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1.
We study the Jupiter family comet (JFC) population assumed to come from the Scattered Disk and transferred to the Jupiter’s zone through gravitational interactions with the Jovian planets. We shall define as JFCs those with orbital periods and Tisserand parameters in the range 2<T?3.1, while those comets coming from the same source, but that do not fulfill the previous criteria (mainly because they have periods ) will be called ‘non-JFCs’. We performed a series of numerical simulations of fictitious comets with a purely dynamical model and also with a more complete dynamical-physical model that includes besides nongravitational forces, sublimation and splitting mechanisms. With the dynamical model, we obtain a poor match between the computed distributions of orbital elements and the observed ones. However with the inclusion of physical effects in the complete model we are able to obtain good fits to observations. The best fits are attained with four splitting models with a relative weak dependence on q, and a mass loss in every splitting event that is less when the frequency is high and vice versa. The mean lifetime of JFCs with radii and is found to be of about 150-200 revolutions (∼. The total population of JFCs with radii within Jupiter’s zone is found to be of 450±50. Yet, the population of non-JFCs with radii in Jupiter-crossing orbits may be ∼4 times greater, thus leading to a whole population of JFCs + non-JFCs of ∼2250±250. Most of these comets have perihelia close to Jupiter’s orbit. On the other hand, very few non-JFCs reach the Earth’s vicinity (perihelion distances ) which gives additional support to the idea that JFCs and Halley-type comets have different dynamical origins. Our model allows us to define the zones of the orbital element space in which we would expect to find a large number of JFCs. This is the first time, to our knowledge, that a physico-dynamical model is presented that includes sublimation and different splitting laws. Our work helps to understand the role played by these erosion effects in the distribution of the orbital elements and lifetimes of JFCs. 相似文献
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P. Székely L.L. Kiss Gy.M. Szabó K. Sárneczky B. Csák M. Váradi Sz. Mészáros 《Planetary and Space Science》2005,53(9):925-936
We present CCD photometric observations of 23 main-belt asteroids, of which 8 have never been observed before; thus, the data of these objects are the first in the literature. The majority showed well-detectable light variations, exceeding 0m1. We have determined synodic periods for 756 Lilliana (936), 1270 Datura (34), 1400 Tirela (1336), 1503 Kuopio (998), 3682 Welther (359), 7505 Furushu (414) and 11436 1969 QR (123), while uncertain period estimates were possible for 469 Argentina (123), 546 Herodias (104) and 1026 Ingrid (53). The shape of the lightcurves of 3682 Welther changed on a short time-scale and showed dimmings that might be attributed to eclipses in a binary system. For the remaining objects, only lower limits of the periods and amplitudes were concluded. 相似文献
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The non-thermal escape of neutral O atoms from Mars at the current epoch is largely due to dissociative recombination of :
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L.A. Rogers 《Planetary and Space Science》2005,53(13):1341-1354
Conventional meteoroid theory assumes that the dominant mode of ablation (which we will refer to as thermal ablation) is by evaporation following intense heating during atmospheric flight. Light production results from excitation of ablated meteoroid atoms following collisions with atmospheric constituents. In this paper, we consider the question of whether sputtering may provide an alternative disintegration process of some importance. For meteoroids in the mass range from 10-3 to and covering a meteor velocity range from 11 to , we numerically modeled both thermal ablation and sputtering ablation during atmospheric flight. We considered three meteoroid models believed to be representative of asteroidal ( mass density), cometary () and porous cometary () meteoroid structures. Atmospheric profiles which considered the molecular compositions at different heights were use in the sputtering calculations. We find that while in many cases (particularly at low velocities and for relatively large meteoroid masses) sputtering contributes only a small amount of mass loss during atmospheric flight, in some cases sputtering is very important. For example, a porous meteoroid at will lose nearly 51% of its mass by sputtering, while a asteroidal meteoroid at will lose nearly 83% of its mass by sputtering. We argue that sputtering may explain the light production observed at very great heights in some Leonid meteors. We discuss methods to observationally test the predictions of these computations. A search for early gradual tails on meteor light curves prior to the commencement of intense thermal ablation possibly represents the most promising approach. The impact of this work will be most dramatic for very small meteoroids such as those observed with large aperture radars. The heights of ablation and decelerations observed using these systems may provide evidence for the importance of sputtering. 相似文献
