Vicinity Of Saturn Research Articles

OLYMPIA-LILBID: A New Laboratory Setup to Calibrate Spaceborne Hypervelocity Ice Grain Detectors Using High-Resolution Mass Spectrometry

The coupling of an Orbitrap-based mass analyzer to the laser-induced liquid beam ion desorption (LILBID) technique has been investigated, with the aim to reproduce the mass spectra recorded by Cassini's Cosmic Dust Analyzer (CDA) in the vicinity of Saturn's icy moon Enceladus. LILBID setups are usually coupled with time-of-flight (TOF) mass analyzers, with a limited mass resolution (∼800 m/Δm). Thanks to the Orbitrap technology, we developed a unique analytical setup that is able to simulate hypervelocity ice grains' impact in the laboratory (at speeds in the range of 15-18 km/s) with an unprecedented high mass resolution of up to 150 000 m/Δm (at m/z 19 for a 500 ms signal duration). The results will be implemented in the LILBID database and will be useful for the calibration and future data interpretation of the Europa Clipper's SUrface Dust Analyzer (SUDA), which will characterize the habitability of Jupiter's icy moon Europa.

Generation of oblique electromagnetic wave by hot injection electron beam with parallel AC electric field in the magnetosphere of Saturn

The Cassini radio and plasma wave investigation is designed to study radio emissions, plasma waves, thermal plasma, and dust in the vicinity of Saturn. Plasma particles, waves and fields are the major factors on which present knowledge about Saturn’s magnetosphere is based from the three flyby missions. For the present work, our examination is focused to study those waves who have frequency lower than electron cyclotron frequency in Saturnian magnetosphere. In this paper, we investigate the growth rate of whistler mode waves in magneto-plasma of Saturn. A dispersion relation for parallel propagating whistler mode waves has been applied to the magnetosphere of Saturn. The effect of electron density, temperature anisotropy, number density and A.C frequency on the temporal growth rate of the whistler mode emission is studied. For the given magnetic field and external electric field configuration, the general dispersion relation has been derived.

Upper-hybrid wave-driven Alfvénic turbulence in magnetized dusty plasmas

The nonlinear dynamics of coupled electrostatic upper-hybrid (UH) and Alfvén waves (AWs) is revisited in a magnetized electron-ion plasma with charged dust impurities. A pair of nonlinear equations that describe the interaction of UH wave envelopes (including the relativistic electron mass increase) and the density as well as the compressional magnetic field perturbations associated with the AWs are solved numerically to show that many coherent solitary patterns can be excited and saturated due to modulational instability of unstable UH waves. The evolution of these solitary patterns is also shown to appear in the states of spatiotemporal coherence, temporal as well as spatiotemporal chaos, due to collision and fusion among the patterns in stochastic motion. Furthermore, these spatiotemporal features are demonstrated by the analysis of wavelet power spectra. It is found that a redistribution of wave energy takes place to higher harmonic modes with small wavelengths, which, in turn, results in the onset of Alfvénic turbulence in dusty magnetoplasmas. Such a scenario can occur in the vicinity of Saturn's magnetosphere as many electrostatic solitary structures have been observed there by the Cassini spacecraft.

Rayleigh Taylor instability in a dusty plasma

A non-local theory is developed to study the Rayleigh–Taylor instability in an inhomogeneous dusty plasma in the presence of a uniform magnetic field. Using the fluid equations and the dynamics of charged dust grains, an eigenmode equation is derived to study the stability of the mode. Typical space parameters in the vicinity of Saturn are used to investigate the stability of the mode. The growth rate of the Rayleigh–Taylor is found to be drastically higher in a dusty plasma than in a pure plasma. This might have a wider consequence in explaining the origin of low-frequency modes in various situations in the space atmosphere. The effects of the dust charge, and the dust density on the growth rate of the instability are also studied.

