Anonymous

SO: Science (Washington), vol. 311, pp. 1416-1418, 10 Mar. 2006.

During Cassini's close flyby of Enceladus on 14 July 2005, the High Rate Detector of the Cosmic Dust Analyzer registered micron-sized dust particles enveloping this satellite. The dust impact rate peaked about 1 minute before the closest approach of the spacecraft to the moon. This asymmetric signature is consistent with a locally enhanced dust production in the south polar region of Enceladus. Other Cassini experiments revealed evidence for geophysical activities near Enceladus' south pole: a high surface temperature and a release of water gas. Production or release of dust particles related to these processes may provide the dominant source of Saturn's E ring.

Anonymous

SO: Science (Washington), vol. 311, pp. 1401-1405, 10 Mar. 2006.

The Cassini spacecraft completed three close flybys of Saturn's enigmatic moon Enceladus between February and July 2005. On the third and closest flyby, on 14 July 2005, multiple Cassini instruments detected evidence for ongoing endogenic activity in a region centered on Enceladus' south pole. 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 kelvin 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 kelvin are required.

Anonymous

SO: Science (Washington), vol. 311, pp. 1419-1422, 10 Mar. 2006.

The Cassini spacecraft passed within 168.2 kilometers of the surface above the southern hemisphere at 19:55:22 universal time coordinated on 14 July 2005 during its closest approach to Enceladus. Before and after this time, a substantial atmospheric plume and coma were observed, detectable in the Ion and Neutral Mass Spectrometer (INMS) data set out to a distance of over 4000 kilometers from Enceladus. INMS data indicate that the atmospheric plume and coma are dominated by water, with significant amounts of carbon dioxide, an unidentified species with a mass-to-charge ratio of 28 daltons (either carbon monoxide or molecular nitrogen), and methane. Trace quantities (<1%) of acetylene and propane also appear to be present. Ammonia is present at a level that does not exceed 0.5%. The radial and angular distributions of the gas density near the closest approach, as well as other independent evidence, suggest a significant contribution to the plume from a source centered near the south polar cap, as distinct from a separately measured more uniform and possibly global source observed on the outbound leg of the flyby.

Anonymous

SO: Science (Washington), vol. 311, pp. 1393-1401, 10 Mar. 2006.

Cassini has identified a geologically active province at the south pole of Saturn's moon Enceladus. In images acquired by the Imaging Science Subsystem (ISS), this region is circumscribed by a chain of folded ridges and troughs at 655'S latitude. The terrain southward of this boundary is distinguished by its albedo and color contrasts, elevated temperatures, extreme geologic youth, and narrow tectonic rifts that exhibit coarse-grained ice and coincide with the hottest temperatures measured in the region. Jets of fine icy particles that supply Saturn's E ring emanate from this province, carried aloft by water vapor probably venting from subsurface reservoirs of liquid water. The shape of Enceladus suggests a possible intense heating epoch in the past by capture into a 1:4 secondary spin/orbit resonance.

Anonymous

SO: Science (Washington), vol. 311, pp. 1425-1428, 10 Mar. 2006.

Observations of Saturn's satellite Enceladus using Cassini's Visual and Infrared Mapping Spectrometer instrument were obtained during three flybys of Enceladus in 2005. Enceladus' surface is composed mostly of nearly pure water ice except near its south pole, where there are light organics, CO2, and amorphous and crystalline water ice, particularly in the region dubbed the "tiger stripes." An upper limit of 5 precipitable nanometers is derived for CO in the atmospheric column above Enceladus, and 2% for NH3 in global surface deposits. Upper limits of 140 kelvin (for a filled pixel) are derived for the temperatures in the tiger stripes.

Anonymous

SO: Nature, vol. 441, pp. 614-616, 1 June 2006.

Enceladus is a small icy satellite of Saturn. Its south polar region consists of young, tectonically deformed terrain and has an anomalously high heat flux. This heat flux is probably due to localized tidal dissipation within either the ice shell or the underlying silicate core. The surface deformation is plausibly due to upwelling of low-density material (diapirism) as a result of this tidal heating. Here we show that the current polar location of the hotspot can be explained by reorientation of the satellite's rotation axis because of the presence of a low-density diapir. If the diapir is in the ice shell, then the shell must be relatively thick and maintain significant rigidity (elastic thickness greater than 60.5 km); if the diapir is in the silicate core, then Enceladus cannot possess a global subsurface ocean, because the core must be coupled to the overlying ice for reorientation to occur. The reorientation generates large (610 MPa) tectonic stress patterns that are compatible with the observed deformation of the south polar region. We predict that the distribution of impact craters on the surface will not show the usual leading hemisphere- trailing hemisphere asymmetry. A low-density diapir also yields a potentially observable negative gravity anomaly.

