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1.
This study shows dependence of the surface diurnal tidal stress on the internal structure of Europa. Its purpose is to investigate possibility of cracking of the icy shell. The stress is evaluated under the plausible model of the internal structure constrained by the gravity field data. The possible effective stress at the sub-Jovian point decreases with thickening of the shell, while it does not depend on the core radius and the thickness of the H2O layer. The range of this value is from 0.095 to 0.161 MPa, which does not exceed the tensile strength of ice. The stress required for the surface cracking would be mainly due to longer period deformations, especially non-synchronous rotation. And/or the actual strength of the ice at the surface would be smaller because of the preexisting cracks than that at a laboratory of the same temperature. 相似文献
2.
A process to make massive ice in the martian regolith using long-term diffusion and thermal cracking
David Andrew Fisher 《Icarus》2005,179(2):387-397
This paper describes a “simple standard” model of water transport through regolith that includes diffusive migration and phase changes driven by damped seasonal temperature waves. A hitherto unused first-order process is then added, that can produce ice densities much greater than those allowed by the initial dry porosity. Voids are produced in cooling icy regolith when tensile stresses exceed the cracking threshold . These stresses build up through an interaction of thermal contraction and elastic-plastic response. When the cracks open up after tensile failure there is purely thermal void enhancement and subsequent reduction as the regolith warms again. When the cracks are open the porosity is increased and they partially fill with ice crystals. Thus the void reduction on warming cannot go back to the original zero point and the bulk density of ice is increased with each temperature cycle. The cracking and thermal adjustment happen at scales of meters to millimeters. The large cracks can occur in pure ice and/or homogeneous icy material and the smaller cracks are produced by rock cobbles, pebbles, and grains having a much smaller coefficient of thermal expansion than ice. Thus a hierarchy of cracks and voids forms each temperature cycle and augments the ice content. The process can take the upper few meters of a pore-saturated icy soil from 28% by mass ice content to 70% in 10 Ma. This mechanism and the seasonal temperature cycle can plausibly produce massive ice deposits in the upper few meters of Mars' high-latitude regolith by diffusion and also keep the massive-ice regolith effectively porous to water vapor transport. The obliquity cycle can produce tensile stresses nearing 2 MPa down to depth so even deeper cracking could be happening. 相似文献
3.
At martian mid-to-high latitudes, the surfaces of potentially ice-rich features, including concentric crater fill, lobate debris aprons, and lineated valley fill, typically display a complex texture known as “brain terrain,” due to its resemblance to the complex patterns on brain surfaces. In order to determine the structure and developmental history of concentric crater fill and overlying latitude-dependent mantle (LDM) material, “brain terrain” and polygonally-patterned LDM surfaces are analyzed using HiRISE images from four craters in Utopia Planitia containing concentric crater fill. “Brain terrain” and mantle surface textures are classified based on morphological characteristics: (1) closed-cell “brain terrain,” (2) open-cell “brain terrain,” (3) high-center mantle polygons, and (4) low-center mantle polygons. A combined glacial and thermal-contraction cracking model is proposed for the formation and modification of the “brain terrain” texture of concentric crater fill. A similar model, related to thermal contraction cracking and differential sublimation of underlying ice, is proposed for the formation and development of polygonally patterned mantle material. Both models require atmospheric deposition of ice, likely during periods of high obliquity, but do not require wet active layer processes. Crater dating of “brain terrain” and mantled surfaces suggests a transition at martian mid-latitudes from peak “glacial” conditions occurring within the past ∼10-100 My to a quiescent period followed by a cold-desert “periglacial” period during the past ∼1-2 My. 相似文献
4.
