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1.
We present a comprehensive examination of Jupiter's “gossamer” rings based on images from Voyager, Galileo, the Hubble Space Telescope and the W.M. Keck Telescope. We compare our results to the simple dynamical model of Burns et al. [Burns, J.A., Showalter, M.R., Hamilton, D.P., Nicholson, P.D., de Pater, I., Ockert-Bell, M., Thomas, P., 1999. Science 284, 1146-1150] in which dust is ejected from Amalthea and Thebe and then evolves inward under Poynting-Robertson drag. The ring follows many predictions of the model rather well, including a linear reduction in thickness with decreasing radius. However, some deviations from the model are noted. For example, additional material appears to be concentrated just interior to the orbits of the two moons. At least in the case of Amalthea's ring, that material is in the same orbital plane as Amalthea's inclined orbit and may be trapped at the Lagrange points. Thebe's ring shows much larger vertical excursions from the model, which may be related to perturbations by several strong Lorentz resonances. Photometry is consistent with the dust obeying a relatively flat power-law size distribution, very similar to dust in the main ring. However, the very low backscatter reflectivity of the ring, and the flat phase curve of the ring at low phase angles, require that the ring be composed of distinctly non-spherical particles.  相似文献   

2.
We present a total of 289 new astrometric observations of the inner jovian satellites, Amalthea and Thebe, obtained using the Cassini ISS narrow angle camera. Observations were made using image sequences from 2000 December 11-12 (inbound) and 2001 January 15-16 (outbound), at phase angles of approximately 2° and 122°, respectively. Target distances were of order 284 RJ, giving a maximum resolution of approximately 100 km/pixel. Centroided line and sample values for 239 observations of Amalthea and 50 of Thebe are provided, together with estimated camera pointing information for each image. Orbit fitting using a uniformly precessing Keplerian ellipse model, taking into account the oblateness of Jupiter up to terms in J6, gave RMS fit residuals of 0.364 and 0.443 pixel for Amalthea and Thebe, respectively (equivalent to 0.450 and 0.547 arcsec). RMS residuals relative to the JPL JUP230 ephemeris were 0.306 and 0.604 pixel (equivalent to 0.378 and 0.746 arcsec), for Amalthea and Thebe. The fitted orbital parameters confirm the relatively high inclinations of these satellites (0.374°±0.002° and 1.076°±0.003°, respectively), equivalent to maximum vertical displacements above Jupiter's equatorial plane of 1188±6 and 4240±12 km, respectively, consistent with current estimates of the half-thicknesses of the Amalthea and Thebe gossamer rings [Ockert-Bell, M.E., Burns, J.A., Dauber, I.J., Thomas, P.C., Veverka, J., Belton, M.J.S., Klaasen, K.P., 1999. Icarus 138, 188-213].  相似文献   

3.
Galileo's Solid State Imaging experiment (SSI) obtained 36 visible wavelength images of Jupiter's ring system during the nominal mission (Ockert-Bell et al., 1999, Icarus 138, 188-213) and another 21 during the extended mission. The Near Infrared Mapping Spectrometer (NIMS) recorded an observation of Jupiter's main ring during orbit C3 at wavelengths from 0.7 to 5.2 μm; a second observation was attempted during orbit E4. We analyze the high phase angle NIMS and SSI observations to constrain the size distribution of the main ring's micron-sized dust population. This portion of the population is best constrained at high phase angles, as the light scattering behavior of small dust grains dominates at these geometries and contributions from larger ring particles are negligible. High phase angle images of the main ring obtained by the Voyager spacecraft covered phase angles between 173.8° and 176.9° (Showalter et al., 1987, Icarus 69, 458-498). Galileo images extend this range up to 178.6°. We model the Galileo phase curve and the ring spectra from the C3 NIMS ring observation as the combination of two power law distributions. Our analysis of the main ring phase curve and the NIMS spectra suggests the size distribution of the smallest ring particles is a power law with an index of 2.0±0.3 below a size of ∼15 μm that transitions to a power law with an index of 5.0±1.5 at larger sizes. This combined power law distribution, or “broken power law” distribution, yields a better fit to the NIMS data than do the power law distributions that have previously been fit to the Voyager imaging data (Showalter et al., 1987, Icarus 69, 458-498). The broken power law distribution reconciles the results of Showalter et al. (1987, Icarus 69, 458-498) and McMuldroch et al. (2000, Icarus 146, 1-11), who also analyzed the NIMS data, and can be considered as an obvious extension of a simple power law. This more complex size distribution could indicate that ring particle production rates and/or lifetimes vary with size and may relate to the physical processes that control their evolution. The significant near arm/far arm asymmetry reported elsewhere (see Showalter et al., 1987, Icarus 69, 458-498; Ockert-Bell et al., 1999, Icarus 138, 188-213) persists in the data even after the main ring is isolated in the SSI images. However, the sense of the asymmetry seen in Galileo images differs from that seen in Voyager images. We interpret this asymmetry as a broad-scale, azimuthal brightness variation. No consistent association with the magnetic field of Jupiter has been observed. It is possible that these longitudinal variations may be similar to the random brightness fluctuations observed in Saturn's F ring by Voyager (Smith et al., 1982, Science 215, 504-537) and during the 1995 ring plane crossings (Nicholson et al., 1996, Science 272, 509-515; Bosh and Rivkin, 1996, Science 272, 518-521; Poulet et al., 2000, Icarus 144, 135-148). Stochastic events may thus play a significant role in the evolution of the jovian main ring.  相似文献   

