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1.
Estimates of tidal damping times of the orbital eccentricities of Saturn's inner satellites place constraints on some satellite rigidities and dissipation functions Q. These constraints favor rock-like rather than ice-like properties for Mimas and probably Dione. Photometric and other observational data are consistent with relatively higher densities for these two satellites, but require lower densities for Tethys, Enceladus, and Rhea. This leads to a nonmonotonic density distribution for Saturn's inner satellites, apparently determined by different mass fractions of rocky materials. In spite of the consequences of tidal dissipation for the orbital eccentricity decay and implications for satellite compositions, tidal heating is not an important contributor to the thermal history of any Saturnian satellite. 相似文献
2.
In a previous work (Callegari and Yokoyama, Celest. Mech. Dyn. Astr. 98:5–30, 2007), the main features of the motion of the
pair Enceladus–Dione were analyzed in the frozen regime, i.e., without considering the tidal evolution. Here, the results
of a great deal of numerical simulations of a pair of satellites similar to Enceladus and Dione crossing the 2:1 mean-motion
resonance are shown. The resonance crossing is modeled with a linear tidal theory, considering a two-degrees-of-freedom model
written in the framework of the general three-body planar problem. The main regimes of motion of the system during the passage
through resonance are studied in detail. We discuss our results comparing them with classical scenarios of tidal evolution
of the system. We show new scenarios of evolution of the Enceladus–Dione system through resonance not shown in previous approaches
of the problem. 相似文献
3.
We analyze the interactions between Saturn's coorbital satellites, Janus and Epimetheus, and the outer edge of the A ring, which is presumably maintained by these moons at their 7:6 resonance. Using two distinct but conceptually related methods, we show that ring torques are driving these satellites into a tighter lock. Unless there is a counterbalancing force which we have neglected, their orbital configuration will evolve from the current horseshoe-type lock to one of tadpole orbits around a single Lagrange point in ~20 myr. This finding adds an additional member to the list of short time scale problems associated with the interactions between Saturn's rings and its inner moons 相似文献
4.
5.
The thermal histories of two geologically active satellites of Saturn—Titan and Enceladus—are discussed. During the Cassini mission, it was found that there are both nitrogen-containing compounds—NH3 and N2-and CO2 and CH4 in the water plumes of Enceladus; at that, ammonia is the prevailing form. This may testify that during evolution, the material
of the satellite was warmed up to T ∼ 500–600 K, when NH3 (the form of nitrogen capable of being accreted) could only be partly converted into N2. Contrary to Enceladus, the temperature inside Titan probably reached values higher than 800 K or even higher than 1000 K,
since the process of the chemical dissociation of ammonia was completely finished on this satellite and its atmosphere contains
only molecular nitrogen. While the internal heating of Titan up to high temperatures can be explained by its large mass, the
heating source for Enceladus’ interior is far from evident. Such traditional heating sources as the energy of gravitational
differentiation and the radiogenic heating due to shortliving 26Al and 60Fe could not be effective. The first one is because of the small size of Enceladus (RE ≈ 250 km), and the inefficiency of the second one is caused by the fact that the satellite was formed not earlier than 8–10
Myr after the formation of calcium and aluminum-enriched inclusions in carbonaceous chondrites (CAI), i.e., after 26Al had completely decayed. In the present paper, we propose other heating mechanisms-the heat of long-living radioactive elements
and tidal heat, which could provide the observed chemical composition of the water plumes of Enceladus rather than only the
differentiation of its protomatter into the ironstone core and the ice mantle. 相似文献
6.
Richard Greenberg 《Icarus》1978,33(1):62-73
Orbital resonances tend to force bodies into noncircular orbits. If a body is also under the influence of an eccentricity-reducing medium, it will experience a secular change in semimajor axis which may be positive or negative depending on whether its orbit is exterior or interior to that of the perturbing body. Thus a dissipative medium can promote either a loss or a gain in orbital energy. This process may explain the resonant structure of the asteroid belt and of Saturn's rings. For reasonable early solar system parameters, it would clear a gap near the 2:1 resonance with Jupiter on a time scale of a few thousand years; the gap width would be comparable to the Kirkwood gap presently at the location in the asteroid belt. Similarly, a gap comparable in width to Cassini's division would be cleared in Saturn's rings at the 2:1 resonance with Mimas in ~106 yr. Most of the material from the gap would be deposited at the outer edge of ring B. The process would also affect the radial distribution of preplanetary material. Moreover, it provides an explanation for the large amplitude of the Titan-Hyperion libration. Consideration of the effects of dissipation on orbits near the stable L4 and L5 points of the restricted three-body problem indicates that energy loss causes particles to move away from these points. This results explains the large amplitude of Trojan asteroids about these points and the possible capture of Trojan into orbit about Jupiter. 相似文献
7.