Anonymous

SO: Science (Washington), vol. 311, pp. 1391-1392, 10 Mar. 2006.

Instruments on the Cassini spacecraft reveal that a heat source within Saturn's moon Enceladus powers a great plume of water ice particles and dust grains, a geyser that jets outward from the south polar regions and most likely serves as the dominant source of Saturn's E ring. The interaction of flowing magnetospheric plasma with the plume modifies the particle and field environment of Enceladus. The structure of Saturn's magnetosphere, the extended region of space threaded by magnetic-field lines linked to the planet, is shaped by the ion source at Enceladus, and magnetospheric dynamics may be affected by the rate at which fresh ions are created.

Anonymous

SO: Science (Washington), vol. 311, pp. 1412-1415, 10 Mar. 2006.

The bombardment of Saturn's moon Enceladus by >20-kiloelectron volt magnetospheric particles causes particle flux depletions in regions magnetically connected to its orbit. Irrespective of magnetospheric activity, proton depletions are persistent, whereas electron depletions are quickly erased by magnetospheric processes. Observations of these signatures by Cassini's Magnetospheric Imaging Instrument allow remote monitoring of Enceladus' gas and dust environments. This reveals substantial outgassing variability at the moon and suggests increased dust concentrations at its Lagrange points. The characteristics of the particle depletions additionally provide key radial diffusion coefficients for energetic electrons and an independent measure of the inner magnetosphere's rotation velocity.

Anonymous

SO: Science (Washington), vol. 311, pp. 1422-1425, 10 Mar. 2006.

The Cassini spacecraft flew close to Saturn's small moon Enceladus three times in 2005. 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.

Anonymous

SO: Science (Washington), vol. 311, pp. 1389-1391, 10 Mar. 2006.

The exploration of Saturn by the Cassini/Huygens mission has yielded a rich collection of data about the planet and its rings and moons, in particular its small satellite Enceladus and giant satellite Titan. Once believed too small to be active, Enceladus has been found to be one of the most geologically dynamic objects in the solar system. Among the surprises are a watery, gaseous plume; a south polar hot spot; and a surface marked by deep canyons and thick flows.

Anonymous

SO: Science (Washington), vol. 311, pp. 1406-1409, 10 Mar. 2006.

The Cassini magnetometer has detected the interaction of the magnetospheric plasma of Saturn with an atmospheric plume at the icy moon Enceladus. This unanticipated finding, made on a distant flyby, was subsequently confirmed during two follow-on flybys, one very close to Enceladus. The magnetometer data are consistent with local outgassing activity via a plume from the surface of the moon near its south pole, as confirmed by other Cassini instruments.

Anonymous

SO: Science (Washington), vol. 311, pp. 1409-1412, 10 Mar. 2006.

During the 14 July 2005 encounter of Cassini with Enceladus, the Cassini Plasma Spectrometer measured strong deflections in the corotating ion flow, commencing at least 27 Enceladus radii (27 x 252.1 kilometers) from Enceladus. The Cassini Radio and Plasma Wave Science instrument inferred little plasma density increase near Enceladus. These data are consistent with ion formation via charge exchange and pickup by Saturn's magnetic field. The charge exchange occurs between neutrals in the Enceladus atmosphere and corotating ions in Saturn's inner magnetosphere. Pickup ions are observed near Enceladus, and a total mass loading rate of about 100 kilograms per second (3 x 1027 H2O molecules per second) is inferred.

Anonymous

SO: Comptes Rendus Physique, vol. 6, pp. 1176-1181, 2005.

The first pass of the Cassini probe in the vicinity of Saturn, above the E- ring, demonstrated a plasma consisting of water group ions (H+, O+, OH+, H2O+) with a small N+ ion component (3%). Using a simple model for the transport of magnetospheric ions, we show that the N+ ions can be traced back to the Enceladus satellite. Such a result can be explained by the existence in this icy satellite, supposed to be still geologically active, of volatile components such as ammonia NH3, or by the previous implantation of N+ ions of external origin on its surface. To cite this article: M. Bouhram et al., C. R. Physique 6 (2005).

