discovering a substantial plume of icy material emanating from the south pole. Preliminary conclusions on the composition were performed on data obtained by the Ion and Neutral Mass Spectrometer (INMS) showing clear signs of H2O, CH4, H2, CO2, an unidentified peak at mass 28 (N2 or CO) and trace quantities of simple organics. Recent Cassini flybys through the Enceladus plume during the spring and fall of 2008 have exceeded expectations of signal strength, solidifying the existence of complex organic material and offering answers to many questions arising from the initial encounter. New analysis techniques have been developed to take advantage of the multiple data sets, enabling a more thorough investigation. INMS data from all relevant observations are combined and presented here for a comprehensive analysis of the neutral composition of Enceladus' plume.
P23B-1379
Enceladus¡¦ Gas Tori of Carbon-bearing and Nitrogen-bearing Molecules
The INMS instrument detected the presence of gas plumes during its passage over Enceladus¡¦ South Pole region on July 14, 2005. The neutral composition measurement showed that the atmospheric composition is characterized by 91±3% H2O, 3.2±0.6% CO2, 4±1% N2 or CO and 1.6±0.4% CH4. A lot of works have been done on modeling and analysis of the structure and evolution of the water gas cloud emitted from Enceladus. In comparison, not much attention has been given to the other gas components like CO2, CO (or N)2 and CH4 even though they represent all together about 15% of the atmospheric loss by mass of this icy satellite. In this work we will show preliminary models on the spatial distributions of the ¡§parent molecules¡¨ and their ¡§daughter molecules and atoms¡¨ created by photodissociation and other processes.
P23B-1380
Thermal Ion Flow Velocities: Signatures of the Enceladus Torus?
For the middle/outer (> 5.5 RS) magnetosphere of Saturn, Cassini Plasma Spectrometer (CAPS) ion counting data provides thermal ion moments obtained assuming Maxwellian distributions for each ion species [Wilson et al., 2008]. However, in the inner magnetosphere (< 5.5 RS) within Saturn's extended neutral cloud and proposed Enceladus torus [Johnson et al., 2006], there is fresh ion production via charge exchange [Tokar et al., 2008], yielding a complex ion velocity distribution. In this study, a new technique is investigated to analyze the CAPS data in the inner region. The technique concentrates on finding the ion flow velocity using only the assumption that the ions are gryotropic. The available ion velocity space viewed by CAPS in a slice perpendicular to the magnetic field is translated within that plane to find the optimum flow velocity that yields gyrotropic ions. Near-equatorial trajectories of Cassini in the inner region are surveyed to determine both azimuthal and radial ion flow velocity. The azimuthal flow velocity profile with radial distance, a proxy for fresh ion production, is compared with models for both the Enceladus torus and extended neutral cloud. References: Wilson, R.J. et al., J. Geophys. Res. in press, doi:10.1029/2008ja013486 Johnson, R.E. et al., The Astrophysical Journal, pg L137, 20 June 2006. Tokar, R.L. et al, Geophys. Res. Letts., vol. 35, L14202, doi:10.1029/2008GL034749
P23B-1381
MHD Study of the Variation of the Mass Loading Sources near Enceladus
The major mass loading source in Saturn's magnetosphere, Enceladus, is studied with seven Cassini flybys during 2005 and 2008. The observations have revealed that the primary mass loading source originates from an extensive and asymmetric water plume centered at the South polar region of this icy moon. Such a persistent gas production results in a plasma torus centered at the Enceladus orbit and possibly extended by other moons of comparatively smaller gas production. To understand the interaction between Enceladus and the corotational torus plasma, the Cassini magnetometer data is investigated with the help of 3-D MHD simulations to infer the size, structures and time variations of the plume. A numeric model is applied to this plasma interaction region, considering self-consistently the photoionization, electron impact ionization and charge exchange as the mass loading source. In addition, the torus-moon surface interaction is studied with different types of inner boundary conditions. We investigate the possibility of charged dust affecting the flowing magnetized plasma at Enceladus.