Electrostatic solitary structures observed at Saturn

We report observations of electric field solitary structures, measured by the Cassini Radio and Plasma Wave Science (RPWS) instrument in the vicinity of Saturn's magnetosphere with ambient magnetic fields that range from ∼0.1 nT to 8000 nT. The peak‐to‐peak electric field amplitudes of the observed solitary structures range from a few μV/m to 10 mV/m and show a slight trend toward larger amplitude electric field pulses being associated with larger ambient magnetic fields. The time durations of the pulses range from a few hundred μs to a few 10's of milliseconds. The solitary waves appear in plasma boundary regions or in regions with abrupt changes in the magnetic field. The solitary waves tend to be observed when the dipole antenna is aligned with the magnetic field.

A model of the ionosphere of Saturn's rings and its implications

The detection of cold O + and O + ions in the vicinity of Saturn’s rings during the Cassini Orbiter orbit insertion confirmed expectations that the rings would have a water product atmosphere and ionosphere. These observations prompted a new look at their origin and nature by Johnson et al. [Johnson, R.E., Luhmann, J.G., Tokar, R.L., Bouhram, M., Berthelier, J.J., Sittler, E.C., Cooper, J.F., Hill, T.W., Crary, F.J., Young, D.T., 2006. Icarus 180, 393–402], but also raised questions about the ionosphere’s spatial distribution and fate that inspired the ionospheric model described in this report. Here a test particle model with some Monte Carlo aspects is used to consider the behavior of the O + and O + ions produced in the atmosphere of Saturn’s rings. Key features of these calculations include the Johnson et al. description of the production of the ring atmosphere, and the effects of the offset dipole magnetic field of Saturn. The results suggest that the latter should produce some possibly observable asymmetries in both the inner ring ionosphere and the precipitation of ring ions into the atmosphere of Saturn. Further in situ observations of the rings are not currently planned, but remote sensing instruments on Cassini may provide future observational tests of the model.  2005 Elsevier Inc. All rights reserved.

Le satellite Encelade source d'ions N + dans la magnétosphère de Saturne

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 +, H 2O +) 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 NH 3, 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).

Cassini observations of the thermal plasma in the vicinity of Saturn's main rings and the F and G rings

The ion mass spectrometer on Cassini detected enhanced ion flux near Saturn's main rings that is consistent with the presence of atomic and molecular oxygen ions in the thermal plasma. The ring “atmosphere” and “ionosphere” are likely produced by UV photosputtering of the icy rings and subsequent photoionization of O2. The identification of the O+ and O2+ ions is made using time‐of‐flight analysis and densities and temperatures are derived from the ion counting data. The ion temperatures over the main rings are a minimum near synchronous orbit and increase with radial distance from Saturn as expected from ion pick up in Saturn's magnetic field. The O2+ temperatures provide an estimate of the neutral O2 temperature over the main rings. The ion mass spectrometer also detected significant O2+ outside of the main rings, near the F ring. It is concluded that between the F and G rings, the heavy ion population most likely consists of an admixture of O2+ and water group ions O+, OH+, and H2O+.

Oxygen Ions Observed Near Saturn's A Ring

Ions were detected in the vicinity of Saturn's A ring by the Ion and Neutral Mass Spectrometer (INMS) instrument onboard the Cassini Orbiter during the spacecraft's passage over the rings. The INMS saw signatures of molecular and atomic oxygen ions and of protons, thus demonstrating the existence of an ionosphere associated with the A ring. A likely explanation for these ions is photoionization by solar ultraviolet radiation of neutral O2 molecules associated with a tenuous ring atmosphere. INMS neutral measurements made during the ring encounter are dominated by a background signal.

