Inner Satellites Of Saturn Research Articles

Astrometry of Cassini ISS Images of 7 Near-ring Inner Satellites of Saturn Affected by Scattered Light

The Imaging Science Subsystem (ISS) mounted on Cassini spacecraft took images of Saturn’s inner satellites between 2004 and 2017. In some of these images, the satellites are so close to Saturn rings that their images are affected by the scattered light from Saturn rings, which results in poor even impossible measurements of targets. A background removal algorithm is proposed to measure such images. A total of 70 ISS images of seven inner satellites near Saturn rings (Janus, Epimetheus, Atlas, Prometheus, Pandora, Methone, and Anthe) have been reduced by using the proposed method. Compared with the results from the method without removing the background affected by scattered light, the proposed method demonstrates an improvement in accuracy of at least 43%. The final results show that the residuals of these measurements relative to the JPL (Jet Propulsion Laboratory) ephemeris SAT415 have means of about 0.72 km and 2.26 km in right ascension and declination, with standard deviations of 10.99 km and 11.36 km, respectively.

The small inner satellites of Saturn: Shapes, structures and some implications

Properties of the small inner satellites of Saturn are updated using final Cassini image data. The equatorial ridges on Pan and Atlas are morphologically, structurally, and gravitationally distinct from their cores and represent distinct additions to, rather than modifications of, the precursor bodies. Exposures of lineated substrates on Pandora and Epimetheus show a relationship between regolith grooves and structures in underlying bedrock. Structures exposed on Epimetheus suggest formation at depth including filling or annealing of fractures. These structures, and sharp bedrock-regolith boundaries indicate a history far more complex than simple rubble-pile assembly. These exposures of bedrock also indicate past removal of the expected regolith from large fractions of these object's surfaces late in their cratering records.

Energetic charged particle weathering of Saturn's inner satellites

We characterize the relative importance of energetic electrons and protons to the weathering of five of the inner satellites of Saturn. To do this, we present data from the Magnetospheric Imaging Instrument on the Cassini spacecraft, some of which is averaged over the whole mission to date. We also compute averaged proton and electron energy spectra relevant to the distances of these inner satellites. Where data are available, we estimate the power per unit area into a satellite's surface. For electron energy deposition into satellite leading hemispheres, we find the power per unit area is greatest at Mimas and falls off with distance from Saturn. Using fluxes of 1–50MeV protons detected within the sweeping corridors of Mimas and Enceladus, we find the corresponding deposition would be about 2×108 and 3.7×107eV/cm2s.

Cassini spectra and photometry 0.25–5.1 μm of the small inner satellites of Saturn

The nominal tour of the Cassini mission enabled the first spectra and solar phase curves of the small inner satellites of Saturn. We present spectra from the Visual Infrared Mapping Spectrometer (VIMS) and the Imaging Science Subsystem (ISS) that span the 0.25–5.1 μm spectral range. The composition of Atlas, Pandora, Janus, Epimetheus, Calypso, and Telesto is primarily water ice, with a small amount (∼5%) of contaminant, which most likely consists of hydrocarbons. The optical properties of the “shepherd” satellites and the coorbitals are tied to the A-ring, while those of the Tethys Lagrangians are tied to the E-ring of Saturn. The color of the satellites becomes progressively bluer with distance from Saturn, presumably from the increased influence of the E-ring; Telesto is as blue as Enceladus. Janus and Epimetheus have very similar spectra, although the latter appears to have a thicker coating of ring material. For at least four of the satellites, we find evidence for the spectral line at 0.68 μm that Vilas et al. [Vilas, F., Larsen, S.M., Stockstill, K.R., Gaffley, M.J., 1996. Icarus 124, 262–267] attributed to hydrated iron minerals on Iapetus and Hyperion. However, it is difficult to produce a spectral mixing model that includes this component. We find no evidence for CO 2 on any of the small satellites. There was a sufficient excursion in solar phase angle to create solar phase curves for Janus and Telesto. They bear a close similarity to the solar phase curves of the medium-sized inner icy satellites. Preliminary spectral modeling suggests that the contaminant on these bodies is not the same as the exogenously placed low-albedo material on Iapetus, but is rather a native material. The lack of CO 2 on the small inner satellites also suggests that their low-albedo material is distinct from that on Iapetus, Phoebe, and Hyperion.

