Formation of the regular satellites of giant planets in an extended gaseous nebula I: subnebula model and accretion of satellites     

Ignacio Mosqueira  Paul R. Estrada 《Icarus》2003,163(1):198-231

We model the subnebulae of Jupiter and Saturn wherein satellite accretion took place. We expect each giant planet subnebula to be composed of an optically thick (given gaseous opacity) inner region inside of the planet’s centrifugal radius (where the specific angular momentum of the collapsing giant planet gaseous envelope achieves centrifugal balance, located at rCJ ∼ 15R_J for Jupiter and rCS ∼ 22R_S for Saturn) and an optically thin, extended outer disk out to a fraction of the planet’s Roche-lobe (R_H), which we choose to be ∼R_H/5 (located at ∼150 R_J near the inner irregular satellites for Jupiter, and ∼200R_S near Phoebe for Saturn). This places Titan and Ganymede in the inner disk, Callisto and Iapetus in the outer disk, and Hyperion in the transition region. The inner disk is the leftover of the gas accreted by the protoplanet. The outer disk may result from the nebula gas flowing into the protoplanet during the time of giant planet gap-opening (or cessation of gas accretion). For the sake of specificity, we use a solar composition “minimum mass” model to constrain the gas densities of the inner and outer disks of Jupiter and Saturn (and also Uranus). Our model has Ganymede at a subnebula temperature of ∼250 K and Titan at ∼100 K. The outer disks of Jupiter and Saturn have constant temperatures of 130 and 90 K, respectively.Our model has Callisto forming in a time scale ∼10⁶ years, Iapetus in 10⁶-10⁷ years, Ganymede in 10³-10⁴ years, and Titan in 10⁴-10⁵ years. Callisto takes much longer to form than Ganymede because it draws materials from the extended, low density portion of the disk; its accretion time scale is set by the inward drift times of satellitesimals with sizes 300-500 km from distances ∼100R_J. This accretion history may be consistent with a partially differentiated Callisto with a ∼300-km clean ice outer shell overlying a mixed ice and rock-metal interior as suggested by Anderson et al. (2001), which may explain the Ganymede-Callisto dichotomy without resorting to fine-tuning poorly known model parameters. It is also possible that particulate matter coupled to the high specific angular momentum gas flowing through the gap after giant planet gap-opening, capture of heliocentric planetesimals by the extended gas disk, or ablation of planetesimals passing through the disk contributes to the solid content of the disk and lengthens the time scale for Callisto’s formation. Furthermore, this model has Hyperion forming just outside Saturn’s centrifugal radius, captured into resonance by proto-Titan in the presence of a strong gas density gradient as proposed by Lee and Peale (2000). While Titan may have taken significantly longer to form than Ganymede, it still formed fast enough that we would expect it to be fully differentiated. In this sense, it is more like Ganymede than like Callisto (Saturn’s analog of Callisto, we expect, is Iapetus). An alternative starved disk model whose satellite accretion time scale for all the regular satellites is set by the feeding of planetesimals or gas from the planet’s Roche-lobe after gap-opening is likely to imply a long accretion time scale for Titan with small quantities of NH₃ present, leading to a partially differentiated (Callisto-like) Titan. The Cassini mission may resolve this issue conclusively. We briefly discuss the retention of elements more volatile than H₂O as well as other issues that may help to test our model.  相似文献   