The Cassini Radio and Plasma Wave Investigation

The Cassini radio and plasma wave investigation is designed to study radio emissions, plasma waves, thermal plasma, and dust in the vicinity of Saturn. Three nearly orthogonal electric field antennas are used to detect electric fields over a frequency range from 1 Hz to 16 MHz, and three orthogonal search coil magnetic antennas are used to detect magnetic fields over a frequency range from 1 Hz to 12 kHz. A Langmuir probe is used to measure the electron density and temperature. Signals from the electric and magnetic antennas are processed by five receiver systems: a high frequency receiver that covers the frequency range from 3.5 kHz to 16 MHz, a medium frequency receiver that covers the frequency range from 24 Hz to 12 kHz, a low frequency receiver that covers the frequency range from 1 Hz to 26 Hz, a five-channel waveform receiver that covers the frequency range from 1 Hz to 2.5 kHz in two bands, 1 Hz to 26 Hz and 3 Hz to 2.5 kHz, and a wideband receiver that has two frequency bands, 60 Hz to 10.5 kHz and 800 Hz to 75 kHz. In addition, a sounder transmitter can be used to stimulate plasma resonances over a frequency range from 3.6 kHz to 115.2 kHz. Fluxes of micron-sized dust particles can be counted and approximate masses of the dust particles can be determined using the same techniques as Voyager. Compared to Voyagers 1 and 2, which are the only spacecraft that have made radio and plasma wave measurements in the vicinity of Saturn, the Cassini radio and plasma wave instrument has several new capabilities. These include (1) greatly improved sensitivity and dynamic range, (2) the ability to perform direction-finding measurements of remotely generated radio emissions and wave normal measurements of plasma waves, (3) both active and passive measurements of plasma resonances in order to give precise measurements of the local electron density, and (4) Langmuir probe measurements of the local electron density and temperature. With these new capabilities, it will be possible to perform a broad range of studies of radio emissions, wave-particle interactions, thermal plasmas and dust in the vicinity of Saturn.

Statistical study of Saturnian electrostatic discharges observed by the Voyager planetary radio astronomy experiment

Abstract:During the two Voyager-Saturn missions, the Planetary Radio Astronomy experiment observed sporadic broadband (20 KHz to at least 40 MHz) emissions during a few days around the closest approaches.These emissions are interpreted to arise from short-lived electrostatic discharges in the vicinity of Saturn. Their characteristics have been studied for the experiment's high band (>1.2 MHz).A difference between the periodicity of these Saturn electrostatic discharges (SED) is found, when comparing Voyager 1 and 2 observations.

Statistical study of Saturn electrostatic discharges

During the Voyager Saturn mission the Planetary Radio Astronomy (PRA) experiment observed sporadic broadband emissions (20 KHz to at least 40 MHz) during a few days around the Voyager 1 and 2 closest approaches to the planet. These emissions are interpreted to arise from short‐lived electrostatic discharges in the vicinity of Saturn. We have made a statistical study of their characteristics from the Voyager 1 and 2 high‐band data of the PRA experiment (1.2 MHz to 40.2 MHz). In this paper, we compare the results obtained for the two encounters and describe in detail the appearance, intensity, and spectrum of the Saturn electrostatic discharges (SED). We show that the SED spectrum is likely smooth in the frequency range 1–40 MHz and extends down to 20 KHz. We have determined the period of the SED occurrence, which seems to be slightly different for Voyager 1 and 2 encounters (10 hours 9 min for the Voyager 1 encounter and 10 hours 00 min for the Voyager 2 encounter). An attempt to interpret the statistical results is presented.

Voyager 2 data on Saturn analyzed

Whereas the fundamentals of Saturn's rings remain a mystery, new results from data obtained last August when Voyager 2 encountered the vicinity of Saturn are beginning to offer a number of explanations. According to results discussed at the Saturn Conference held May 11–15 in Tucson and sponsored by the Lunar and Planetary Institute of Houston, there are a series of gravitationally stratified sequences of particles that orbit in resonance with Saturn's satellites (Science, June 11, 1982). Thus at least a few of the ring features may be caused by resonances originating at the satellites. An astronomy concept, the spiral density wave, has been postulated as particles orbit in inside rings and periodically come into identity and pass a satellite, suffering gravitational perturbation.

Charged particle sputtering of ice surfaces in Saturn's magnetosphere

Following recent investigations of the Saturn system in situ by Pioneer 11 and Voyager 1 and 2 and remotely by the International Ultraviolet Explorer, we have reconsidered the sputtering of water ice surfaces by magnetospheric ions as a possible source of hydrogen atoms and heavy ions. We use results from laboratory measurements of water ice sputtering and dissociation rates as a function of ice temperature. We also use Voyager charged particle data from the low energy charged particle experiment to obtain more accurate assessments of energetic ion fluxes and loss rates. Ion phase space densities show that, if charged particle sputtering is an important physical process, it will occur predominantly in the vicinity of Saturn's E ring and the moons Dione and Tethys, not at the outer edge of the A ring as previously suggested prior to the availability of in situ data. Charged particle sputtering of ice surfaces between 4.5 and 8 RS, followed by the ionization of H2O and dissociation fragments, may be an important local source of the heavy ion plasma reported in the magnetosphere by Pioneer 11. We review several existing theories for the ring atmosphere and conclude that difficulties remain with all of them.