Anonymous

SO: Geophysical Research Letters, vol. 32, pp. [vp], Oct. 2005.

We present evidence of two icy satellite microsignatures in the Cassini LEMMS data. Just upstream of Enceladus, a deep and narrow decrease in the flux of several MeV electrons is consistent with a recent absorption by that moon. This microsignature is collocated with a deep depletion in the MeV proton flux. The proton feature is much wider than the satellite diameter, suggesting multiple interactions and/or losses to the E Ring and neutral gas. An observed increase in proton flux toward the planet suggests a possible inner magnetospheric source. A decrease in the low energy electron intensity downstream of Tethys is also consistent with a microsignature approximately the size of the satellite that has drifted slightly toward the planet near midnight. Modeling suggests that microsignatures near Tethys' orbit would persist for more than a complete rotation of Saturn and the radial diffusion coefficient is about 10-9 RS 2/s.

J. P. Emery, D. M. Burr, D. P. Cruikshank, R. H. Brown and J. B. Dalton.

Astronomy & Astrophysics. Vol. 435, no. 1, pp. 353-362. 3 May 2005

Spectral measurements from the ground in the time leading up to the Cassini mission at Saturn provide important context for the interpretation of the forthcoming spacecraft data. Whereas ground-based observations cannot begin to approach the spatial scales Cassini will achieve, they do possess the benefits of better spectral resolution, a broader possible time baseline, and unique veiewing geometries not obtained by spacecraft (i.e., opposition). In this spirit, we present recent NIR reflectance spectra of four icy satellites of Saturn measured with the SpeX instrument at the IRTF. These measurements cover the range 0.8 to 4.0 microns of both the leading and trailing sides of Tethys and the leading side of Rhea. The L-band region (2.8 to 4.0 microns) offers new opportunities for searches of minor components on these objects. Additionally, these data include 0.8 to 2.5 micron spectra of both the leading and trailing sides of Mimas and of the (mostly) trailing side of Enceladus. The spectrum of Enceladus shows activity near 2.25 microns that we interpret as a possible signature of NH3 ice. The presence of ammonia in the Saturn system is not unexpected, and may help explain the apparent recent geologic activity of Enceladus. Analysis of leading/trailing differences in H2O band depths, spectral slopes, and albedo imply a separate regime of surface modification for Mimas and Enceladus than for the more distant icy satellites (Tethys, Dione, Rhea). Aside from the potential NH3 on Enceladus, no other minor constituents are detected in these icy surfaces.

Anonymous

SO: Lunar and Planetary Science XXXVI.

The Cassini Ultraviolet Imaging Spectrograph (UVIS) has collected ultraviolet observations of many of Saturn's icy moons since Cassini's insertion into orbit around Saturn. We will report on results from Enceladus, Tethys and Dione, orbiting in the Saturn system at distances of 3.95, 4.88 and 6.26 Saturn radii, resp. Icy satellite science objectives of the UVIS include investigations of surface age and evolution, surface composition and chemistry, and tenuous exospheres. We address these objectives by producing albedo maps, and reflection and emission spectra, and observing stellar occultations. UVIS has four channels: EUV: Extreme Ultraviolet (55 nm to 110 nm), FUV: Far Ultraviolet (110 to 190 nm), HSP: High Speed Photometer, and HDAC: Hydrogen-Deuterium Absorption Cell. The EUV and FUV spectrographs image onto a 2-dimensional detector, with 64 spatial rows by 1024 spectral columns. To-date we have focused primarily on the far ultraviolet data acquired with the low resolution slit width (4.8 A spectral resolution).