4.
We present new observations of Jupiter's ring system at a wavelength of 2.2 μm obtained with the 10-m W.M. Keck telescopes on three nights during a ring plane crossing: UT 19 December 2002, and 22 and 26 January 2003. We used conventional imaging, plus adaptive optics on the last night. Here we present detailed radial profiles of the main ring, halo and gossamer rings, and interpret the data together with information extracted from radio observations of Jupiter's synchrotron radiation. The main ring is confined to a 800-km-wide annulus between 128,200 and 129,000 km, with a ∼5000 km extension on the inside. The normal optical depth is 8×10−6, 15% of which is provided by bodies with radii a?5 cm. These bodies are as red as Metis. Half the optical depth, τ≈4×10−6, is attributed to micron-sized dust, and the remaining τ≈3×10−6 to grains tens to hundreds of μm in size. The inward extension consists of micron-sized (a?10 μm) dust, which probably migrates inward under Poynting-Robertson drag. The inner limit of this extension falls near the 3:2 Lorentz resonance (at orbital radius r=122,400 km), and coincides with the outer limit of the halo. The gossamer rings appear to be radially confined, rather than broad sheets of material. The Amalthea ring is triangularly shaped, with a steep outer dropoff over ∼5000 km, extending a few 1000 km beyond the orbit of Amalthea, and a more gradual inner dropoff over 15,000-20,000 km. The inner edge is near the location of the synchronous orbit. The optical depth in the Amalthea ring is ∼5×10−7, up to 20% of which is comprised of macroscopic material. The optical depth in the Thebe ring is a factor of 3 smaller.  相似文献   

5.
Cassini's Imaging Science Subsystem (ISS) instrument took nearly 1200 images of the Jupiter ring system during the spacecraft's 6-month encounter with Jupiter (Porco et al., 2003, Science 299, 1541-1547). These observations constitute the most complete data set of the ring taken by a single instrument, both in phase angle (0.5°-120° at seven angles) and wavelength (0.45-0.93 μm through eight filters). The main ring was detected in all targeted exposures; the halo and gossamer rings were too faint to be detected above the planet's stray light. The optical depth and radial profile of the main ring are consistent with previous observations. No broad asymmetries within the ring were seen; we did identify possible hints of 1000 km-scale azimuthal clumps within the ring. Cassini observations taken within 0.02° of the ring plane place an upper limit on the ring's full thickness of 80 km at a phase angle of 64°. We have combined the Cassini ISS and VIMS (Visible and Infrared Mapping Spectrometer) observations with those from Voyager, HST (Hubble Space Telescope), Keck, Galileo, Palomar, and IRTF (Infrared Telescope Facility). We have fit the entire suite of data using a photometric model that includes microscopic silicate dust grains as well as larger, long-lived ‘parent bodies’ that engender this dust. Our best-fit model to all the data indicates an optical depth of small particles of τs=4.7×10−6 and large bodies τl=1.3×10−6. The dust's cross-sectional area peaks near 15 μm. The data are fit significantly better using non-spherical rather than spherical dust grains. The parent bodies themselves must be very red from 0.4-2.5 μm, and may have absorption features near 0.8 and 2.2 μm.  相似文献   