X-ray and energetic neutral particle emission from Saturn's magnetosphere

A description is presented of observations of the Voyager 1 spacecraft obtained a few days before encounter of the Saturn magnetosphere in November 1980. As Voyager 1 has no special experiment onboard for X-ray and neutral particle detection, the sensitivity and directional measurements of the low energy charged particle (LECP) analyzer is used to identify the neutral radiation above the charged particle background. The LECP telescope uses a silicon detector of 96.6 micrometers thickness and 0.08 sq cm area to accumulate counts in eight separate sectors near the ecliptic plane. The existence of radiation above the detector background emanating from the vicinity of Saturn, upstream from the bow shock is shown. The excess counts are most plausibly attributed to neutral radiation, that is, X rays and/or neutral energetic particles. However, in connection with quantitative considerations the observed excess counts are probably not due to X-rays. It is concluded that charge exchange of energetic ions with satellite tori is an important loss mechanism at Saturn as well as at Jupiter.

Saturn's sweeper moons predicted

The recent observation of the absorption of radiation belts in the vicinity of Saturn's bright rings and historical observations of the ring system make the following related results apparent: - The gaps in the rings are caused by the presence of at least 6 small, extremely dense and probably electrically charged ‘sweeper’ moons which effectively sweep the ring matter clean from the gaps. This is known due to the fading of the inner ring edges whereas the outer edges are well defined. Their orbital periods will differ from the expected Keplerian periods if the moons and Saturn do possess electric fields. - Absorption of radiation belts near the rings (of Jupiter also) implies that the ring particles themselves are not absorbing the radiation but the small moons are. This is consistent with the observed radiation belt absorption near the outer Saturnian moons. - If electric fields of the sweeper moons cause the ring edge fading as observed (and not simply gravitational), then Saturn itself must maintain an electric field in its vicinity by way of a sizeable proton wind to affect the uneven ring edge fading and will be surrounded by an H+ cloud at least to approximately the A-ring. this is consistent with the detection of an H+ cloud surrounding Saturn (Weiseret al., 1977, p. 755). The other possibility is that these moons are extremely dense and have very large internal magnetic fields. - Because of their location, these moons must be captured and if very dense as believed, may be core remnants of a nova.

Lyman-alpha observations in the vicinity of Saturn with Copernicus

For the first time, high-resolution Ly-alpha observations of the Saturn vicinity were completed with the Princeton spectrometer on board the Copernicus satellite. They showed that near a minimum solar activity the emissions related to several sources are 250 + or - 50 rayleighs for the interplanetary medium in a near-downwind direction, less than 100 rayleighs for the rings, 200 + or - 100 rayleighs for a torus linked to the Titan orbit, and 1400 + or - 450 rayleighs for the disk of Saturn. These results induce some constraints through the corresponding theoretical evaluations: the B ring as the primary source of the atoms for the ring emissions; an efficient production mechanism for hydrogen atoms in the Titan torus; and a slightly larger eddy diffusion coefficient in the Saturn atmosphere than in the Jupiter atmosphere near solar minimum.

On Saturnian cosmic ray cutoff rigidities

The theory of charged particle motion in a dipole field has been well developed, however because of the importance to the geomagnetic field of contributions other than dipolar, interest in the subject has remained primarily pedagogical over the past decade. Recent measurements by Ness et al. and Smith et al. have shown that the magnetic field of Saturn is essentially dipolar, with the axis of the dipole approximately aligned with the planetary rotation axis. This note briefly sketches the dipole geomagnetic cutoff theory and demonstrates the scaling required for its applicability to energetic particle measurements in the vicinity of Saturn.