E. Martínez-Gómez H. J. Durand-Manterola H. Pérez de Tejada 《Astrophysics and Space Science》2007,310(3-4):211-230
A model is presented to describe the energization of charged particles in planetary magnetospheres. The model is based on
the stochastic acceleration produced by a random electric field that is induced by the magnetic field fluctuations measured
within the magnetospheres. The stochastic behavior of the electric field is simulated through a Monte Carlo method. We solve
the equation of motion for a single charged particle—which comprises the stochastic acceleration due to the stochastic electric
field, the Lorentz acceleration (containing the local magnetic field and the corotational electric field) and the gravitational
planetary acceleration of the particle—under several initial conditions. The initial conditions include the ion species and
the velocity distribution of the particles which depends on the sources they come from (solar wind, ionospheres, rings and
satellites). We applied this model to Saturn’s inner magnetosphere using a sample of particles (H+, H2O+, N+, O+ and OH+) initially located on Saturn’s north pole, above the C-Ring, on the south pole of Enceladus, in the north pole of Dione and
above the E-Ring. The results show that the particles tend to increase the value of their energy with time reaching several
eV in a few seconds and the large energization is observed far from the planet. We can distinguish three main energization
regions within Saturn’s inner magnetosphere: minimum (Saturn’s ionosphere), intermediate (Dione) and high-energy (Enceladus
and the E-ring). The resulting energy spectrum follows a power-law distribution (>1 keV), a logistic, an exponential decay
or an asymmetric sigmoidal (<1 keV). 相似文献
8.
Joseph A. Burns 《Icarus》1976,28(4):453-458
Mercury, currently rotating very slowly, probably rotated faster in the past. If Mercury's rotation period had been near 8 hours initially, similar to that of most solar system bodies today, it would have been flattened by a few percent. As Mercury was slowed by solar tides, craters which were circular when they were emplaced would have been distorted by the same few percent. Substantial surface stresses, well above the fracture stress, would have been produced unless stress relief occurred; these stresses should have produced tensional fractures near the poles and two intersecting sets of shear planes in equatorial regions. Satellite orbits about the slowly spinning Mercury have been shown to collapse onto its surface: the impact craters resulting from these hypothetical lost satellites should be elongated along the orbit paths, which probably lie near the equator. However, none of these features has been found on the Mariner 10 images. They may be obscured by the effects of tidal heating that should cause an overall internal temperature increase of about 100°K although the increase would be substantially more in certain regions. Radial tides, sometimes called push-pull tides, are important at the present time because Mercury's large orbital eccentricity causes the planet to undergo significant tidal flexing each orbital period; the contemporary tidal heating due to this mechanism is estimated at more than 1016 erg/sec. 相似文献
9.
Jack J. Lissauer 《Icarus》1985,62(3):433-447
The surface mass density profiles at four locations within Saturn's rings are calculated using Voyager spacecraft images of spiral bending waves. Bending waves are vertical corrugations in Saturn's rings which are excited at vertical resonances of a moon, e.g., Mimas, whose orbit is inclined with respect to the mean plane of the rings. Bending waves propagate toward Saturn by virtue of the rings' self-gravity; their wavelength depends on the local surface mass density of the rings. Observations of bending waves can thus be used to determine the surface density in regions of Saturn's rings near vertical resonances. The average surface density of the outer B ring near Mimas' 4:2 inner vertical resonance is 54 ± 10 g cm?2. Surface density in this region probably varies by ~ 30% over radial length scales of tens of kilometers; and irregular radial structure is present on similar length scales in this region. Surface densities ranging from 24 g cm?2 to 45 g cm?2 are found in the A ring. Small scale variations in surface density are not seen in the A ring, consistent with its more uniform optical appearance. 相似文献
10.
Jack J. Lissauer 《Icarus》1984,57(1):63-71
Ejecta from impacts of micrometeoroids on Saturn's ring particles will, in most cases, remain in orbit about Saturn and eventually be reaccreted by the rings, possibly at a different radial location. The resulting mass transport has been suggested as the cause of some of the features observed in Saturn's rings. Previous attempts to model this transport have used numerical simulations which have not included the effects of the angular momentum transport coincident with mass transport. An analytical model for ballistic mass transport in Saturn's rings is developed. The model includes the effects of angular momentum advection and shows that the net material movement due to angular momentum advection is comparable to that caused by direct ballistic mass transport. 相似文献
11.
