Two dynamical classes of Centaurs

The Centaurs are a transient population of small bodies in the outer Solar System whose orbits are strongly chaotic. These objects typically suffer significant changes of orbital parameters on timescales of a few thousand years, and their orbital evolution exhibits two types of behaviors described qualitatively as random walk and resonance-sticking. We have analyzed the chaotic behavior of the known Centaurs. Our analysis has revealed that the two types of chaotic evolution are quantitatively distinguishable: (1) the random walk type behavior is well described by so-called generalized diffusion in which the rms deviation of the semimajor axis grows with time t as ∼t^H, with Hurst exponent H in the range 0.22-0.95, however (2) orbital evolution dominated by intermittent resonance sticking, with sudden jumps from one mean motion resonance to another, has poorly defined H. We further find that these two types of behavior are correlated with Centaur dynamical lifetime: most Centaurs whose dynamical lifetime is less than ∼22 Myr exhibit generalized diffusion, whereas most Centaurs of longer dynamical lifetimes exhibit intermittent resonance sticking. We also find that Centaurs in the diffusing class are likely to evolve into Jupiter-family comets during their dynamical lifetimes, while those in the resonance-hopping class do not.     

Ion irradiation of H2O ice on top of sulfurous solid residues and its relevance to the Galilean satellites

In this paper we present the results of new experiments of ion irradiation of water ice deposited on top of a solid sulfurous residue to study the potential formation of SO₂ at the interface ice/refractory material and discuss the possibility that this mechanism accounts for the sulfur dioxide ice detected on the surfaces of the Galilean satellites. In situ infrared spectroscopy was the used experimental technique. We have irradiated a thin film of H₂O frost on a sulfurous layer with 200 keV of He⁺ at 80 K. The used sulfurous residue was obtained by irradiation of frozen SO₂ at 16 K and it is used as a template of sulfur bearing solid materials. We have not found evidences of the efficient formation of SO₂ after irradiation of H₂O ice on top of the sulfurous residue. An upper limit to the production yield of SO₂, of interface area for each 100 eV of energy absorbed in 1 cm³ of ice-covered residue, has been estimated. These results have relevance in the context of the surfaces of the icy Galilean satellites in which SO₂ was detected. Our results show that radiolysis of mixtures of water ice and refractory sulfurous materials is not the primary formation mechanism responsible for the SO₂ present on the surfaces of the Galilean satellites.     

Expression of Cassini's third law for Callisto, and theory of its rotation

The rotation of the main natural satellites of the Solar System is widely assumed to be synchronous, because this corresponds to an equilibrium state. In the case of the Moon, 3 laws have been formulated by Cassini, assuming a spin-orbit resonance and a 1:1 nodal resonance. The recent gravitational data collected by the spacecrafts Galileo (in the jovian system) and Cassini (in the saturnian system) allows us to study the rotation of other natural satellites, and to check the universality of Cassini's laws. This paper deals with the rotation of the Galilean satellites of Jupiter J-4 Callisto. In this study we use both analytical (like Lie transforms) and numerical methods (numerical detection of chaos, numerical integration, frequency analysis) to first check the reliability of Cassini Laws for Callisto, and then to give a first theory of its rotation, Callisto's being considered as a rigid body. We first show that the Third Cassini Law (i.e. the nodal resonance), is not satisfied in every reference frame, in particular in the most natural one (i.e. the J2000 jovian equator). The difference of the nodes presents a chaotic-like behavior, that we prove to be just a geometrical illusion. Moreover, we give a mathematical condition ruling the choice of an inertial reference frame in which the Third Cassini Law is fulfilled. Secondly, we give a theory of Callisto's rotation in the International Celestial Reference Frame (ICRF). We highlight a small motion (i.e. <200 m) of its rotation axis about its body figure, a 11.86-yr periodicity in Callisto's length-of-day, and the proximity of a resonance that forces 182-yr librations in Callisto's obliquity.     