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We investigate the effects of atmospheric gravity waves on the vertical and horizontal structure of the ionosphere of Jupiter. The presented non-linear, two-dimensional model of the jovian ionosphere allows for spatially and temporally varying neutral wind and temperature fields and tracks the time evolution of six ionospheric species, , and . An analytical approach is used to validate the model results for linear, small-amplitude waves and to elucidate the mechanisms that leads to perturbations in the density of the main ion species, H+ and . We demonstrate that the long-lived H+ ions are perturbed directly by wave dynamics whereas short-lived ions such as are perturbed by chemical interactions with other perturbed ion species. The model is then applied using larger gravity wave amplitudes consistent with observations. Atmospheric gravity waves propagating at high altitudes create layers of enhanced electron density similar to the system of layers observed during the J0-ingress radio occultation of the Galileo spacecraft. Our best fit to the J0-ingress observation is achieved using an 82 min period forcing wave with horizontal and vertical wavelengths of 500 km and 60 km respectively, and peaks at 510 km above the 1 bar pressure level. We further investigate the effects of the wave-induced ion flux on the background ionospheric structure and demonstrate that in the presence of a gravity wave the background density profiles of the H+ and ions are significantly modified. We also find that the column density of has variations that can exceed 10% as the wave propagates. 相似文献
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The production of energetic and escaping neutral O atoms at the current epoch in the martian thermosphere is thought to be dominated by the dissociative recombination process:
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The giant planets of our solar system possess envelopes consisting mainly of hydrogen and helium but are also significantly enriched in heavier elements relatively to our Sun. In order to better constrain how these heavy elements have been delivered, we quantify the amount accreted during the so-called “late heavy bombardment”, at a time when planets were fully formed and planetesimals could not sink deep into the planets. On the basis of the “Nice model”, we obtain accreted masses (in terrestrial units) equal to for Jupiter, and for Saturn. For the two other giant planets, the results are found to depend mostly on whether they switched position during the instability phase. For Uranus, the accreted mass is with an inversion and without an inversion. Neptune accretes in models in which it is initially closer to the Sun than Uranus, and otherwise. With well-mixed envelopes, this corresponds to an increase in the enrichment over the solar value of 0.033±0.001 and 0.074±0.007 for Jupiter and Saturn, respectively. For the two other planets, we find the enrichments to be 2.1±1.4 (w/ inversion) or 1.2±0.7 (w/o inversion) for Uranus, and 2.0±1.2 (w/ inversion) or 2.7±1.6 (w/o inversion) for Neptune. This is clearly insufficient to explain the inferred enrichments of ∼4 for Jupiter, ∼7 for Saturn and ∼45 for Uranus and Neptune. 相似文献
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Michael Mueller Franck Marchis Joshua P. Emery Stefano Mottola Jérome Berthier 《Icarus》2010,205(2):505-515
We present mid-infrared observations of the binary L5-Trojan system (617) Patroclus-Menoetius before, during, and after two shadowing events, using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. For the first time, we effectively observe changes in asteroid surface temperature in real time, allowing the thermal inertia to be determined very directly. A new detailed binary thermophysical model is presented which accounts for the system’s known mutual orbit, arbitrary component shapes, and thermal conduction in the presence of eclipses.We obtain two local thermal-inertia values, representative of the respective shadowed areas: and . The average thermal inertia is estimated to be , potentially with significant surface heterogeneity. This first thermal-inertia measurement for a Trojan asteroid indicates a surface covered in fine regolith. Independently, we establish the presence of fine-grained (<a few μm) silicates on the surface, based on emissivity features near 10 and similar to those previously found on other Trojans.We also report V-band observations and report a lightcurve with complete rotational coverage. The lightcurve has a low amplitude of peak-to-peak, implying a roughly spherical shape for both components, and is single-periodic with a period equal to the period of the mutual orbit, indicating that the system is fully synchronized.The diameters of Patroclus and Menoetius are 106±11 and , respectively, in agreement with previous findings. Taken together with the system’s known total mass, this implies a bulk mass density of , significantly below the mass density of L4-Trojan asteroid (624) Hektor and suggesting a bulk composition dominated by water ice.All known physical properties of Patroclus, arguably the best studied Trojan asteroid, are consistent with those expected in icy objects with devolatilized surface (extinct comets), consistent with what might be implied by recent dynamical modeling in the framework of the Nice Model. 相似文献
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Thomas K. Greathouse G.R. Gladstone S.A. Stern R.J. Vervack Jr. M.H. Versteeg L.A. Young H. Throop 《Icarus》2010,208(1):293-305