E. C. Sittler, R. E. Johnson and S. Jurac, et al.

Journal of Geophysical Research. A. Space Physics. Vol. 109, no. A1. Jan. 2004

Voyager images of the icy satellites of Saturn, Dione and Enceladus, suggest that they may have been geologically active and are not only composed of ice. Recent observations by the Hubble Space Telescope have shown the presence of ozone at both Dione and Rhea, which also implies the presence of molecular oxygen at these bodies. Observations of Ariel, Europa, Ganymede, and Callisto indicate the presence of CO2, so its presence on the Saturnian satellites is also expected. The Cassini Plasma Spectrometer (CAPS) will provide the capability to determine the global composition of these bodies by measuring the pickup ions produced by the ionization of their sputter-produced atmospheres. We will present a model of these atmospheres and associated pickup ions and demonstrate CAPS ability to distinguish the freshly produced picked up ions from the ambient plasma. Such ions are expected to form a ring distribution that will have a uniquely different energy-angle dependence than the ambient plasma ions. In the case of Dione we expect the potential for a moderate strength interaction for which both Voyager 1 and Pioneer 11 spacecraft measured ion cyclotron waves centered on the Dione L shell and near the equatorial plane. SKR radio emissions also displayed emissions occurring at the orbital period of Dione which could indicate some intrinsic activity due to Dione. So again, something interesting may be going on at Dione. Since Enceladus, or material in orbit near Enceladus, may be the source of the E-ring, some surprises may be encountered during its close encounter with the Cassini spacecraft. In the case of Dione we will show that a wake pass at 500 km altitude is more than an order of magnitude better than an upstream pass at 500 km altitude. Pickup ion detection for minor ion species such as NH3(+) is possible for 500 km altitude wake pass but not for 500 km altitude upstream pass at closest approach. For navigation reasons a 100 km pass is not allowed. Therefore it is essential to have a wake pass to maximize the science return for a targeted flyby with Dione. The CAPS observations when combined with magnetometer, plasma wave and energetic particle observations will allow us to estimate the source of ions into Saturn's magnetosphere due to these two bodies and to characterize the nature of the interaction with Saturn's magnetosphere.

A. Verbiscer, R. French and C. McGhee.

Solar System Remote Sensing; 01 Jan. 2002

The innermost of Saturn's classical satellites, Mimas and Enceladus, have historically proven challenging observing targets for Earth-based telescopes due to their proximity to the bright planet and rings. Since the ring plane crossings in 1995 and 1996, in preparation for the upcoming Cassini mission in 2004, the Hubble Space Telescope's Wide-Field Planetary Camera (WFPC2) has been monitoring the Saturn system at each opposition as well as near quadrature each year. This observing program has provided numerous images of Saturn's classical satellites in each of the five WFPC2 wideband UVBRI filters (F336W, F439W, F555W, F675W, and F814W) and occasionally in the F255W, F785LP, and F1042M filters at solar phase angles between 0.26 deg and 6.4 deg.

E. Sittler, R. E. Johnson and S. Jurac, et al.

Conference Proceedings, Solar System Remote Sensing; 01 Jan. 2002

Voyager images of the icy satellites of Saturn, Dione and Enceladus, suggest they have been geologically active and are not only composed of ice. Recent observations by HST have shown the presence of ozone at both Dione and Rhea which also implies the presence of molecular oxygen at these bodies. The Cassini Plasma Spectrometer (CAPS) will provide the capability to determine the global composition of these bodies by measuring the pickup ions produced by the ionization of their sputter produced atmospheres. We will present a model of these atmospheres and associated pickup ions and demonstrate CAPS ability to distinguish the freshly produced picked up ions from the ambient plasma. Such ions are expected to form a ring distribution that will have a uniquely different energy-angle dependence than the ambient plasma ions. In the case of Dione we expect the potential for a moderate strength interaction for which both Voyager 1 and Pioneer 11 spacecraft measured ion cyclotron waves centered on the Dione L shell and near the equatorial plane. Since Enceladus may be the source of the E-ring, some surprises may be encountered during its close encounter with the Cassini spacecraft. In the case of Dione we will show that a wake pass at 500 km altitude is more than an order of magnitude better than an upstream pass at 500 km altitude. Pickup ion detection for minor ion species such as NH3+ is possible for 500 km altitude wake pass but not for a 500 km altitude upstream pass at closest approach. For navigation reasons a 100 km pass is not allowed and therefore it is essential to have a wake pass to maximize the science return for a targeted flyby with Dione. The CAPS observations when combined with magnetometer, plasma wave and energetic particle observations will allow us to estimate the source of ions into Saturn's magnetosphere due to these two bodies and to characterize the nature of the interaction with Saturn's magnetosphere.

Wing-Huen Ip.

IEEE Transactions on Plasma Science. Vol. 29, pt. 1, no. 2, pp. 271-273. Apr. 2001

The physical consequence of the collisional interaction of the Saturnian E-ring particles with the icy satellite Enceladus is examined. It is suggested that Enceladus should be surrounded by an extended halo of dust particles. In association with the generation of impact vapor clouds, an abundance of tiny smoke particles of 10-100 A diameter might be created and subsequently picked up by the corotating magnetosphere. The Saturnian system could therefore be immersed in a disc of fine dust particles. The E-ring particle impact process is also important in providing a mechanism for surface mass transport and removal at Enceladus. For example, because of impact, erosion craters of diameters less than 2 km could be significantly eroded in regions where the surface age is only 100 million years old. (Author)

Frank Spahn, Kai-Uwe Thiessenhusen, Joshua E. Colwell, Ralf Srama and Eberhard Gruen.