P23B-1382
A Comparison of CAPS Composition Measurements of the Plasma near Enceladus with Predictions of a Chemical Model
The Cassini spacecraft has flown by Saturn's moon Enceladus several times, with the most recent (designated 080 EN) on August 11, 2008 at an altitude of 54 km. At least two additional close flybys are planned. The surprising discovery of the jets of water/ice from the southern hemisphere of this moon has stimulated considerable discussion regarding the possible nature and especially temperature of the internal reservoir for these jets. A possible explanation that has been suggested for the apparently liquid water internal to such a cold, icy body involves the possible presence of small amounts of species such as ammonia that would affect the melting point. Measurements during previous flybys have hinted at the existence of such species. We have been analyzing the ion composition data from the Cassini Plasma Spectrometer (CAPS) Ion Mass Spectrometer obtained during the recent Enceladus flybys and comparing the results with those of a chemical network model (Boice, this session). This model can serve as an aid in interpreting the CAPS measurements in cases where there is some ambiguity in species assignment in the data. In addition to the ubiquitous water group ions we find nitrogen and some of its hydrides.
P23B-1383
On the Long-Term Abundance of Energetic Sodium Ions in Saturn's <20 Rs Magnetosphere
Saturn's icy magnetospheric environment presents a rather easily accessible astrophysical laboratory, possibly reflecting some conditions similar to those present in the early solar nebula. The recent discovery of a Sodium-rich water ice population in Saturn's E ring, with Sodium (Na) concentrations of ~10-5 [Postberg and Kempf, Abstracts for "Saturn After Cassini-Huygens" Symposium, Imperial College, London, 28.July-1.August, 2008, http://www.saturnaftercassini.org/] has prompted a search for Na in Saturn's magnetospheric energetic charged particle population. This Na dust measurement leads Postberg and Kemp to the possibility of a salty sea/ocean on Enceladus. We use measurements from MIMI/CHEMS, the Charge- Energy-Mass Spectrometer, to analyze the ion composition in the range 73-220 keV/e taking full note of the of the concurrent, background magnetospheric charged particle population. CHEMS, one of three sensors comprising the MIMI investigation on Cassini, determines the mass and charge state of ions. Measurements made throughout the solar system by instruments similar to CHEMS have shown that both dust and gas can be sources of energetic charged particles through photoionization and/or charged particle impact. We examine energetic, singly-charged heavy ions in the near-Saturn magnetosphere in order to determine and/or set an upper limit to the relative abundance of Na+1 to other heavy ion species for Cassini's near-planet equatorial orbits early in the mission (mid-2004 to mid-2006) and from selected intervals to the present. If the Na is released as NaCl, for example, we should have two ionized species to search for. However, in our measurements, Na+1 is obscured partially by the more abundant water-group (O+1 , OH+1 , and H2O+1) ions and Cl+1 fully by diatomic oxygen (O2+1) ions. The Na+1 is easier to separate. In the equatorial-orbits data set, the level of Na+1 is at or below background levels.
P23B-1384
Water Group Composition Near the Orbit of Enceladus
We present magnetospheric ion composition results from the Cassini CAPS IMS instrument. The data set is averaged over a four year period. Data from the equatorial plane are selected and binned into radial, local time and longitude bins with a focus on radial distances between 3-10 Saturn radii. The data analysis process necessitates fitting the instrument response functions to the raw data and so methods of assessment of data uncertainty are also presented. Water group ions (O+, OH+, H2O+, H3O+; or collectively W+), presented as mixing ratios dominate over H+ in this radial range and O+ is the dominant water group species. Temporal variations within the study period are presented. The energy distribution of the ion composition is also examined and the flow velocity is compared to the corotation velocity of the plasma.
P23B-1385
Cassini/RPWS Dust Measurements During E3 and E4 Flybys
The Cassini spacecraft completed two very close flybys of Enceladus on days 072 and 224, 2008. Both flybys were designed to pass through the water vapor plume near the south pole of this tiny but energetic moon. At closest approach, the spacecraft was merely 50 kilometers away from the surface. The Radio and Plasma Wave Science (RPWS) instrument on board the spacecraft obtained a great opportunity to measure the parameters of micron-sized particles in the plume. As the spacecraft approached the water plume, the RPWS dipole antenna started recording impulses indicating dust impacts on the spacecraft. The peak impact rate is about 500/s and the corresponding number density is about 5.7 × 10-2 m-3. The dust particles are thought to have radii of about 1 to 10 micrometers, which is within the RPWS sensitivity range. Some saturation effects occurred near the peak flux that complicates the analysis in this region. In this presentation we will discuss the variation of dust flux as a function of distance from the south pole of Enceladus, the mass and size distribution, and the optical depth computations.