On the orbits and masses of the satellites of the Pluto-Charon system     

Man Hoi Lee  S.J. Peale 《Icarus》2006,184(2):573-583

Two small satellites of Pluto, S/2005 P1 (hereafter P1) and S/2005 P2 (hereafter P2), have recently been discovered outside the orbit of Charon, and their orbits are nearly circular and nearly coplanar with that of Charon. Because the mass ratio of Charon-Pluto is ∼0.1, the orbits of P2 and P1 are significantly non-Keplerian even if P2 and P1 have negligible masses. We present an analytic theory, with P2 and P1 treated as test particles, which shows that the motion can be represented by the superposition of the circular motion of a guiding center, the forced oscillations due to the non-axisymmetric components of the potential rotating at the mean motion of Pluto-Charon, the epicyclic motion, and the vertical motion. The analytic theory shows that the azimuthal periods of P2 and P1 are shorter than the Keplerian orbital periods, and this deviation from Kepler's third law is already detected in the unperturbed Keplerian fit of Buie and coworkers. In this analytic theory, the periapse and ascending node of each of the small satellites precess at nearly equal rates in opposite directions. From direct numerical orbit integrations, we show the increasing influence of the proximity of P2 and P1 to the 3:2 mean-motion commensurability on their orbital motion as their masses increase within the ranges allowed by the albedo uncertainties. If the geometric albedos of P2 and P1 are high and of order of that of Charon, the masses of P2 and P1 are sufficiently low that their orbits are well described by the analytic theory. The variation in the orbital radius of P2 due to the forced oscillations is comparable in magnitude to that due to the best-fit Keplerian eccentricity, and there is at present no evidence that P2 has any significant epicyclic eccentricity. However, the orbit of P1 has a significant epicyclic eccentricity, and the prograde precession of its longitude of periapse with a period of 5300 days should be easily detectable. If the albedos of P2 and P1 are as low as that of comets, the large inferred masses induce significant short-term variations in the epicyclic eccentricities and/or periapse longitudes on the 400-500-day timescales due to the proximity to the 3:2 commensurability. In fact, for the maximum inferred masses, P2 and P1 may be in the 3:2 mean-motion resonance, with the resonance variable involving the periapse longitude of P1 librating. Observations that sample the orbits of P2 and P1 well on the 400-500-day timescales should provide strong constraints on the masses of P2 and P1 in the near future.  相似文献   

Celestial-mechanical peculiarities of Uranus’s satellite system     

M. A. Vashkov’yak 《Astronomy Letters》2001,27(7):464-469

We consider the structural peculiarities of Uranus’s satellite system associated with its separation into two groups: inner equatorial satellites moving in nearly circular orbits and distant irregular satellites with retrograde motion in highly elliptical orbits. The intermediate region is free from satellites in a wide range of semimajor axes. By analyzing the evolution of satellite orbits under the combined effect of solar attraction and Uranus’s oblateness, we offer a celestial-mechanical explanation for the absence of equatorial satellites in this region. M.L. Lidov’s studies during 1961–1963 have served as a basis for our analysis.  相似文献   

Final tidal evolution of orbit-orbit resonances     

Th. Pauwels 《Celestial Mechanics and Dynamical Astronomy》1994,59(2):101-127

In this paper we describe a model for the tidal evolution of an orbit-orbit resonance between two satellites of the same planet. We let the system evolve till infinity or until the resonance is destroyed. We find that there are asymptotic values for the eccentricities and inclinations. We list the possible final stages that a resonance can achieve, we give a few examples, and finally we discuss the limitations of the model and its possible applications to real systems.  相似文献   

The dependence of protoplanet migration rates on co-orbital torques     

G. D'Angelo  M. R. Bate  S. H. Lubow 《Monthly notices of the Royal Astronomical Society》2005,358(2):316-332

  相似文献   

Three-dimensional calculations of high- and low-mass planets embedded in protoplanetary discs     

M. R. Bate  S. H. Lubow  G. I. Ogilvie  K. A. Miller 《Monthly notices of the Royal Astronomical Society》2003,341(1):213-229

We analyse the non-linear, three-dimensional response of a gaseous, viscous protoplanetary disc to the presence of a planet of mass ranging from 1 Earth mass (1 M_⊕) to 1 Jupiter mass (1 M_J) by using the zeus hydrodynamics code. We determine the gas flow pattern, and the accretion and migration rates of the planet. The planet is assumed to be in a fixed circular orbit about the central star. It is also assumed to be able to accrete gas without expansion on the scale of its Roche radius. Only planets with masses   M _p≳ 0.1 M_J  produce significant perturbations in the surface density of the disc. The flow within the Roche lobe of the planet is fully three-dimensional. Gas streams generally enter the Roche lobe close to the disc mid-plane, but produce much weaker shocks than the streams in two-dimensional models. The streams supply material to a circumplanetary disc that rotates in the same sense as the orbit of the planet. Much of the mass supply to the circumplanetary disc comes from non-coplanar flow. The accretion rate peaks with a planet mass of approximately 0.1 M_J and is highly efficient, occurring at the local viscous rate. The migration time-scales for planets of mass less than 0.1 M_J, based on torques from disc material outside the Roche lobes of the planets, are in excellent agreement with the linear theory of type I (non-gap) migration for three-dimensional discs. The transition from type I to type II (gap) migration is smooth, with changes in migration times of about a factor of 2. Starting with a core which can undergo runaway growth, a planet can gain up to a few M_J with little migration. Planets with final masses of the order of 10 M_J would undergo large migration, which makes formation and survival difficult.  相似文献   

