A WKBJ spectral method for computation of SV synthetic seismograms in a cylindrically symmetric medium     

Chang-Eob Baag  Charles A. Langston 《Geophysical Journal International》1985,80(2):387-417

Summary. A method of synthetic seismogram computation for teleseismic SV -waves is developed in order to treat quantitatively SV -waves in problems of body wave source inversion and source—receiver structure studies. The method employs WKBJ theory for a generalized ray in a vertically inhomogeneous half-space and the propagator matrix technique for waves in near-surface homogeneous layers. Wavenumber integration is done along the real axis of the wavenumber plane and anelasticity is included by using complex velocity in all regions of the earth model. The near-surface source structure is taken into account in the computation for the case of the shallow source by allowing a point source to be located in the homogeneous layers. Source and receiver area structures are also allowed to differ. A general moment tensor point source is considered.  相似文献   

A perturbative treatment of motion near the 3/1 commensurability     

Jack Wisdom 《Icarus》1985,63(2):272-289

A semianalytic perturbation theory for motion near the 3/1 commensurability in the planar elliptic restricted three-body problem is presented. The predictions of the theory are in good agreement with the features found on numerically generated surfaces of section; a global understanding of the phase space is achieved. The unusual features of the motion discovered by J. Wisdom (1982, Astron. J.87, 577–593; 1983a, Icarus56, 51–74) are explained. The principal cause of the large chaotic zone near the 3/1 commensurability is identified, and a new criterion for the existence of large-scale chaotic behavior is presented.  相似文献   

Age and early metamorphic history of the Sanbagawa belt: Lu–Hf and P–T constraints from the Western Iratsu eclogite     

S. ENDO  S.WALLIS  T. HIRATA  R. ANCZKIEWICZ  J. PLATT  M. THIRLWALL  Y. ASAHARA 《Journal of Metamorphic Geology》2009,27(5):371-384

Two distinct age estimates for eclogite-facies metamorphism in the Sanbagawa belt have been proposed: (i) c.  120–110 Ma based on a zircon SHRIMP age for the Western Iratsu unit and (ii) c.  88–89 Ma based on a garnet–omphacite Lu–Hf isochron age from the Seba and Kotsu eclogite units. Despite the contrasting estimates of formation ages, petrological studies suggest the formation conditions of the Western Iratsu unit are indistinguishable from those of the other two units—all ∼20 kbar and 600–650 °C. Studies of the associated geological structures suggest the Seba and Western Iratsu units are parts of a larger semi-continuous eclogite unit. A combination of geochronological and petrological studies for the Western Iratsu eclogite offers a resolution to this discrepancy in age estimates. New Lu–Hf dating for the Western Iratsu eclogite yields an age of 115.9 ± 0.5 Ma that is compatible with the zircon SHRIMP age. However, petrological studies show that there was significant garnet growth in the Western Iratsu eclogite before eclogite facies metamorphism, and the early core growth is associated with a strong concentration of Lu. Pre-eclogite facies garnet (Grt1) includes epidote–amphibolite facies parageneses equilibrated at 550–650 °C and ∼10 kbar, and this is overgrown by prograde eclogite facies garnet (Grt2). The Lu–Hf age of c.  116 Ma is strongly skewed to the isotopic composition of Grt1 and is interpreted to reflect the age of the pre-eclogite phase. The considerable time gap ( c.  27 Myr) between the two Lu–Hf ages suggests they may be related to separate tectonic events or distinct phases in the evolution of the Sanbagawa subduction zone.  相似文献   