P23B-1386
Hybrid Simulations of the Enceladus Plasma Interaction and Comparison with MAG Data
The Cassini Spacecraft has made four close flybys of Saturn's icy moon Enceladus. It became evident that a large water vapour and dust plume is located below Enceladus' south pole. Cassini plasma and magnetic field measurements indicate a source strength exceeding 100 kg/s and show a significant draping of the magnetic field lines. Cassini observations suggest a plume diameter of at least 2 Enceladus radii and a mass loading rate of about 2-3 kg/s. We study the interaction of Enceladus with the Saturnian magnetospheric plasma and magnetic field by using a three-dimensional hybrid model, which treats the electrons as a fluid and the ions as individual particles. Our model includes a self-consistent description of the obstacle's internal conductivity and also considers ion-neutral-collisions. We analyze systematically how the obstacle's internal conductivity as well as the shape and size of the plume contribute to the overall structure of the interaction region. The presence of the plume gives rise to a magnetic cavity downstream of the moon and a magnetic pile-up region, while the field lines are able to pass through the solid body of Enceladus almost unaffected. Our simulation results are in reasonable agreement with observations made by the Cassini magnetometer instrument.
P23B-1387
Hybrid Simulations of Plasma Environment Around Enceladus
The Cassini flybys of the moon Enceladus have established the presence of magnetic field perturbations and plasma deflections due to its interaction with the corotating plasma. These observations show that the nature of the interaction region changes from one flyby to another indicating a time dependent behavior. In general, the field and plasma perturbations can be caused by the body of the moon, its exosphere (if present) and the plumes associated with the escape of neutral gasses from its southern hemisphere. Thus, the time dependency of the interaction region is tied to the variabilities of the neutral densities associated with the exosphere or plumes and also the observed, planetary rotation synched variation of plasma density in the region around the orbit of Enceladus . To understand the nature of plasma interaction with Enceladus we have conducted an investigation using 3-D electromagnetic hybrid simulations in which ions are treated kinetically while the electrons form a massless fluid. In these simulations, charge exchange between corotating ions and neutrals around Enceladus and plasma absorption by the body of the moon are included. The results show that the body of Enceladus by itself leads to an interaction consisting of a density cavity behind the moon with little change in plasma flow velocity and weak magnetic field perturbations. On the other hand, the presence of a dense enough exosphere or a plasma plume modify the nature of the interaction region significantly leading to more complex density and flow structures and generation of Alfvenic and slow magnetosonic wings. In this presentation, we describe these results in detail and show how the body of Enceladus, its exosphere and plumes contribute to the overall interaction region in isolation and collectively. We also compare the results with Cassini flyby data to estimate the neutral densities associated with the plumes and whether they indicate the presence of an exosphere or not.
P23B-1388
The Plume Ionosphere of Enceladus as Seen by the Cassini Ion and Neutral Mass Spectrometer
The Cassini spacecraft passed within 168 km of the surface of Enceladus on 14 July 2005 and passed even closer (50 km) during the E3 encounter on 13 March 2008. During the first encounter (E2), the Ion and Neutral Mass Spectrometer (INMS) detected a plume atmosphere consisting mainly of water but also with some carbon dioxide, carbon monoxide, and nitrogen. Higher quality data was obtained by the INMS during the E3 encounter and included measurements of the ion composition in the plume. A strong signal was present at a mass number of 19 Daltons in the ion mass spectrum of the plume and is interpreted as H3O+. The plume abundances of other ion species (including H2O+, OH+, O+) were at least 8 times lower than for H3O+. The INMS did not observe significant ion abundances near Enceladus just outside the plume. The INMS in its open source ion mode can only detect ions in a narrow volume of phase space (located for this observation at rest with respect to Enceladus), limiting the spectra obtained to "cold" ionospheric-type plasma. The published ion distributions measured by the Cassini plasma spectrometer (CAPS) during E2 just outside the plume appeared as an extended ring distribution covering a larger phase space volume. Ion-neutral cross sections will be used to interpret the INMS measurements of cold H3O+ ions and the almost complete absence of cold H2O+ ions. A plume ionosphere in which ion-neutral chemistry rapidly converts virtually all ions species into H3O+ ions will be discussed.