Journal of Geophysical Research. Vol. 104, no. E10, pp. 24, 111-24, 120. 25 Oct. 1999

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 depend 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. (Author)

Paul M. Schenk and Jeffrey M. Moore.

Journal of Geophysical Research. Vol. 100, no. E9, pp. 19, 009-19, 022. 25 Sept. 1995

The morphology of volcanic features on Ganymede differs significantly from that on the terrestrial planets. Few if any major volcanic landforms, such as thick flows or shield volcanoes, have been identified to date. Using new stereo Voyager images, we have searched Ganymede for relief-generating volcanic constructs. We observed seven major types of volcanic structures, including several not previously recognized. The oldest are broad flat-topped domes partially filling many older craters in dark terrain. Similar domes occur on Enceladus. Together with smooth dark deposits, these domes indicate that the volcanic history of the dark terrain is complex. Bright terrain covers vast areas; smooth bright materials embay and flood older terrains, and may have been emplaced as low-viscosity fluids. Associated with smooth bright material are a number of scalloped-shaped, semienclosed scarps that cut into preexisting terrain. In planform, these structures resemble terrestrial calderas. The youngest volcanic materials identified are a series of small flows that may have flooded the floor of the multiring impact structure Gilgamesh, forming a broad dome. The identification of volcanic constructs up to 1 km thick is the first evidence for extrusion of moderate-to-high viscosity material on Ganymede. (Author)

Stanley F. Dermott and Peter C. Thomas.

Icarus. Vol. 109, no. 2, pp. 241-257. June

If the mass of a satellite is unknown, but the satellite is differentiated and has a deep mantle of known composition, shape measurements alone can facilitate the determination of its mass, mean density, and moment of inertia. Application of this method to Encedalus, assuming a deep mantle of water ice, yields a satellite mean density of 1.00 +/- 0.03 g/cu cm. (AIAA)

RUDOLF DVORAK and MARTIN KARCH.

Journal of Geophysical Research. Vol. 100, no. E9, pp. 19, 009-19, 022. 25 Sept. 1995, In German

The motion of Enceladus and Dione, two of Saturn's satellites whose orbits are in resonance, is studied using the SOS method. The phase curves in phase space are examined, and the consequences of the findings for the stability of celestial mechanical systems is discussed. In particular, it is shown that only the simplest systems can avoid chaos. An integration program for calculating the motion of Enceladus is presented. (C.D.)

BONNIEJ BURATTI and Jet Propulsion Lab., California Inst. of Tech., Pasadena.

Icarus. Vol. 75, pp. 113-126. July 1988

Voyager 1 and 2 images are used to ascertain the photometric properties of Enceladus up to a 43-deg phase angle. The lack of correlation noted between spectrophotometric properties and terrain type indicates that optical characteristics result from a recently-deposited, ubiquitous surface layer that may have originated on the E-ring of Saturn. Enceladus' distinctive phase and photometric functions can be accounted for by the high degree of surface multiple scattering; observations to date indicate that the Enceladus regolith's textural characteristics are similar to those of other icy satellites of Saturn. (O.C.)

M. KARCH and R. DVORAK.

SO: Celestial Mechanics. Vol. 43, no. 1-4, pp. 361-369. 1987-1988

New results are shown for the possible long term evolution of the orbit of Enceladus perturbed by Dione in the 2:1 resonance in the planar elliptic restricted problem. The numerical integrations of the secular variation equations (Ferraz-Mello and Dvorak, 1986) yield different types of chaotic behavior. The onset of chaos for this dynamical system is also studied. Additionally, some illustrative pictures of the whole phase trajectory show the complex structure of phase space. (Author)

JOYCEDASILVA BEVILACQUA and WAGNER SESSIN.