Equipotential surface and normal gravity of the Galilean satellites

In this article, expanded equations of normal gravity on the equipotential surface are proposed for a natural satellite whose orbital plane is close to its equatorial plane. Tidal effects on the normal gravity are also discussed. The authors apply these to the Galilean satellites. Calculations suggest that the tides raised by Jupiter weakly affect the Galilean satellites. The radial displacements of the gravity due to the tides are in the range between 10⁻³ and 10⁻⁵ m s⁻², which are similar to the latitudinal and longitudinal displacements. The variations along the latitude circle are larger than those along the longitude circle. We conclude that the tidal effects on most of the Galilean satellites are larger than those on the Moon.     

CO2 production by ion irradiation of H2O ice on top of carbonaceous materials and its relevance to the Galilean satellites

In this work we report on new experiments of ion irradiation of water ice deposited on top of solid carbonaceous materials to study the production of CO₂ at the interface ice/refractory material and discuss the possibility that this mechanism accounts for the quantity of CO₂ ice detected on the surfaces of the Galilean satellites. The used experimental technique has been in situ infrared spectroscopy. We have irradiated thin films of H₂O frost on carbonaceous layers with 200 keV of He⁺ and Ar⁺, and 30 keV of He⁺ at 16 and 80 K. The used carbonaceous layers have been asphaltite, a natural bitumen, and solid organic residues obtained by irradiation of frozen benzene. In both cases the results show that CO₂ is produced very efficiently after irradiation obtaining a maximum quantity of the order of . These results are, also quantitatively similar, to those recently obtained for water ice deposited on amorphous carbon films [Mennella, V., Palumbo, M.E., Baratta, G.A., 2004. Formation of CO and CO₂ molecules by ion irradiation of water ice covered hydrogenated carbon grains. Astrophys. J. 615, 1073-1080]. Thus we suggest that, whatever is the carbonaceous residue, CO₂ will be produced efficiently by the studied process. These results have interest in the context of the surfaces of the icy Galilean satellites in which CO₂ has been detected mainly trapped in the non-ice material, not in the pure water ice. We suggest that radiolysis of mixtures of water ice and refractory carbonaceous materials is the primary formation mechanism responsible for the CO₂ formation on the surfaces of the Galilean satellites.     

New evidence of precision premium for Galilean satellites from CCD imaging

After a CCD image of the four Galilean satellites of Jupiter is obtained by a long focal length telescope, we can compare the theoretical positions of these satellites with their pixel positions so as to obtain the calibration parameters of the CCD field of view. In theory, when two of the four satellites have small enough separation, their relative positional measurement will have a good accuracy since the error existing in the solved calibration parameters has a direct proportional effect on the separation of the two satellites. The 347 CCD images taken by 1-m long focal length telescope at Yunnan Observatory in 2002-2005 are used to perform the experimental test. After we improve the centroid algorithm for the satellites and our former halo-removal technique, the results show that the positional measurement of two small-separation satellites has an external precision as good as 0.01-0.03 arcsec. This precision has comparability as that from rarely occurring mutual events of the Galilean satellites. This experiment confirms the finding of the “precision premium” firstly presented by Pascu [1994. An appraisal of the USNO program of photographic astrometry of bright planetary satellites. In: Morrison, L.V., Gilmore, G.F., (Eds.), Galatic and Solar System Optical Astrometry, pp. 304-311] using photographic observations. We believe that this type of observations, besides mutual event observations, might also be used to improve our knowledge of the orbital motions of the Galilean satellites because of its much more opportunities.     

The web of three-planet resonances in the outer Solar System: II. A source of orbital instability for Uranus and Neptune

The motion of the giant planets from Jupiter to Neptune is chaotic with Lyapunov time of approximately 10 Myr. A recent theory explains the presence of this chaos with three-planet mean-motion resonances, i.e. resonances among the orbital periods of at least three planets. We find that the distribution of these resonances with respect to the semi-major axes of all the planets is compatible with orbital instability. In particular, they overlap in a region of 10⁻³ AU with respect to the variation of the semi-major axes of Uranus and Neptune. Fictitious planetary systems with initial conditions in this region can undergo systematic variations of semi-major axes. The true Solar System is marginally in this region, and Uranus and Neptune undergo very slow systematic variations of semi-major axes with speed of order 10⁻⁴ AU/Gyr.     