The fate of ejecta from Hyperion

Ejecta from Saturn's moon Hyperion are subject to powerful perturbations from nearby Titan, which control their ultimate fate. We have performed numerical integrations to simulate a simplified system consisting of Saturn (including optical flattening as well as dynamical oblateness), its main ring system (treated as a massless flat annulus), the moons Tethys, Dione, Titan, Hyperion, and Iapetus, and the Sun (treated simply as a massive satellite). At several different points in Hyperion's orbit, 1050 massless particles, more or less evenly distributed over latitude and longitude, were ejected radially outward from 1 km above Hyperion's mean radius at speeds 10% faster than escape speed from Hyperion. Most of these particles were removed within the first few thousand years, but ∼3% of them survived the entire 100,000-year duration of the simulations. Ejecta from Hyperion are much more widely scattered than previously thought, and can cross the orbits of all of Saturn's satellites. About 9% of all the particles escaped from the saturnian system, but Titan accreted ∼78% of the total, while Hyperion reaccreted only ∼5%. This low efficiency of reaccretion may help to account for Hyperion's small size and rugged shape. Only ∼1% of all the particles hit other satellites, and another ∼1% impacted Saturn itself, while ∼3% of them struck its main rings. The high proportion of impacts into Saturn's rings is surprising; these collisions show a broad decline in impact speed with time, suggesting that Hyperion ejecta gradually spread inwards. Additional simulations were used to investigate the dependence of ejecta evolution on launch speed, the mass of Hyperion, and the presence of the Sun. In general, the wide distribution of ejecta from Hyperion suggests that it does contribute to “Population II” craters on the inner satellites of Saturn. Ejecta which escape from a satellite into temporary orbit about its planet, but later reimpact into the same moon or another one produce “poltorary” impacts, intermediate in character between primary and secondary impacts. It may be possible to distinguish poltorary craters from primary and secondary craters on the basis of morphology.

The color of the Uranian rings

Observations of the Uranian rings were made in several color filters by the Voyager Imaging Science experiment in January 1986 for the purpose of determining the color of the rings. Selected images were taken through the Violet ( λ = 0.41 μm), Clear ( λ = 0.48 μm), and Green ( λ = 0.55 μm) filters of the Voyager 2 narrow angle camera. The results of the analysis are consistent with the α, β, η, γ, δ, and ϵ rings being very dark, with flat spectra throughout the visible, and are comparable to the latest Voyager results showing a lack of color for the Uranian satellites. The general lack of color in the ring/satellite system of Uranus is remarkably different than the case of the distinctly reddish systems of Jupiter and Saturn. The unique combination of low absolute reflectivity and flat spectrum which characterizes the Uranian rings supports the concept that the Uranian ring material is compositionally distinct from either the Si- and S-rich Jovian ring and inner satellites, or the water-ice-rich rings and inner satellites of Saturn. Of all cosmically abundant materials, the candidate which best matches the low brightness and flat spectrum of the Uranian rings is carbon.

Limits on the extent of Saturn's hydrogen cloud

The 30‐day reports from the Voyager Ultraviolet Spectrometer team characterize the atomic hydrogen observed near Saturn as a torus with a half width of 7 Rs extending from about 8 to 25 Rs. The source of this torus was hypothesized to be escape of hydrogen from Titan's atmosphere. Several recent papers propose that the source is actually Saturn's exosphere and that the hydrogen cloud extends inwards to the planet. We model the atomic processes occurring in the inner magnetosphere, including sputtering, ionization, charge exchange, ion‐atom interchange, recombination, and transport, and add a neutral hydrogen source to test the proposal that a hydrogen cloud exists in this region. Plasma observations in the inner magnetosphere are found to be inconsistent with the presence of a dense (20 cm−3) atomic hydrogen cloud. The presence of such a cloud in the observed plasma environment would result in a much larger proton density than observed and would remove nearly all heavy ions from the magnetosphere. Thus we conclude that the hydrogen cloud must be a torus as originally reported, and that the cloud does not extend inward to the orbits of the inner satellites of Saturn.