Abstract— Dynamic tensile strengths and fracture strengths of 3 terrestrial rocks, San Marcos gabbro, Coconino sandstone, and Sesia eclogite were determined by carrying out flat‐plate (PMMA and aluminum) impact experiments on disc‐shaped samples in the 5 to 60 m/sec range. Tensile stresses of 125 to 300 MPa and 245 to 580 MPa were induced for gabbro and eclogite, respectively (with duration time of ?1 μs). For sandstone (porosity 25%), tensile stresses normal to bedding of ?13 to 55 MPa were induced (with duration times of 2.4 and ?1.4 μs). Tensile crack failure was detected by the onset of shock‐induced (damage) P and S wave velocity reduction. The dynamic tensile strength of gabbro determined from P and S wave velocity deficits agrees closely with the value of previously determined values by post‐impact microscopic examination (?150 MPa). Tensile strength of Coconino sandstone is 20 MPa for a 14 μs duration time and 17 MPa for a 2.4 μs duration time. For Sesia eclogite, the dynamic tensile strength is ?240 MPa. The fracture strength for gabbro is ?250 MPa, ?500 MPa for eclogite, and ?40 MPa for sandstone. Relative crack‐induced reduction of S wave velocities is less than that of post‐impact P wave velocity reductions for both gabbro and eclogite, indicating that the cracks were predominantly spall cracks. Impacts upon planetary surfaces induce tensile failure within shock‐processed rocks beneath the resulting craters. The depth of cracking beneath impact craters can be determined both by seismic refraction methods for rocks of varying water saturation and, for dry conditions (e.g., the Moon), from gravity anomalies. In principle, depth of cracking is related to the equations‐of‐state of projectile and target, projectile dimension, and impact velocity. We constructed a crack‐depth model applicable to Meteor Crater. For the observed 850 m depth of cracking, our preferred strength scaling model yields an impact velocity of 33 km/s and impactor radius of 9 m for an iron projectile. 相似文献
5.
The proposed past eruption of liquid water on Europa and ongoing eruption of water vapor and ice on Enceladus have led to discussion about the feasibility of cracking a planetary ice shell. We use a boundary element method to model crack penetration in an ice shell subjected to tension and hydrostatic compression. We consider the presence of a region at the base of the ice shell in which the far-field extensional stresses vanish due to viscoelastic relaxation, impeding the penetration of fractures towards a subsurface ocean. The maximum extent of fracture penetration can be limited by hydrostatic pressure or by the presence of the unstressed basal layer, depending on its thickness. Our results indicate that Europa's ice shell is likely to be cracked under 1-3 MPa tension only if it is ?2.5 km thick. Enceladus' ice shell may be completely cracked if it is capable of supporting ∼1-3 MPa tension and is less than 25 km thick. 相似文献
6.
Each of the Galilean satellites, as well as most other satellites whose initial rotations have been substantially altered by tidal dissipation, has been widely assumed to rotate synchronously with its orbital mean motion. Such rotation would require a small permanent asymmetry in the mass distribution in order to overcome the small mean tidal torque. Since Io and Europa may be substantially fluid, they may not have the strenght to support the required permanent asymmetry. Thus, each may rotate at the unknown but slightly nonsynchronous rate that corresponds to zero mean tidal torque. This behaviour may be observable by Galileo spacecraft imaging. It may help explain the longitudinal variation of volcanism on Io and the cracking of Europa's crust. 相似文献
7.
F. A. Levi 《Meteoritics & planetary science》1973,8(3):209-221
Periodic thermal excursions, even in a low-temperature range, may cause internal stresses and give origin to some of the observed plastic and clastic deformations in stony meteorites. An experimental test confirms our hypothesis: lattice distortions, cleavage and cracking were artificially induced in specimens of two H-chondrites (Mills and Bur-Gheluai) by low-temperature “thermal fatigue.” The observed effects appear to be increasing with increasing number of cycles. 相似文献
8.
Sandro Da Silva Fernandes 《Celestial Mechanics and Dynamical Astronomy》1994,58(3):297-308
The classic Lagrange's expansion of the solutionE(e, M) of Kepler's equation in powers of eccentricity is extended to highly eccentric orbits, 0.6627 ... <e<1. The solutionE(e, M) is developed in powers of (e–e*), wheree* is a fixed value of the eccentricity. The coefficients of the expansion are given in terms of the derivatives of the Bessel functionsJ
n
(ne). The expansion is convergent for values of the eccentricity such that |e–e*|<(e*), where the radius of convergence (e*) is a positive real number, which is calculated numerically. 相似文献
9.
The magnetic field in the solar corona plays an important role in coronal heating, flaring activity and many other phenomena studied on the Sun. Magnetic topology is frequently used to understand complicated coronal magnetic fields. By calculating the skeleton of a field, it is possible to build up a sophisticated representation of the key elements of a field’s configuration. This paper determines a simple relation between the numbers of separators (X), coronal null points (Nc), flux domains (D) and flux sources (S) in such a configuration: D=X+S−Nc−1. This equation is used to explain the behaviour of some of the bifurcations found in Magnetic Charge Topology, and to show that a one-to-one relationship exists between the number of circuits in the domain graph and the augmented null graph. Finally, it is shown that in quiet-Sun regions, the number of separators is approximately proportional to the number of flux sources. 相似文献
10.