Origin of chaos in the Prometheus-Pandora system

We demonstrate that the chaotic orbits of Prometheus and Pandora are due to interactions associated with the 121:118 mean motion resonance. Differential precession splits this resonance into a quartet of components equally spaced in frequency. Libration widths of the individual components exceed the splitting, resulting in resonance overlap which causes the chaos. Mean motions of Prometheus and Pandora wander chaotically in zones of width 1.8 and 3.1 deg yr⁻¹, respectively. A model with 1.5 degrees of freedom captures the essential features of the chaotic dynamics. We use it to show that the Lyapunov exponent of 0.3 yr⁻¹ arises because the critical argument of the dominant member of the resonant quartet makes approximately two separatrix crossings every 6.2 year precessional cycle.     

Cassini VIMS observations of the Galilean satellites including the VIMS calibration procedure

The Visual and Infrared Mapping Spectrometer (VIMS) observed the Galilean satellites during the Cassini spacecraft's 2000/2001 flyby of Jupiter, providing compositional and thermal information about their surfaces. The Cassini spacecraft approached the jovian system no closer than about 126 Jupiter radii, about 9 million kilometers, at a phase angle of <90°, resulting in only sub-pixel observations by VIMS of the Galilean satellites. Nevertheless, most of the spectral features discovered by the Near Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft during more than four years of observations have been identified in the VIMS data analyzed so far, including a possible ¹³C absorption. In addition, VIMS made observations in the visible part of the spectrum and at several new phase angles for all the Galilean satellites and the calculated phase functions are presented. In the process of analyzing these data, the VIMS radiometric and spectral calibrations were better determined in preparation for entry into the Saturn system. Treatment of these data is presented as an example of the VIMS data reduction, calibration and analysis process and a detailed explanation is given of the calibration process applied to the Jupiter data.     

Physisorption of CO2 on non-ice materials relevant to icy satellites

CO₂ is known to adsorb onto clay and other minerals when a significant atmospheric pressure is present. We have found that CO₂ can also adsorb onto some clays when the CO₂ partial pressure is effectively zero under ultra-high vacuum (UHV) if cooled to the surface temperatures of the icy satellites of Jupiter and Saturn. The strength of adsorption and the spectral characteristics of the adsorbed CO₂ infrared (IR) ν₃ absorption band near 4.25 μm depend on the composition and temperature of the adsorbent. CO₂ remains adsorbed onto the clay mineral montmorillonite for >10 s of min when exposed to a vacuum of ∼1×¹⁰⁻⁸ Torr at ∼125 K. CO₂ does not adsorb onto serpentine, goethite, or palagonite under these conditions. A small amount may adsorb onto kaolinite. When heated above 150 K under vacuum, the CO₂ desorbs from the montmorillonite within a few minutes. The ν₃ absorption band of CO₂ adsorbed onto montmorillonite at 125 K is similar to that of the CO₂ detected on the saturnian and Galilean satellites and is markedly different from CO₂ adsorbed onto montmorillonite at room temperature. We infer the adsorption process is physisorption and postulate that this mechanism may explain the presence and spectral characteristics of the CO₂ detected in the surfaces of these outer satellites.     

Tidally Induced Volcanism

The dissipation of tidal energy causes the ongoing silicate volcanism on Jupiter's satellite, Io, and cryovolcanism almost certainly has resurfaced parts of Saturn's satellite, Enceladus, at various epochs distributed over the latter's history. The maintenance of tidal dissipation in Io and the occurrence of the same on Enceladus depends crucially on the maintenance of the respective orbital eccentricities by the existence of mean motion resonances with nearby satellites. A formation of the resonances among the Galilean satellites by differential expansion of the satellite orbits from tides raised on Jupiter by the satellites means the onset of the volcanism on Io could be relatively recent. If, on the other hand, the resonances formed by differential migration from resonant interactions of the satellites with the disk of gas and particles from which they formed, Io would have been at least intermittently volcanically active throughout its history. Either means of assembling the Galilean satellite resonances lead to the same constraint on the dissipation function of Jupiter Q _J 10⁶, where the currently high heat flux from Io seems to favor episodic heating as Io's eccentricity periodically increases and decreases. Either of the two models might account for sufficient tidal dissipation in the icy satellite Enceladus to cause at least occasional cryovolcanism over much of its history. However, both models are assumption-dependent and not secure, so uncertainty remains on how tidal dissipation resurfaced Enceladus.     