The Alice ultraviolet spectrograph onboard the New Horizons spacecraft observed two occultations of the bright star χ Ophiucus by Jupiter’s atmosphere on February 22 and 23, 2007 during the approach phase of the Jupiter flyby. The ingress occultation probed the atmosphere at 32°N latitude near the dawn terminator, while egress probed 18°N latitude near the dusk terminator. A detailed analysis of both the ingress and egress occultations, including the effects of molecular hydrogen, methane, acetylene, ethylene, and ethane absorptions in the far ultraviolet (FUV), constrains the eddy diffusion coefficient at the homopause level to be cm2 s−1, consistent with Voyager measurements and other analyses (Festou, M.C., Atreya, S.K., Donahue, T.M., Sandel, B.R., Shemansky, D.E., Broadfoot, A.L. [1981]. J. Geophys. Res. 86, 5717-5725; Vervack Jr., R.J., Sandel, B.R., Gladstone, G.R., McConnell, J.C., Parkinson, C.D. [1995]. Icarus 114, 163-173; Yelle, R.V., Young, L.A., Vervack Jr., R.J., Young, R., Pfister, L., Sandel, B.R. [1996]. J. Geophys. Res. 101 (E1), 2149-2162). However, the actual derived pressure level of the methane homopause for both occultations differs from that derived by
[Festou et al., 1981] and [Yelle et al., 1996] from the Voyager ultraviolet occultations, suggesting possible changes in the strength of atmospheric mixing with time. We find that at 32°N latitude, the methane concentration is cm−3 at 70,397 km, the methane concentration is cm−3 at 70,383 km, the acetylene concentration is cm−3 at 70,364 km, and the ethane concentration is cm−3 at 70,360 km. At 18°N latitude, the methane concentration is cm−3 at 71,345 km, the methane concentration is cm−3 at 71,332 km, the acetylene concentration is cm−3 at 71,318 km, and the ethane concentration is cm−3 at 71,315 km. We also find that the H2 occultation light curve is best reproduced if the atmosphere remains cold in the microbar region such that the base of the thermosphere is located at a lower pressure level than that determined by in situ instruments aboard the Galileo probe (Seiff, A., Kirk, D.B., Knight, T.C.D., Young, R.E., Mihalov, J.D., Young, L.A., Milos, F.S., Schubert, G., Blanchard, R.C., Atkinson, D. [1998]. J. Geophys. Res. 103 (E10), 22857-22889) - the Sieff et al. temperature profile leads to too much absorption from H2 at high altitudes. However, this result is highly model dependent and non-unique. The observations and analysis help constrain photochemical models of Jupiter’s atmosphere. 相似文献
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We have obtained numerically integrated orbits for Saturn's coorbital satellites, Janus and Epimetheus, together with Saturn's F-ring shepherding satellites, Prometheus and Pandora. The orbits are fit to astrometric observations acquired with the Hubble Space Telescope and from Earth-based observatories and to imaging data acquired from the Voyager spacecraft. The observations cover the 38 year period from the 1966 Saturn ring plane crossing to the spring of 2004. In the process of determining the orbits we have found masses for all four satellites. The densities derived from the masses for Janus, Epimetheus, Prometheus, and Pandora in units of g cm−3 are , , , and , respectively. 相似文献
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M. Grott 《Planetary and Space Science》2009,57(1):71-77
A measurement of the martian planetary heat flow requires the determination of the subsurface temperature gradient, which is affected by surface insolation. I investigate the propagation of thermal disturbances caused by lander shadowing and derive measurement requirements for in situ heat flow experiments. I find that for short term measurements spanning 180 sol, a measurement depth of at least 2 m is needed to guarantee a stable thermal environment directly underneath the lander for Moon-like thermal conductivities of . For extremely large conductivities of , this depth needs to be increased to 4 m, but if the probe can be deployed outside the lander structure, the respective depths can be decreased by 1 m. For long term measurements spanning at least a full martian year heat flow perturbations are smaller than 5% below a depth of 3 m directly underneath the lander. Outside the lander structure, essentially unperturbed measurements may be conducted at depths of 0.5 and 1.5 m for thermal conductivities of 0.02 and , respectively. 相似文献
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Stéphane Erard Bruno Bézard Alain Doressoundiram Daniela Despan 《Planetary and Space Science》2011,59(15):1842-1852
Mercury has been observed in June 2006 with the SofI instrument at NTT. Resolved spectroscopy was performed in the range. The observations were acquired close to a favorable quadrature with maximum elongation, and span mostly the southern hemisphere in the 235–315°E longitude area.In contrast to recent observations from IRTF, the present study does not indicate any pyroxene absorption in the near-infrared range, neither in the region nor at . Detailed spectral analysis allows to derive an upper limit of 1.5% type-A pyroxene (hedenbergite) or 1.1% enstatite (assuming minimum lunar iron content) in the regions observed, at the 700 km scale. Assuming lunar-like maturity effects on band depth, this corresponds to an upper limit of 0.15 and 0.6 wt.% FeO content in silicates, an order of magnitude less than previous estimates. 相似文献
16.