Satellite tori at Saturn

The inner satellites of Saturn are icy bodies imbedded in a plasma environment in which they are continuously bombarded by energetic ions, corotating plasma, and solar radiation. Laboratory sputtering experiments indicate that this should result in the injection of substantial amounts of neutral H, H2, OH, H2O, and O2 into the magnetosphere. We model the atomic processes affecting these neutrals and the neutrals and ions formed from them and calculate the steady state neutral and ion densities expected in the plasma tori of Enceladus, Dione‐Tethys, and Rhea. Comparison with observations shows that recombination can limit the Enceladus and Dione‐Tethys tori to the observed densities but that transport rates of at least 4×10−8 Rs²/s are required to limit torus densities at Rhea to the observed values.

General characteristics of hot plasma and energetic particles in the Saturnian magnetosphere: Results from the Voyager spacecraft

The low energy charged particle (LECP) experiment on the Voyager 1 and 2 spacecraft made measurements of the intensity, energy spectra, and spatial distributions of ions (30 keV ≲ E ≲ 150 MeV) and electrons (22 keV ≲ E ≲ 20 MeV) during encounters with the Saturnian magnetosphere in November 1980 and August 1981, respectively. Detailed analysis of the data has revealed the following: (1) Energetic ions are present in the interplanetary medium both upstream (to ∼200 Rs) and off the dawn bow shock (to ∼400 Rs) of the magnetosphere, with maximum energies ∼100 keV. (2) Low‐energy (≳22 keV) electrons are generally depleted inward of L ∼ 10 Rs, while low‐energy (≳30 keV) ions are greatly enhanced in the same region. (3) The composition of low‐energy ions is most likely dominated by protons in the outer magnetosphere but is consistent with oxygen in the inner (L ≲ 9) magnetosphere. (4) The ion spectrum is described well by the κ distribution with characteristic temperatures kTH ranging from ∼15 to ∼55 keV; the hot plasma region is generally confined between the L shells of Tethys and Rhea but exhibits substantial variability. (5) The electron energy spectrum at L ≲ 10 develops a secondary peak at E ≳ 200 keV that shifts to higher (∼1 MeV) energies inside the orbits of Enceladus and Mimas, indicative of electron resonance interactions with the planetary satellites. (6) There is a noon‐dawn asymmetry in ion and electron intensities with peak fluxes near the Rhea‐Dione L shells at local morning; this is the region in local time where Saturn kilometric radiation is modulated by the presence of Dione. (7) The ion energy density (≳30 keV) represents a significant fraction of the field energy density in the outer magnetosphere of the planet (L ≳ 13 Rs), with values of β ranging from 0.1 up to ∼4, when projected to the equator. (8) Comparison of electron and ion intensities measured by Voyagers 1 and 2 in the inner (L ≲ 6) magnetosphere at common points in B, L space shows that the radiation belts are substantially stable over periods of ∼9 months; both ion and electron intensities compared well with Pioneer 11 observations in 1979. It is evident from the results that the inner satellites of Saturn play a dominant role in the determination of intensity and spectral features of energetic particles at L ≲ 10. These aspects of the data are discussed in the context of proposed physical mechanisms expected to be operating within the magnetosphere of Saturn.

Findings on rings and inner satellites of Saturn by pioneer 11

The introductory part of this paper gives a short account of the theory of absorption by planetary rings and satellites of energetic charged particles that are trapped in a planet's magnetic field and describes the observable consequences of such absorption processes. The previously published University of Iowa observations of absorption features during Pioneer 11's passage through Saturn's inner radiation belt on 1 September 1979 are critically reanalyzed and related to other evidence on rings and satellites inside 2.9 R s, especially that from the imaging system on Voyagers 1 and 2. It is found (a) that satellites 1979 S1, 1979 S2, and 1980 S3 are almost certainly identical; (b) that the evidence for 1979 S4 is weak to nonexistent; (c) that 1979 S5 and two other nearby absorption features and 1979 S6 and one other nearby absorption feature are probably caused by longitudinal and radial structure of Ring F and not by satellites; and (d) that absorption feature 1979 S3 at 169,200 ± 600 km is identified with the opically observed Ring G.