Chaotic Dynamics of Satellite Systems

We consider the problem of calculating the Lyapunov time (the characteristic time of predictable dynamics) of chaotic motion in the vicinity of separatrices of orbital resonances in satellite systems. The primary objects of study are the chaotic regimes that have occurred in the history of the orbital dynamics of the second and fifth Uranian satellites (Umbriel and Miranda) and the first and third Saturnian satellites (Mimas and Tethys). We study the dynamics in the vicinity of separatrices of the resonance multiplets corresponding to the 3 : 1 commensurability of mean motions of Miranda and Umbriel and the multiplets corresponding to the 2 : 1 commensurability of mean motions of Mimas and Tethys. These chaotic regimes have most probably contributed much to the long-term orbital evolution of the two satellite systems. The equations of motion have been numerically integrated to estimate the Lyapunov time in models corresponding to various epochs of the system evolution. Analytical estimates of the Lyapunov time have been obtained by a method (Shevchenko, 2002) based on the separatrix map theory. The analytical estimates have been compared to estimates obtained by direct numerical integration.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 4, 2005, pp. 364–374.Original Russian Text Copyright © 2005 by Mel’nikov, Shevchenko.     

Transfer of mass from Io to Europa and beyond due to cometary impacts

We simulate the production and orbital evolution of escaping ejecta due to cometary impacts on Io. The model includes the four Galilean satellites, Amalthea, Thebe, Jupiter's gravitational moments, Saturn and the Sun. Five scenarios are examined: an impact at the apex, the sub-jovian point, the anti-jovian point, the antapex, and at the south pole of Io. We estimate that on average a cometary impact injects thrice its mass (in the form of Io surface material) into jovicentric orbit. The majority of the escaping debris comes back to Io, but a sizeable fraction (between 5.0 and 8.7%) manages to reach Europa, and a smaller fraction Ganymede (between 1.5 and 4.6%). Smaller fractions reached Amalthea Thebe, Callisto, and Jupiter itself. For million year time scales, the mass transfer to Europa is estimated as 1.8-3.1×¹⁰¹⁴ g/Myr. The median time for transfer of ejecta from Io to Europa is ∼56 years.     

伽利略卫星的等势面和正常重力场

给出了轨道面接近赤道面的轨旋同步卫星的正常重力场在等势面上分布的展开式，并讨论了潮汐对其正常重力场的影响，利用这一方法，讨论了伽利略卫星正常重力场及其在等势面上的分布，以及木星的潮汐对伽利略卫星的正常重力场的影响，计算表明，潮汐对伽利略卫星的正常重力场影响不大，其径向的影响grt大约是10^-3-10^-5m/s^2的量级，与重力场在经度和纬度方向的分量接近，通过估算，月球的重力场所受到的潮汐影响要比绝大多数伽利略卫星受到的潮汐影响小。     

Orbital resonances in the inner neptunian system: II. Resonant history of Proteus, Larissa, Galatea, and Despina

We investigate the orbital history of the small neptunian satellites discovered by Voyager 2. Over the age of the Solar System, tidal forces have caused the satellites to migrate radially, bringing them through mean-motion resonances with one another. In this paper, we extend our study of the largest satellites Proteus and Larissa [Zhang, K., Hamilton, D.P., 2007. Icarus 188, 386-399] by adding in mid-sized Galatea and Despina. We test the hypothesis that these moons all formed with zero inclinations, and that orbital resonances excited their tilts during tidal migration. We find that the current orbital inclinations of Proteus, Galatea, and Despina are consistent with resonant excitation if they have a common density . Larissa's inclination, however, is too large to have been caused by resonant kicks between these four satellites; we suggest that a prior resonant capture event involving either Naiad or Thalassa is responsible. Our solution requires at least three past resonances with Proteus, which helps constrain the tidal migration timescale and thus Neptune's tidal quality factor: 9000<QN<36,000. We also improve our determination of Qs for Proteus and Larissa, finding 36<QP<700 and 18<QL<200. Finally, we derive a more general resonant capture condition, and work out a resonant overlap criterion relevant to satellite orbital evolution around an oblate primary.     