We report on observations of the dust trail of Comet 67P/Churyumov-Gerasimenko (CG) in visible light with the Wide Field Imager at the ESO/MPG 2.2 m telescope at 4.7 AU before aphelion, and at with the MIPS instrument on board the Spitzer Space Telescope at 5.7 AU both before and after aphelion. The comet did not appear to be active during our observations. Our images probe large dust grains emitted from the comet that have a radiation pressure parameter β<0.01. We compare our observations with simulated images generated with a dynamical model of the cometary dust environment and constrain the emission speeds, size distribution, production rate and geometric albedo of the dust. We achieve the best fit to our data with a differential size distribution exponent of −4.1, and emission speeds for a β=0.01 particle of 25 m/s at perihelion and 2 m/s at 3 AU. The dust production rate in our model is on the order of 1000 kg/s at perihelion and 1 kg/s at 3 AU, and we require a dust geometric albedo between 0.022 and 0.044. The production rates of large (>) particles required to reproduce the brightness of the trail are sufficient to also account for the coma brightness observed while the comet was inside 3 AU, and we infer that the cross-section in the coma of CG may be dominated by grains of the order of . 相似文献
17.
The reaction kinetics of the butadinyl radical, C4H, with various hydrocarbons detected in the atmosphere of Titan (methane, ethane, propane, acetylene, ethene and methylacetylene) are studied over the temperature range of 39-298 K using the Rennes CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus. Kinetic measurements were made using the pulsed laser photolysis—laser induced fluorescence technique. The rate coefficients, except for the reaction with methane, all show a negative temperature dependence and can be fitted with the following expressions over the temperature range of this study: ; ; , , . These expressions are not intended to be physically meaningful but rather to provide an easy way to introduce experimental results in photochemical models. They are only valid over the temperature range of the experiments. Possible channels of these reactions are discussed as well as possible consequences of these results for the production of large molecules and hazes in the atmosphere of Titan. These results should also be considered for the photochemistry of Giant Planets. 相似文献
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The cumulative effects of weak resonant and secular perturbations by the major planets produce chaotic behavior of asteroids on long timescales. Dynamical chaos is the dominant loss mechanism for asteroids with diameters in the current asteroid belt. In a numerical analysis of the long-term evolution of test particles in the main asteroid belt region, we find that the dynamical loss history of test particles from this region is well described with a logarithmic decay law. In our simulations the loss rate function that is established at persists with little deviation to at least . Our study indicates that the asteroid belt region has experienced a significant amount of depletion due to this dynamical erosion—having lost as much as ∼50% of the large asteroids—since 1 Myr after the establishment of the current dynamical structure of the asteroid belt. Because the dynamical depletion of asteroids from the main belt is approximately logarithmic, an equal amount of depletion occurred in the time interval 10-200 Myr as in 0.2-4 Gyr, roughly ∼30% of the current number of large asteroids in the main belt over each interval. We find that asteroids escaping from the main belt due to dynamical chaos have an Earth-impact probability of ∼0.3%. Our model suggests that the rate of impacts from large asteroids has declined by a factor of 3 over the last 3 Gyr, and that the present-day impact flux of objects on the terrestrial planets is roughly an order of magnitude less than estimates currently in use in crater chronologies and impact hazard risk assessments. 相似文献