Motion of trapped electrons and protons in Saturn's inner magnetosphere

A summary is given of basic formulas for the guiding center motion of energetic charged particles trapped in a dipolar magnetic field. These formulas for longitudinal drift rates, latitudinal bounce periods, equatorial gyroradii, and equatorial gyroperiods are then stated in convenient numerical form for electrons and protons as functions of kinetic energy E, magnetic shell parameter L, and equatorial pitch angle α0 for a slightly simplified model of the observed magnetic field of Saturn. To aid in the study of the interaction of charged particles with the rings and inner satellites of Saturn, additional formulas are given for the time interval between successive encounters of charged particles with a satellite in a circular prograde orbit and for the energy of electrons whose longitudinal angular velocity is resonant, or synchronous, with the Keplerian angular velocity of such a satellite.

Review of the Theories of Motion of the Natural Satellites

Most of the natural satellites of the planets of the solar system may be put into one of three main groups, according as to which of three main influences dominate the perturbation of their motion from Keplerian motion about the primary planet. The first of these is the attraction of the Sun, which governs the perturbations of the Moon's motion about the Earth, and those of the outer satellites of Jupiter (satellites VI to XIII), and Saturn's satellite Phoebe. The second is the departure of the gravitational field of the planet from that of a spherically symmetric body (the “figure terms”), and this governs the perturbations of the two satellites of Mars, Jupiter's satellite Amalthea (V), Neptune's satellite Triton, is probably the most important influence on Uranus' satellites, and is important, though not dominant, for the inner satellites of Saturn. The third influence is the mutual attraction of the satellites themselves. An order of magnitude argument suggests that periodic perturbations from this cause could scarcely be expected to be measureable from Earth, were it not that the frequent appearance of small-integer near-commensurabilities of pairs of orbital periods, and the consequent argumentation of the associated perturbations by a variety of types of resonance effects, in the systems of Jupiter and Saturn, causes mutual perturbations to dominate the orbital theories of three of the four great satellites of Jupiter, and six of the nine satellites of Saturn, and enables the masses of most of the satellites involved to be determined with otherwise unexpected relative precision (in some favourable cases, of the order of one per-cent) from Earth based data. Let us now consider the satellite systems of each of the outer planets in a little more detail.

Study of the inner satellites of Saturn by photographic photometry

Good photographs of Saturn and its five inner satellites were obtained on January 2, 1974 with the 105 cm telescope at Pic du Midi Observatory with exposure times of 45 sec. The spread function is constant over the field, and isotropic. The true photometric profiles of the satellites are obtained after deriving a model for the stray light coming from the rings. The magnitudes, computed by integration, are in good agreement with published values except for Mimas, which is nearly one magnitude fainter then previously believed.

Albedos and densities of the inner satellites of Saturn

Broad-band radiometry at 20 μm is presented of Rhea and Dione; the measured flux densities, together with visual photometry, indicate that both satellites have geometric albedos near 0.6 and that their radii are, respectively, 800 ± 125 and 575 ± 100 km. The density of Dione is 1.4 ± 0.6 gcm −3; for Tethys, Enceladus, and Mimas, whose densities have not been measured, I define a “photometric density” from the available data and show that their densities are probably near unity. These satellites must therefore all be composed primarily of ices.