未来的国际天球参考架     

金文敬  李东明  唐正宏  王叔和 《紫金山天文台台刊》2003,(1)

本文简述了国际天球参考架的发展历史和现在射电参考架的现状—基准源选择的标准和参考架的稳定性。描述了地面上光学观测在依巴谷参考架的维持和加密的一系列工作。介绍由天体测量卫星GAIA和SIM给出的天球参考架可能逵到的精度。详述了在今后十年中地面天体测量的作用以及正在开展有关天球参考架的研究课题 ,同时也列出了我国正在和即将开展天体测量的几个研究课题  相似文献   

Observations with the Visual and Infrared Mapping Spectrometer (VIMS) during Cassini's flyby of Jupiter     

R.H. Brown  K.H. Baines  J.-P. Bibring  F. Capaccioni  R.N. Clark  D.P. Cruikshank  V. Formisano  Y. Langevin  T.B. McCord  V. Mennella  P.D. Nicholson  C. Sotin  M.A. Chamberlain  G. Hansen  M. Showalter 《Icarus》2003,164(2):461-470

The Cassini Visual and Infrared Mapping Spectrometer (VIMS) is an imaging spectrometer covering the wavelength range 0.3-5.2 μm in 352 spectral channels, with a nominal instantaneous field of view of 0.5 mrad. The Cassini flyby of Jupiter represented a unique opportunity to accomplish two important goals: scientific observations of the jovian system and functional tests of the VIMS instrument under conditions similar to those expected to obtain during Cassini's 4-year tour of the saturnian system. Results acquired over a complete range of visual to near-infrared wavelengths from 0.3 to 5.2 μm are presented. First detections include methane fluorescence on Jupiter, a surprisingly high opposition surge on Europa, the first visual-near-IR spectra of Himalia and Jupiter's optically-thin ring system, and the first near-infrared observations of the rings over an extensive range of phase angles (0-120°). Similarities in the center-to-limb profiles of H⁺₃ and CH₄ emissions indicate that the H⁺₃ ionospheric density is solar-controlled outside of the auroral regions. The existence of jovian NH₃ absorption at 0.93 μm is confirmed. Himalia has a slightly reddish spectrum, an apparent absorption near 3 μm, and a geometric albedo of 0.06±0.01 at 2.2 μm (assuming an 85-km radius). If the 3-μm feature in Himalia's spectrum is eventually confirmed, it would be suggestive of the presence of water in some form, either free, bound, or incorporated in layer-lattice silicates. Finally, a mean ring-particle radius of 10 μm is found to be consistent with Mie-scattering models fit to VIMS near-infrared observations acquired over 0-120° phase angle.  相似文献   

北斗三星无源导航定位技术研究     

方兆宝  夏哲仁  赵培海  周琼华  田震华  许泽明 《海洋测绘》2007,27(2):15-17

简述北斗三星无源导航定位的原理及定位误差源。探讨用最优估计理论和方法,即卡尔曼滤波技术,消除北斗三星无源定位数据中随机误差的方法。对华南沿海北斗三星无源定位实验数据进行滤波处理,结果表明:(1)北斗三星无源定位单点定位精度在纬度方向为±58m,经度方向为±4.3m,经卡尔曼滤波后其单点定位精度在纬度方向为±43m,经度方向为±3.6m;(2)北斗三星定位误差主要分布在纬度方向;(3)通过卡尔曼滤波可使北斗三星无源定位精度提高25%。  相似文献   

On the tidal variation of the geopotential     

José M. Ferrándiz  Juan Getino 《Celestial Mechanics and Dynamical Astronomy》1993,57(1-2):279-292

In this paper analytical expressions are derived for the temporal variations ofJ ₂ andJ ₂₂ due to the tides of the solid Earth, taking into account only the deformation of the mantle, and employing a procedure already used by the authors in their Hamiltonian theory of the Earth's rotation, which obtain the necessary parameters in a direct way by integration of those provided by a selected model of Earth interior.Numerical tables giving the periodic variation of coefficients are given, as well as a new prediction for UT1. For J ₂ and J ₂₂ the amplitudes reach such a magnitude that both two variations should not be ignored in studies involving the analysis of highly precise satellite tracking data. Moreover, the possibility of improving our knowledge of the value of those harmonic coefficients in only a more exact digit appears as to be strongly dependent on the limitations in the theoretical modeling of the variations of the inertia tensor due to solid tides.  相似文献