Origin of Jupiter's Magnetic Field

Plausible theories of non-thermal radio-noise from Jupiter on decimetre and decametre wavelengths invoke a strong and nearly dipolar Jovian magnetic field and an associated system of van Allen-type radiation belts of electrically-charged particles extending beyond and interacting with the first Galilean satellite 10. Data from the recent Pioneer I0 fly-by provide direct evidence for the radiation belts and magnetic field and confirm the values for the strength, position and orientation of the equivalent Jovian magnetic dipole inferred previously by radio astronomers (Smith et al. 1974; Stannard 1975). The apparent absence of a non-thermal component in Saturn’s radio-emission implies that radiation belts cannot form around that planet, possibly because Saturn is non-magnetic, or, more likely, because charged particles in the vicinity of Saturn are rapidly removed through interactions with the rocky fragments that make up Saturn’s rings. Considerations of possible sources of Jupiter’s magnetic field-which theories of Jupiter’s internal constitution and energy sources must now take properly into account -indicate that the most likely mechanism is a magnetohydrodynamic dynamo associated with thermally-driven fluid motions in the electrically-conducting parts of Jupiter’s interior (Hide 1965; see also Runcorn 1968; Smoluchowski 1971 ; Stevenson 1974). It has not yet proved possible in dynamo theory to establish a precise relationship between the strength of the observed magnetic field and other parameters (density, planetary radius, electrical conductivity, rotation rate, etc.), but it proves instructive to compare the observed strength of Jupiter’s magnetic field with approximate theoretical relationships based on general considerations of the equations of magnetohydrodynamics of rotating fluids (Hide 1974) or on more specific dynamo models such as Parker’s (Levy 1972; Roberts 1975).

Jupiter and Saturn

Jupiter, the largest planet, and Saturn, the second largest, contain nine-tenths of the material of the solar system outside the Sun and most of the angular momentum of the solar system is associated with their orbital motion. Both planets rotate very rapidly (rotation periods ~ 10 h) and possess rich satellite systems. Owing to their strong gravitational fields and low surface temperatures, Jupiter and Saturn may, unlike the ‘terrestrial’ planets, be fairly close in chemical composition to the primordial material out of which the solar system originally formed; they consist mainly of hydrogen, much of which is compressed to a metallic form. Jupiter is the only planet other than Earth showing evidence of a general magnetic field. Absorption of incident solar energy accounts for less than one-half the estimated total thermal (infrared) radiation emitted by Jupiter and Saturn. The balance is probably due to internal heat sources and could be accounted for in terms of a gravitational contraction at about 0.1 cm/year. The outward flow of heat should maintain the atmospheric temperature gradients close to their adiabatic values, which is a significant result for theories of atmospheric motions (see appendix A). These theories are largely concerned with explaining the rough alinement of clouds in bands parallel to the equator, the presence of strong eastward equatorial currents, the occurrence of transient spots and other irregular markings and, in the case of Jupiter, the nature of the enigmatic Great Red Spot. Jupiter, unlike Saturn, is a strong emitter of non-thermal radio noise on decametre and decimetre wavelengths. Plausible theories of this radio emission invoke a strong Jovian dipole magnetic field and an associated system of van Allen-type ‘radiation’ belts of electrically-charged particles extending beyond and interacting with the first Galilean satellite Io. The most likely source of the Jovian magnetic field - which theories of Jupiter’s internal constitution must now take properly into account - is a hydromagnetic dynamo (see appendix B) associated with fluid motions in the electrically-conducting parts of Jupiter’s interior. The absence of a non-thermal component in Saturn’s radio spectrum implies that radiation belts cannot form around that planet, possibly because Saturn is non-magnetic or, if it is magnetic, because charged particles in the vicinity of Saturn are rapidly removed through interactions with Saturn’s rings. Modern research on Jupiter and Saturn is based on a rich variety of data, soon to be augmented by observations from space-craft. Future progress with the theoretical interpretation of these data in terms of improved models of the structure and evolution of the giant planets will involve not only the further application of a wide range of established knowledge but also the development of new ideas in several areas of basic science. The paper ends with two appendices, on the dynamics of rapidly rotating non-homogeneous fluids and on hydromagnetic dynamos.