Revista Mexicana de Astronomia y Astrofisica, Vol. 14, May 1987, pp. 627-630

A new orbit for the pair of Saturn's satellites Enceladus and Dione, is constructed based on the intermediate solution obtained by Salgado and Sessin (1985). Secular terms are aggregated to the Hamiltonian when all first order perturbations are taken into account. Consequently, the periods of the new orbit generated by this Hamiltonian are better determined. (Author)

B. BURATTI, J. VEVERKA and Cornell Univ., Ithaca, NY.

Washington Repts. of Planetary Geol. Program, 1984, pp. 23-25

Voyager imaging observations of the satellites of Jupiter and Saturn provide an excellent test for various photometric theories that were proposed to describe the scattering properties of planetary and satellite surfaces. Not only does the Voyager data set include observations of surfaces ranging widely in albedo, but it provides measurements (in both disc-integrated and disc-resolved forms) over a wide range of phase angles. A detailed comparison of the above models with Voyager data for Europa, Enceladus, Rhea, and Mimas was described. These satellites were selected because they cover a range of reflectances from 0.65 to 1.0 and because for them the Voyager photometric data sets are most complete.

K. D. PANG, C. C. VOGE, J. W. RHOADS, J. M. AJELLO and Jet Propulsion Lab., California Inst. of Tech., Pasadena.

Journal of Geophysical Research. Vol. 89, pp. 9459-9470. 10 Oct. 1984.

Photographic imagery from Pioneer and Voyager flyby satellites has been used to study the microphysical properties of Saturn's outermost E-ring, and to determine the physical relationship between the E-ring and the satellite Enceladus. The optical and infrared characteristics of the E-ring are explained in terms of Mie scattering of ice spheres with an effective diameter of 2 to 2.25 microns and an effective variance of 0.1 to 1.5. It is suggested that the E-ring is continuously replenished by volcanic eruptions on Enceladus, and recent tectonic evidence is cited in support of this hypothesis. A number of similarities in the relationship between the E-ring and Enceladus and Io and its torus are discussed, within the framework of a general model of outer solar system volcanism and planetary ring interaction.

J. J. LISSAUER, S. J. PEALE, J. N. CUZZI and National Aeronautics and Space Administration. Ames Research Center, Moffett Field, CA.

Icarus. Vol. 58, pp. 159-168. May 1984

The absence of craters noted on Voyager 2 images of Encedalus indicates geologically recent resurfacing, probably due to internal melting; heating mechanism calculations, however, yield heating rates too small to cause melting. If Janus, whose orbital mean motion is currently decreasing as Janus' orbit evolves outward due to resonant torques from Saturn's rings, were ever to become locked into a stable 2:1 orbital commensurability with Encedalus, the resulting angular momentum transfer could have sufficiently enhanced the eccentricity of Encedalus' orbit for the ensuing tidal heating to have melted Encedalus' interior. However, the predicted rapid time scale for ring evolution due to resonant torques from Saturn's inner moons remains a major problem.

B. BURATTI, J. VEVERKA and Jet Propulsion Lab., California Inst. of Tech., Pasadena.

Icarus. Vol. 58, pp. 254-264. May 1984

Voyager imaging observations provide new photometric data on Saturn's satellites at large phase angles (up to 133 deg in the case of Mimas) not observable from earth. Significant new results include the determination of phase integrals ranging from 0.7 in the case of Rhea to 0.9 for Enceladus. For Enceladus an average geometric albedo 1.04 and a Bond albedo of 0.9 are found. The data indicate an orbital lightcurve with an amplitude of 0.2 mag, the trailing side being the brighter. For Mimas, the lightcurve amplitude is probably less than 0.1 mag. The value of the geometric albedo of Mimas reported here, is definitely higher than the currently accepted value of about 0.5. For Dione, the Voyager data show a well-defined orbital lightcurve of amplitude about 0.6 mag, with the leading hemisphere brighter than the trailing one.

M. E. DAVIES, F. Y. KATAYAMA and RAND Corp., Santa Monica, CA.

Icarus. Vol. 53, pp. 332-340. Feb. 1983

A bundle-type analytical triangulation program is employed to compute control networks for Mimas, whose network encircles the satellite with 110 points measured on 32 Voyager 1 pictures, and Enceladus, whose network does not completely encircle the satellite and contains 71 points measured on 22 Voyager 2 pictures. Many of the control points are identified on illustrations and by name, and their coordinates are presented in tabular form. The analytical triangulation program was used to solve for the mean radii and three principal axes of best-fit ellipsoids. The mean radius of Mimas is 197 + or - 3 km, while that of Enceladus is 251 + or - 5 km.