Classical resonance theory is extended to include corrections due to Saturn's oblateness. A single classical resonance splits into a band structure, with individual resonances almost evenly spaced in radius from the planet. When applied to Saturn's rings this theory predicts, in detail, the structure of Cassini's division. 相似文献
12.
Voyager imaging observations provide new photometric data on Saturn's satellites at large phase angles (up to 133° 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 we find an average geometric albedo pv = 1.04 ± 0.15 and Bond albedo of 0.9 ± 0.1. 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, pv = 0.77 ± 0.15 (corresponding to a mean opposition magnitude V0 = +12.5) 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. 相似文献
13.
Thermal histories of the small icy Saturnian satellites Mimas, Tethys, Dione, Rhea, and Iapetus are constructed by assuming that they formed as homogeneous ice-silicate mixtures. The models include effects of radiogenic and accretional heating, conductive and subsolidus convective heat transfer, and lithospheric growth. Accretional heating is unlikely to have melted the water ice in the interiors of these bodies and solid state creep of the predominately ice material precludes melting by radiogenic heating. Mimas is so small that its thermal evolution is essentially purely conductive; at present it is a cold, nearly isothermal body. Any subsolidus convection or thermal activity in Mimas would have been confined to a brief period in its early history and would have been due to a warm formation. The four largest satellites are big enough and contain sufficient heat-producing silicates that solid state convection beneath a rigid lithosphere is inevitable independent of initial conditions. Dione and Rhea have convective interiors for most of their thermal histories, while Tethys and Iapetus have mainly conductive thermal histories with early periods of convective 0activity. The thermal histories of the five satellites for the last 4 by are independent of initial conditions; at present they have cold, conductive interiors. The model thermal histories are qualitatively consistent with the appearances of these satellites: Mimas has an ancient heavily cratered surface, Tethys and probably Iapetus have both heavily cratered and more lightly cratered areas, and Dione and Rhea have extensively modified surfaces. Because of their similar sizes and densities, Mimas and Enceladus are expected to have similar surfaces and thermal histories, but instead Enceladus has the most modified surface of all the small icy Saturnian satellites. Our results suggest a heat source for Enceladus, in addition to radiogenic and accretional heating; tidal dissipation is a possibility. Because the water ice in these bodies does not melt, resurfacing must be accomplished by the melting of a low-melting-temperature minor component such as ammonia hydrate. 相似文献
14.
We describe a direct simulation Monte Carlo (DSMC) model of Enceladus’ neutral cloud and compare its results to observations of OH and O orbiting Saturn. The OH and O are observed far from Enceladus (at 3.95 RS), as far out as 25 RS for O. Previous DSMC models attributed this breadth primarily to ion/neutral scattering (including charge exchange) and molecular dissociation. However, the newly reported O observations and a reinterpretation of the OH observations (Melin, H., Shemansky, D.E., Liu, X. [2009] Planet. Space Sci., 57, 1743-1753, PS&S) showed that the cloud is broader than previously thought. We conclude that the addition of neutral/neutral scattering (Farmer, A.J. [2009] Icarus, 202, 280-286), which was underestimated by previous models, brings the model results in line with the new observations. Neutral/neutral collisions primarily happen in the densest part of the cloud, near Enceladus’ orbit, but contribute to the spreading by pumping up orbital eccentricity. Based on the cloud model presented here Enceladus maybe the ultimate source of oxygen for the upper atmospheres of Titan and Saturn. We also predict that large quantities of OH, O and H2O bombard Saturn’s icy satellites. 相似文献
15.
Tidal evolution of Mimas, Enceladus, and Dione 总被引:2,自引:0,他引:2
The tidal evolution through several resonances involving Mimas, Enceladus, and/or Dione is studied numerically with an averaged resonance model. We find that, in the Enceladus-Dione 2:1 e-Enceladus type resonance, Enceladus evolves chaotically in the future for some values of k2/Q. Past evolution of the system is marked by temporary capture into the Enceladus-Dione 4:2 ee′-mixed resonance. We find that the free libration of the Enceladus-Dione 2:1 e-Enceladus resonance angle of 1.5° can be explained by a recent passage of the system through a secondary resonance. In simulations with passage through the secondary resonance, the system enters the current Enceladus-Dione resonance close to tidal equilibrium and thus the equilibrium value of tidal heating of 1.1(18,000/QS) GW applies. We find that the current anomalously large eccentricity of Mimas can be explained by passage through several past resonances. In all cases, escape from the resonance occurs by unstable growth of the libration angle, sometimes with the help of a secondary resonance. Explanation of the current eccentricity of Mimas by evolution through these resonances implies that the Q of Saturn is below 100,000. Though the eccentricity of Enceladus can be excited to moderate values by capture in the Mimas-Enceladus 3:2 e-Enceladus resonance, the libration amplitude damps and the system does not escape. Thus past occupancy of this resonance and consequent tidal heating of Enceladus is excluded. The construction of a coherent history places constraints on the allowed values of k2/Q for the satellites. 相似文献
16.