Bijan Saha 《Astrophysics and Space Science》2007,312(1-2):3-11
We consider a system of nonlinear spinor and a Bianchi type I gravitational fields in presence of viscous fluid. The nonlinear
term in the spinor field Lagrangian is chosen to be λ
F, with λ being a self-coupling constant and F being a function of the invariants I an J constructed from bilinear spinor forms S and P. Self-consistent solutions to the spinor and BI gravitational field equations are obtained in terms of τ, where τ is the volume scale of BI universe. System of equations for τ and ε, where ε is the energy of the viscous fluid, is deduced. This system is solved numerically for some special cases.
相似文献
11.
The equation of radiative acoustics is derived by taking into account the effect of a transverse magnetic field, which is quite similar to the acoustic equation derived in Paper I. The only difference is that theadiabatic, isothermal, andisentropic speeds of sound and theradiation-acoustic speed are replaced by theadiabatic, isothermal, andisentropic magnetoacoustic speeds and theradiation-magnetoacoustic speed, respectively. The main results shown in Paper I are valid even in the presence of a transverse magnetic field. 相似文献
12.
A series of solar cm-radio bursts are analyzed by a new inverse method estimating spatial changes of the superthermal electron
distribution in solar cm-radio burst sources. It is found that the measure of the spatial change of superthermal electrons
in the radio source ν
n
is always greater than that for the magnetic field ν
B
and it is linearly dependent on the spectral index of the electrons δ as ν
n
≈0.5δ. This relation is explained in the simplified flare-loop model integrating the analytical solutions of the Fokker – Planck
equation. The mean value of ν
B
is found to be 0.36±0.04, which is very close to the value of ν
B
=0.38±0.02 derived from the dependence of the magnetic field strength on the height in the active region measured by RATAN-600. 相似文献
13.
Kuniaki Masai 《Astrophysics and Space Science》1984,106(2):391-407
The effect of X-ray irradiation on the line-driven stellar wind in X-ray binary systems is studied. The product of the X-ray luminosityL
x and the densityn in the wind is a measure to distinguish between the optically thin and thick wind regimes. For an X-ray source of the radiation temperature of 10 keV, the critical value(L
x
n)
c
, is of the order of 1037 erg s–1 cm–3; hence, most of wind-fed X-ray sources lie in the optically thick wind regime because ofL
x
n>(L
x
n)
c
. Then the wind structure is determined not only by the parameter =(L
x
/nR
2),R the distance from the X-ray source, but also by the optical thickness due to helium. The formation of fully ionized helium region depends onL
x
andn in a way different from that of the Strömgren sphere. In such an optically thick wind, the region behind the He II ionization boundary is little affected by X-ray irradiation and the trace elements remain to be responsible for wind acceleration. Thus, its location is important for the structure of the wind and the interpretation of various phenomena in objects such as wind-fed X-ray pulsars. 相似文献
14.
Three wavelet functions: the Morlet wavelet, the Paul wavelet, and the DOG wavelet have been respectively performed on both
the monthly Wolf sunspot numbers (Rz) from January 1749 to May 2004 and the monthly group sunspot numbers (Rg) from June 1795 to December 1995 to study the evolution of the Gleissberg and Schwabe periods of solar activity. The main
results obtained are (1) the two most obvious periods in both the Rz and Rg are the Schwabe and Gleissberg periods. The Schwabe period oscillated during the second half of the eighteenth century and
was steady from the 1850s onward. No obvious drifting trend of the Schwabe period exists. (2) The Gleissberg period obviously
drifts to longer periods the whole consideration time, and the drifting speed of the Gleissberg period is larger for Rz than for Rg. (3) Although the Schwabe-period values for Rz and Rg are about 10.7 years, the value for Rz seems slightly larger than that for Rg. The Schwabe period of Rz is highly significant after the 1820s, and the Schwabe period of Rg is highly significant over almost the whole consideration time except for about 20 years around the 1800s. The evolution
of the Schwabe period for both Rz and Rg in time is similar to each other. (4) The Gleissberg period in Rz and Rg is highly significant during the whole consideration time, but this result is unreliable at the two ends of each of the time
series of the data. The evolution of the Gleissberg period in Rz is similar to that in Rg. 相似文献
15.