J. B. PLESCIA, J. M. BOYCE and Jet Propulsion Lab., California Inst. of Tech., Pasadena.

Nature. Vol. 301, pp. 666-670. 24 Feb. 1983

The surfaces of the Saturn satellites Tethys, Iapetus and Encedalus display surfaces which indicate active geological processes and therefore suggest a degree of internal evolution. By contrast, the Saturn satellite Hyperion and the coorbitals 1980S1 and 1980S3 show no trace of geological activity and may be fragments of once-larger bodies. Activity on Iapetus appears to have been confined to the dark terrain, and offers no clue as to its timing and extent. The widest terrain type and crater number variations are those of Encedalus, which indicate the most prolonged period of geological activity of any of the satellites studied.

S. W. SQUYRES, R. T. REYNOLDS, P. M. CASSEN, S. J. PEALE and National Aeronautics and Space Administration. Ames Research Center, Moffett Field, CA.

Icarus. Vol. 53, pp. 319-331. Feb. 1983 .

Evidence is adduced for several episodes of geologic resurfacing and extensional tectonism spreading over much of the history of the small, icy Saturn moon Enceladus. Resurfacing was the product of fresh material eruptions that may have contained ammonia, which may also have made melting in the interior more likely. Tidal dissipation seems to be the only heating mechanism capable of melting Enceladus. For the thermal properties of pure H2O, the orbital eccentricity would have to be higher than the present value of 0.0044 by a factor of 5-7 in order to maintain a molten interior, and may have to be greater by a factor of 20 in order to cause melting in an initially frozen body. Removal of eccentricity forcing would result in rapid eccentricity damping, freezing, and the cessation of tectonic activity.

P. K. HAFF, G. L. SISCOE, A. EVIATAR and California Inst. of Tech., Pasadena.

Icarus. Vol. 56, pp. 426-438. Dec. 1983 .

The E ring associated with the Kronian moon Enceladus has a lifetime of only a few thousand years against sputtering by slow corotating O ions. The existence of the ring implies the necessity for a continuous supply of matter. Possible particle source mechanisms on Enceladus include meteoroidal impact ejection and geysering. Estimates of ejection rates of particulate debris following small meteoroid impact are on the order of 3 x 10 to the -18th g/(sq cm sec), more than an order of magnitude too small to sustain the ring. A geyser source would need to generate a droplet supply at a rate of approximately 10 to the -16th g/(sq cm sec) in order to account for a stable ring. Enceladus and the ring particles also directly supply both plasma and vapor to space via sputtering. The absence of a 60 eV plasma at the Voyager 2 Enceladus L-shell crossing, such as might have been expected from sputtering, cannot be explained by absorption and moderation of plasma ions by ring particles, because the ring is too diffuse. Evidently, the effective sputtering yield in the vicinity of Enceladus is on the order of, or smaller than, 0.4, about an order of magnitude less than te calculated value. Small scale surface roughness may account for some of this discrepancy.

J. P. POIRIER, L. BOLOH and P. CHAMBON.

Icarus. Vol. 55, pp. 218-230. Aug. 1983.

Tidal dissipation is investigated in a viscoelastic homogeneous sphere having the orbital and physical characteristics of the icy inner satellite of Saturn, Enceladus. The dissipated power is calculated for Kelvin-Voigt and Maxwell rheologies, whose dissipation function can be expressed in terms of viscosity. Expressions for the dissipated power as a function of viscosity is calculated in both cases and compared to the expression found for a lossy elastic body. A physical law relating viscosity of water ice to temperature and grain size is introduced and the feedback between dissipated power and temperature is investigated. It is found that tidal dissipation with current orbital eccentricity alone canot account for the surface activity observed on Enceladus, if it is composed of water ice.

Q. R. PASSEY and California Inst. of Tech., Pasadena.

Icarus. Vol. 53, pp. 105-120. Jan. 1983

Regions of the Enceladus surface are shown by high resolution Voyager II images to be highly cratered, as if by heavy bombardment, with crater forms similar to those of fresh lunar surfaces but often shallower in depth. The flattening of these craters and the bowing up of their floors indicate viscous relaxation of the topography. Viscosity at the top of the lithosphere is suggested by crater form analysis to lie between 10 to the 24th and 10 to the 25th P. The zones where flattened craters occur may be regions of past or present heat flow that is higher than in adjacent terrains. Encedalus probably has a mixture of ammonia ice and water ice in the lithosphere, while the lithospheres of Ganymede and Callisto are primarily composed of water ice

O. G. FRANZ and R. L. MILLIS.

Icarus. Vol. 24, pp. 433-442. Apr. 1975

UBV measurements of Dione, Tethys, and Enceladus were made with an area-scanning photometer on several nights during the 1972/73 and 1973/74 apparitions of Saturn. The observed brightness variations have been separated into two components - one a function of orbital position, the other a function of solar phase angle. Dione and Tethys are brightest near greatest eastern elongation and faintest near greatest western elongation. The reverse is true of Enceladus. Opposition surges are observed for Dione and Tethys.