The discovery of faint irregular satellites of Uranus

We report the discovery of four new uranian irregular satellites in our deep, m_R∼25.4, optical search around that planet. The orbital properties of these satellites are diverse. There is some grouping of inclinations and one of the satellites appears to be inside the Kozai resonant zone of Uranus. Further, we find that the differential size distribution of satellites is rather shallow compared to objects in the asteroid and Kuiper belts, going as ∼r^−2.4. We also report a strong coupling between semi-major axis and orbital eccentricity. We comment on the apparent paradox between the inclination grouping, shallow size distribution, and orbital correlation as they relate to the likelihood of a collisional origin for the uranian irregulars. The currently observed irregulars appear to be consistent with a disruptive formation process and a collisional origin for Uranus' obliquity.     

Chaotic motions of prometheus and pandora

Recent HST images of the saturnian satellites Prometheus and Pandora show that their longitudes deviate from predictions of ephemerides based on Voyager images. Currently Prometheus is lagging and Pandora leading these predictions by somewhat more than 20°. We show that these discrepancies are fully accounted for by gravitational interactions between the two satellites. These peak every 24.8 days at conjunctions and excite chaotic perturbations. The Lyapunov exponent for the Prometheus-Pandora system is of order 0.3 year⁻¹ for satellite masses based on a nominal density of 0.63 g cm⁻³. Interactions are strongest when the orbits come closest together. This happens at intervals of 6.2 years when their apses are antialigned. In this context, we note the sudden changes of opposite signs in the mean motions of Prometheus and Pandora at the end of 2000 occurred around the time their apsidal lines were antialigned.     

Orbital resonances in the inner neptunian system: I. The 2:1 Proteus-Larissa mean-motion resonance

We investigate the orbital resonant history of Proteus and Larissa, the two largest inner neptunian satellites discovered by Voyager 2. Due to tidal migration, these two satellites probably passed through their 2:1 mean-motion resonance a few hundred million years ago. We explore this resonance passage as a method to excite orbital eccentricities and inclinations, and find interesting constraints on the satellites' mean density () and their tidal dissipation parameters (Qs>10). Through numerical study of this mean-motion resonance passage, we identify a new type of three-body resonance between the satellite pair and Triton. These new resonances occur near the traditional two-body resonances between the small satellites and, surprisingly, are much stronger than their two-body counterparts due to Triton's large mass and orbital inclination. We determine the relevant resonant arguments and derive a mathematical framework for analyzing resonances in this special system.     

On the chaotic orbital dynamics of the planet in the system 16 Cyg

The chaotic orbital dynamics of the planet in the wide visual binary star system 16 Cyg is considered. The only planet in this system has a significant orbital eccentricity, e = 0.69. Previously, Holman et al. suggested the possibility of chaos in the orbital dynamics of the planet due to the proximity of 16 Cyg to the separatrix of the Lidov–Kozai resonance. We have calculated the Lyapunov characteristic exponents on the set of possible orbital parameters for the planet. In all cases, the dynamics of 16 Cyg is regular with a Lyapunov time of more than 30 000 yr. The dynamics is considered in detail for several possible models of the planetary orbit; the dependences of Lyapunov exponents on the time of their calculation and the time dependences of osculating orbital elements have been constructed. Phase space sections for the system dynamics near the Lidov–Kozai resonance have been constructed for all models. A chaotic behavior in the orbital motion of the planet in 16 Cyg is shown to be unlikely, because 16 Cyg in phase space is far from the separatrix of the Lidov–Kozai resonance at admissible orbital parameters, with the chaotic layer near the separatrix being very narrow.