D. C. Mourão O. C. Winter T. Yokoyama R. R. Cordeiro 《Monthly notices of the Royal Astronomical Society》2006,372(4):1614-1620
In the present work, we study the stability of hypothetical satellites that are coorbital with Enceladus and Mimas. We performed numerical simulations of 50 particles around the triangular Lagrangian equilibrium points of Enceladus and Mimas taking into account the perturbation of Mimas, Enceladus, Tethys, Dione, Titan and the oblateness of Saturn. All particles remain on tadpole orbits after 10 000 yr of integration. Since in the past the orbit of Enceladus and Mimas expanded due to the tidal perturbation, we also simulated the system with Enceladus and Mimas at several different values of semimajor axes. The results show that in general the particles remain on tadpole orbits. The exceptions occur when Enceladus is at semimajor axes that correspond to 6:7, 5:6 and 4:5 resonances with Mimas. Therefore, if Enceladus and Mimas had satellites librating around their Lagrangian triangular points in the past, they would have been removed if Enceladus crossed one of these first-order resonances with Mimas. 相似文献
17.
We study the orbital behavior of Saturn’s satellites Enceladus and Dione during their passage through the 2:1 mean-motion resonances to constrain their interior structures, parameterized by the quantity k2/Q (assumed constant). Enceladus’ evolution after escape from the second-order e-Enceladus e-Dione resonance requires that (k2/Q)Enceladus<8×10-4, for that QSaturn>18,000. This result is in agreement with [Meyer, J., Wisdom, J., 2008b. Icarus 193, 213-223]. The present-day libration amplitude of Enceladus requires that (k2/Q)Enceladus>1.2×10-4, assuming that QSaturn<105. Dione’s present-day eccentricity indicates that (k2/Q)Dione?3×10-4 for QSaturn>18,000. Assuming Maxwellian viscoelastic behavior, we find that for Enceladus a convective ice shell overlying an ocean is too dissipative to match the orbital constraints. We conclude that a conductive shell overlying an ocean is more likely, and discuss the implications of this result. Dione’s ice shell is also likely to be conductive, but our results are less constraining. 相似文献
18.
Observations of Saturn's rings during passage of the Earth through the ring plane, coupled with those of others, suggest a ring thickness of 1.3 ± 0.3 km. The wide disparity in the optical depth of Cassini's division found by other investigators is resolved, and for conservative isotropic single scattering, a normal optical depth for Cassini's division of 0.060 ± 0.006 is obtained. We find the mean normal optical depth of ring C to be 0.074 ± 0.007. Analysis of all available observations of faint objects near Saturn indicates the presence of at least one previously undiscovered satellite of Saturn. The orbit for Janus determined by Dollfus is supported. These satellites may be major members of an extended ring. 相似文献
19.
Harris (Icarus24, 190–192) has suggested that the maximum size of particles in a planetary ring is controlled by collisional fragmentation rather than by tidal stress. While this conclusion is probably true, estimated radius limits must be revised upward from Harris' values of a few kilometers by at least an order of magnitude. Accretion of particles within Roche's limit is also possible. These considerations affect theories concerning the evolution of Saturn's rings, of the Moon, and of possible former satellites of Mercury and Venus. In the case of Saturn's rings, comparison of various theoretical scenarios with available observational evidence suggests that the rings formed from the breakup of larger particles rather than from original condensation as small particles. This process implies a distribution of particle sizes in Saturn's rings possibly ranging up to ~100 km but with most cross-section in cm-scale particles. 相似文献
20.
Observations of Saturn's satellites and external rings during the 1980 edge-on presentation were obtained with a focal coronograph. A faint satellite traveling in the orbit of Dione and leading it by 72° has been detected, together with the two inner satellites already suspected (cf. J. W. Fountain and S. M. Larson, 1978,Icarus36, 92–106). The external ring has been observed on both east and west sides; it may extend up to Saturn radii, and appears structured. 相似文献