Strength of the lithosphere of the Galilean satellites     

Karen Luttrell  David Sandwell 《Icarus》2006,183(1):159-167

Several approaches have been used to estimate the ice shell thickness on Callisto, Ganymede, and Europa. Here we develop a method for placing a strict lower bound on the thickness of the strong part of the shell (lithosphere) using measurements of topography. The minimal assumptions are that the strength of faults in the brittle lithosphere is controlled by lithostatic pressure according to Byerlee's law and the shell has relatively uniform density and thickness. Under these conditions, the topography of the ice provides a direct measure of the bending moment in the lithosphere. This topographic bending moment must be less than the saturation bending moment of the yield strength envelope derived from Byerlee's law. The model predicts that the topographic amplitude spectrum decreases as the square of the topographic wavelength. This explains why Europa is rugged at shorter wavelengths (∼10 km) but extremely smooth, and perhaps conforming to an equipotential surface, at longer wavelengths (>100 km). Previously compiled data on impact crater depth and diameter [Schenk, P.M., 2002. Nature 417, 419-421] on Europa show good agreement with the spectral decrease predicted by the model and require a lithosphere thicker than 2.5 km. A more realistic model, including a ductile lower lithosphere, requires a thickness greater than 3.5 km. Future measurements of topography in the 10-100 km wavelength band will provide tight constraints on lithospheric strength.  相似文献   

Thick and thin models of the evolution of carbon dioxide on Mars     

Curtis V. Manning  Christopher P. McKay 《Icarus》2006,180(1):38-59

We present results from a new simulation code that accounts for the evolution of the reservoirs of carbon dioxide on Mars, from its early years to the present. We establish a baseline model parameter set that produces results compatible with the present (i.e., Patm?6.5 mbar with permanent CO₂ ice cap) for a wide range of initial inventories. We find that the initial inventory of CO₂ broadly determines the evolutionary course of the reservoirs of CO₂. The reservoirs include the atmosphere, ice cap, adsorbed CO₂ in the regolith, and carbonate rocks. We track the evolution of the free inventory: the atmosphere, ice cap and regolith. Simulations begin at 4.53 Gyr before present with a rapid loss of free inventory to space in the early Noachian. Models that assume a relatively small initial inventory (?5 bar) have pronounced minima in the free inventory of CO₂ toward the end of the Noachian. Under baseline parameters, initial inventories below ∼4.5 bar result in a catastrophic loss of the free inventory to space. The current free inventory would be then determined by the balance between outgassing, sputtering losses and chemical weathering following the end of the late bombardment. We call these “thin” models. They generically predict small current free inventories in line with expectations of a small present CO₂ ice cap. For “thick” models, with initial inventories ?5 bar, a surplus of 300-700 mbar of free CO₂ remains during the late-Noachian. The histories of free inventory in time for thick models tend to converge within the last 3.5 Gyr toward a present with an ice cap plus atmospheric inventory of about 100 mbar. For thick models, the convergence is largely due to the effects of chemical weathering, which draws down higher free inventories more rapidly than the low. Thus, thick models have ?450 mbar carbonate reservoirs, while thin models have ?200 mbar. Though both thick and thin scenarios can reproduce the current atmospheric pressure, the thick models imply a relatively large current CO₂ ice cap and thin models, little or none. While the sublimation of a massive cap at a high obliquity would create a climate swing of greenhouse warming for thick models, under the thin model, mean temperatures and pressures would be essentially unaffected by increases in obliquity.  相似文献   

Constraints on a planetary origin for the gap in the protoplanetary disc of GM Aurigae   总被引：1，自引：0，他引：1  

W. K. M. Rice  Kenneth Wood  P. J. Armitage  B. A. Whitney  J. E. Bjorkman 《Monthly notices of the Royal Astronomical Society》2003,342(1):79-85

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Old Pulsators: White Dwarfs and Their Immediate Precursors     

M. Sean O'Brien 《Astrophysics and Space Science》2003,284(1):45-52

There are now fully six classes of pulsators among white dwarfsand their immediate precursors among central stars of planetarynebulae and on the extended horizontal branch.In this review, we outline those observational and theoreticalconsiderations that link them together and set them apart fromother kinds of pulsating stars.We summarize some select astrophysical puzzles to which studiesof such pulsators might speak, and we discuss current applicationsin the fields of atomic, nuclear, and neutrino physics.Finally, we suggest how future observing programs might solve somegeneral problems common not only to the white dwarf and pre-whitedwarf pulsators but to many types of variable stars.  相似文献   

Astrometric signatures of self-gravitating protoplanetary discs     

W. K. M. Rice  P. J. Armitage  M. R. Bate  I. A. Bonnell 《Monthly notices of the Royal Astronomical Society》2003,338(1):227-232

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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