W. H. JEFFERYS, L. M. RIES and Austin Texas Univ.

American Astronautical Society and American Institute of Aeronautics and Astronautics, Astrodynamics Specialist Conference, Nassau, Bahamas; United States; 28-30 July 1975

The orbits of the satellites of the outer planets are poorly known, due to lack of attention over the past half century. We have been developing a new theory of Saturn's satellites Enceladus and Dione which is literal (all constants of integration appear explicitly), canonically invariant (the Hori-Lie method is used), and which correctly handles the eccentricity-type resonance between the two satellites. The algebraic manipulations are being performed using the TRIGMAN formula manipulation language, and the programs have been developed so that with minor modifications they can be used on the Mimas-Tethys and Titan-Hyperion systems.

W. H. JEFFERYS, L. M. RIES and Austin Texas Univ.

Astronomical Journal. Vol. 80, pp. 876-884. Oct. 1975

A new theory of the motion of Enceladus and Dione is under development. The theory is in literal form, with all constants and parameters appearing explicitly, and is being computed with the aid of the algebraic manipulation system TRIGMAN. The development of the disturbing function; the elimination of the short-period terms; and the resonant Hamiltonian and the constants of integration are discussed. An important feature is that the programs have been written in such a manner that, with only minor modification, they should be able to develop the theories of Mimas-Tethys and Titan-Hyperion as well.

T. NAKAMURA.

Astronomical Society of Japan, Publications. Vol. 26, no. 4, pp. 479-484. 1974 .

W. H. JEFFERYS, L. M. RIES and Texas Univ., Austin. Dept. of Astronomy.

NASA-CR-142980; Pagination 12P, 1974

A theory of Saturn's satellites Enceladus and Dione is discussed which is literal (all constants of integration appear explicitly), canonically invariant (the Hori-Lie method is used), and which correctly handles the eccentricity-type resonance between the two satellites. Algebraic manipulations are designed to be performed using the TRIGMAN formula manipulation language, and computer programs were developed so that, with minor modifications, they can be used on the Mimas-Tethys and Titan-Hyperion systems.

M. RAPAPORT.

Astronomy and Astrophysics. Vol. 22, no. 2, pp. 179-186. Jan. 1973

The satellite pairs Mimas-Tethys and Enceladus-Dione are considered in order to determine certain characteristics affecting their dynamic study. A Hamiltonian formulation of the equations of motion of two interacting satellites within the gravitational potential of Saturn was desired. The von Zeipel-Hori method makes it possible to solve the equations of motion in a systematic way. By making use of the resolution technique used by Struve (1933), a complementary term is found which improves the comparison which he made between theory and observation.

A. Verbiscer, R. French and C. McGhee.

Solar System Remote Sensing; Pittsburgh, PA; Sept. 20-21, 2002

The authors have produced mutually consistent solar phase and rotation curves for Mimas, Enceladus, Tethys, Dione, and Rhea and report the first observation of the steep, narrow opposition effects of Mimas and Enceladus at visible wavelengths, necessitating an increase in currently accepted values of their geometric albedos. Among these satellites, only Rhea was known to have a strong opposition effect in the visible, and 1995 ring plane crossing observations revealed Enceladus' large opposition surge in the near-infrared. Combining these new observations with existing Voyager and telescopic data enables a more precise determination of the physical characteristics of regolith particles comprising the surfaces of these satellites. Application of the Hapke photometric equation to phase curves which include near opposition data yields information on the directional scattering behavior of surface particles, regolith compaction, and the degree to which coherent backscatter plays a role in the scattering of light from the particulate surface of an icy satellite. Comparisons between the derived photometric parameters for each satellite suggest the degree to which location - either interior (Mimas and Enceladus) or exterior (Tethys, Dione, and Rhea) to the densest point in the E-ring - affects the physical properties of regolith particles. Mimas and Enceladus have brighter trailing hemispheres, while Tethys, Dione, and Rhea display the opposite hemispherical albedo dichotomy. (Abstract only).