The Origin of Commensurabilities in the Satellite Systems

The distribution of orbits in the satellite systems of the major planets is definitely nonrandom as there is a marked preference for commensurability among pairs of mean motions in these systems. If this preference is not the result of a formation process then it follows that since the time of satellite formation dynamical evolution has occurred. In this paper (a full version of which is to be submitted to another journal) I show that there is a bulk of evidence in favour of the hypothesis that orbital evolution has occurred as the result of tidal dissipation in the planets. This evidence can be listed as follows:(i) In a tidally evolved satellite system there should be a linear correlation between log (orbital radius) and log (satellite mass). The expected correlation is observed among the inner satellites of Saturn and Uranus, the slope of the log-log plots being consistent with an amplitude and frequency independent Q (the tidal dissipation function). The correlation in Saturn's system must be largely due to either a formation process or possibly an early amplitude and frequency dependent tidal process, but certainly the present satellite distribution is completely consistent with the tidal hypothesis.(ii) If Q is amplitude- and frequency-independent then the Mimas-Tethys and Enceladus-Dione resonances are stable under the action of tidal forces. This is an important result as it indicates that the necessary tidal dissipation occurred in the solid parts of the planets. Whether or not these planets have any solid parts at present is a matter of dispute.(iii) The tidal hypothesis can account for the formation (capture into libration) of the Mimas-Tethys and Enceladus-Dione resonances.(iv) If it is allowed that the age of the Mimas-Tethys resonance is not small (> 108 yr) in comparison with that of the solar system and that orbital evolution since the time of satellite formation has been appreciable then it can be shown that tidal forces (with Q amplitude- and frequency-independent) can account for the present large amplitude of libration of this resonance.(v) If tidal evolution in the satellite systems of Jupiter and Saturn has been appreciable then from the present orbits of Io and Mimas it can be deduced that Q (Jupiter) ~1.1 × 105 and Q (Saturn) ~1.2 × 105. As the masses of the two satellites differ by a factor ~2000 and the amplitudes of the tides they raise on their respective planets differ by a factor ~ 5000 this agreement in the Q-values is somewhat remarkable. But as the stability of the Mimas-Tethys and Enceladus-Dione resonances demands that Q is amplitude- and frequency-independent and as the chemical compositions and structures of Jupiter and Saturn are probably similar then one should expect the Q's of these planets to be similar and thus the result supports the tidal hypothesis.(vi) Finally, as the values of Q are so very high it would be remarkable if tidal evolution had not taken place.

Our Astronomical Column

THE DIAMETERS OF SATURN'S SATELLITES.—Major P. H. Hepburn contributes a paper on this subject to B.A.A. Journ. for March. The only one that is large enough to measure with a micrometer is Titan, for which Barnard and I -owell agreed in finding a diameter of some 2600 miles, say 0.7″ in arc. Major Levin found 3500 miles by the eclipse of Rhea by Titan, April 8, 1921; probably the truth lies between the two. For the other satellites estimates can be made (i) from the masses determined by H. Struve, combined with assumed densities, and (2) from the stellar magnitudes of the satellites determined by Guthnick and at Harvard, combined with assumed albedoes. In practice each method was found to help the other; it was established that all the smaller satellites must have small densities, not much greater than that of Saturn itself, which is £ of the earth's density, or 0.7 of water. In particular, Mimas must have both low density and high albedo, and Major Hepburn suggested, half seriously, that it might be a gigantic snowball; he had in a former paper suggested that the Ring might be composed of ice crystals. The values of the diameters considered most probable are: Mimas, 300 miles; Enceladus, 450; Tethys, 700; Dione, 800; Rhea, 1000. Data are lacking for trustworthy estimates of Hyperion and lapetus. As Jupiter's satellites have also low densities, some colour is given to the old suggestion that the outer parts of the solar system are built of less dense materials than the inner parts. It will be remembered that the inner satellites of Saturn are much the smallest bodies the masses of which have been determined gravitationally, hence determination of their densities throws new light on our knowledge of that of small bodies generally. Bodies that have never been in a molten state might well have interstices between their particles which would be filled up in a molten condition.

Erratum [Observations of the seven inner satellites of Saturn

Erratum [Observations of the seven inner satellites of Saturn

Observations of the seven inner satellites of Saturn

Observations of the seven inner satellites of Saturn

Observations of the relative positions of the inner satellites of Saturn

Observations of the relative positions of the inner satellites of Saturn

Ephemerides of the Five Inner Satellites of Saturn, 1894

Ephemerides of the Five Inner Satellites of Saturn, 1894