Fluid inclusions studies in quartz and calcite in samples from the ICDP‐Chicxulub drill core Yaxcopoil‐1 (Yax‐1) have revealed compelling evidence for impact‐induced hydrothermal alteration. Fluid circulation through the melt breccia and the underlying sedimentary rocks was not homogeneous in time and space. The formation of euhedral quartz crystals in vugs hosted by Cretaceous limestones is related to the migration of hot (>200 °C), highly saline, metal‐rich, hydrocarbon‐bearing brines. Hydrocarbons present in some inclusions in quartz are assumed to derive from cracking of pre‐impact organic matter. The center of the crater is assumed to be the source of the hot quartz‐forming brines. Fluid inclusions in abundant newly‐formed calcite indicate lower cyrstallization temperatures (75–100 °C). Calcite crystallization is likely related to a later stage of hydrothermal alteration. Calcite precipitated from saline fluids, most probably from formation water. Carbon and oxygen isotope compositions and REE distributions in calcites and carbonate host rocks suggest that the calcite‐forming fluids have achieved close equilibrium conditions with the Cretaceous limestones. The precipitation of calcite may be related to the convection of local pore fluids, possibly triggered by impact‐induced conductive heating of the sediments. 相似文献
16.
In this paper we compare outcomes of some extended phantom-like cosmologies with each other and also with ΛCDM and ΛDGP. We
focus on the variation of the luminosity distances, the age of the universe and the deceleration parameter versus the redshift
in these scenarios. In a dynamical system approach, we show that the accelerating phase of the universe in the f(R)-DGP scenario is stable if one consider the curvature fluid as a phantom scalar field in the equivalent scalar-tensor theory, otherwise it is a transient and unstable phenomenon. Up
to the parameters values adopted in this paper, the extended F(R,ϕ)-DGP scenario is closer to the ΛCDM scenario than other proposed models. All of these scenarios explain the late-time cosmic
speed-up in their normal DGP branches, but the redshift at which transition to the accelerating phase occurs are different:
while the ΛDGP model transits to the accelerating phase much earlier, the F(R,ϕ)-DGP model transits to this phase much later than other scenarios. Also, within the parameter spaces adopted in this paper,
the age of the universe in the f(R)-DGP model is larger than ΛCDM, but this age in F(G,ϕ)-DGP is smaller than ΛCDM. 相似文献
17.
Arnold Stokes 《Celestial Mechanics and Dynamical Astronomy》1978,17(2):137-144
When integrating a perturbed two-body problem, very often the propagation of the numerical error is reduced by using a new time variables defined by dt/ds=|q|
n
, (|q| is the radial distance,t the time). This paper introduces a time element for such transformations, i.e., a new variablet
n is defined so that dt
n/ds=1+ (perturbing terms) andt=F
n(tn), whereF
n is a known function. The time element equation should be useful in reducing the error in the determination of the timet.F
n is given explicitly forn=1, 3/2, 2, 5/2 and 3, and a general expression is given for other values.The work was performed while the author was an NRC Senior Research Associate, Goddard Space Flight Center, Greenbelt, Md., U.S.A. 相似文献
18.
The temperature profile of the KTB pilot drillhole, T(z)KTB-PH,is distinctly nonlinear: a temperature deficit ΔT (relative to a linear temperature-depth profile) is especially pronounced in the depth range 500–3500 m. The depth dependence of the deficit, Δ(z) is compared to be anticipated effect of surface paleoclimatic variations, ΔTpc(z), at the drillsite on the temperature profile. The latter can be calculated from available paleo-climatic models. If ΔTpc(z) is added to T(z)KTB-PH, a nearly linear temperature-depth curve results with an average geothermal gradient of 27.9°C/km. This, together with an average vertical thermal conductivity of 3.0 W/mK, estimated from KTB drillcore data, implies a heat flow density at the KTB site of 84 mW/m2. This modelled value is in good agreement with heat flow determinations in the adjacent Eger graben structure (Western Bohemian massif). 相似文献
19.
J. Klačka 《Earth, Moon, and Planets》1993,62(2):145-148
Action of electromagnetic radiation on nonspherical dust particles is discussed. It is stressed that the radiation pressure coefficientQ
PR
cannot be considered to be a scalar quantity, as it is used in all calculations for dynamical studies of interplanetary dust particles. Also the equation <Q
PR
A> = <Q
PR
><A> (A - area of the particle) holds only for perfectly absorbing convex dust particle (Q
PR
= 1) and not even one of these two properties holds for interplanetary dust particles. Plane mirror is discussed in detail - all calculations can be done in this simple case. 相似文献
20.
We discuss the f(R) gravity model in which the origin of dark energy is identified as a modification of gravity. The Noether symmetry with gauge
term is investigated for the f(R) cosmological model. By utilization of the Noether Gauge Symmetry (NGS) approach, we obtain two exact forms f(R) for which such symmetries exist. Further it is shown that these forms of f(R) are stable. 相似文献