6.
In November of 2002, the Galileo spacecraft passed within 250 km of Jupiter's moon Amalthea. An onboard telescope, the star scanner, observed a series of bright flashes near the moon. It is believed that these flashes represent sunlight reflected from 7 to 9 small moonlets located within about 3000 km of Amalthea. From star scanner geometry considerations and other arguments, we can constrain the diameter of the observed bodies to be between 0.5 m to several tens of kilometers. In September of 2003, while crossing Amalthea's orbit just prior to Galileo's destruction in the jovian atmosphere, a single additional body seems to have been observed. It is suspected that these bodies are part of a discrete rocky ring embedded within Jupiter's Gossamer ring system.  相似文献   

7.
Sascha Kempf  Uwe Beckmann 《Icarus》2010,206(2):446-457
Pre-Cassini models of Saturn’s E ring [Horányi, M., Burns, J., Hamilton, D., 1992. Icarus 97, 248-259; Juhász, A., Horányi, M., 2002. J. Geophys. Res. 107, 1-10] failed to reproduce its peculiar vertical structure inferred from Earth-bound observations [de Pater, I., Martin, S.C., Showalter, M.R., 2004. Icarus 172, 446-454]. After the discovery of an active ice-volcanism of Saturn’s icy moon Enceladus the relevance of the directed injection of particles for the vertical ring structure of the E ring was swiftly recognised [Juhász, A., Horányi, M., Morfill, G.E., 2007. Geophys. Res. Lett. 34, L09104; Kempf, S., Beckmann, U., Moragas-Klostermeyer, G., Postberg, F., Srama, R., Economou, T., Schmidt, J., Spahn, F., Grün, E., 2008. Icarus 193, 420-437]. However, simple models for the delivery of particles from the plume to the ring predict a too small vertical ring thickness and overestimate the amount of the injected dust.Here we report on numerical simulations of grains leaving the plume and populating the dust torus of Enceladus. We run a large number of dynamical simulations including gravity and Lorentz force to investigate the earliest phase of the ring particle life span. The evolution of the electrostatic charge carried by the initially uncharged grains is treated selfconsistently. Freshly ejected plume particles are moving in almost circular orbits because the Enceladus orbital speed exceeds the particles’ ejection speeds by far. Only a small fraction of grains that leave the Hill sphere of Enceladus survive the next encounter with the moon. Thus, the flux and size distribution of the surviving grains, replenishing the ring particle reservoir, differs significantly from the flux and size distribution of the particles freshly ejected from the plume. Our numerical simulations reproduce the vertical ring profile measured by the Cassini Cosmic Dust Analyzer (CDA) [Kempf, S., Beckmann, U., Moragas-Klostermeyer, G., Postberg, F., Srama, R., EconoDmou, T., Smchmidt, J., Spahn, F., Grün, E., 2008. Icarus 193, 420-437]. From our simulations we calculate the deposition rates of plume particles hitting Enceladus’ surface. We find that at a distance of 100 m from a jet a 10 m sized ice boulder should be covered by plume particles in 105-106 years.  相似文献   

8.
P.G.J. Irwin  K. Sihra  F.W. Taylor 《Icarus》2005,176(2):255-271
New measurements of the low-temperature near-infrared absorption of methane (Sihra, 1998, Laboratory measurements of near-infrared methane bands for remote sensing of the jovian atmosphere, Ph.D. thesis, University of Oxford) have been combined with existing, longer path-length, higher-temperature data of Strong et al. (1993, Spectral parameters of self- and hydrogen-broadened methane from 2000 to 9500 cm−1 for remote sounding of the atmosphere of Jupiter, J. Quant. Spectrosc. Radiat. Trans. 50, 309-325) and fitted with band models. The combined data set is found to be more consistent with previous low-temperature methane absorption measurements than that of Strong et al. (1993, J. Quant. Spectrosc. Radiat. Trans. 50, 309-325) but covers the same wider wavelength range and accounts for both self- and hydrogen-broadening conditions. These data have been fitted with k-coefficients in the manner described by Irwin et al. (1996, Calculated k-distribution coefficients for hydrogen- and self-broadened methane in the range 2000-9500 cm−1 from exponential sum fitting to band modelled spectra, J. Geophys. Res. 101, 26,137-26,154) and have been used in multiple-scattering radiative transfer models to assess their impact on our previous estimates of the jovian cloud structure obtained from Galileo Near-Infrared Mapping Spectrometer (NIMS) observations (Irwin et al., 1998, Cloud structure and atmospheric composition of Jupiter retrieved from Galileo NIMS real-time spectra, J. Geophys. Res. 103, 23,001-23,021; Irwin et al., 2001, The origin of belt/zone contrasts in the atmosphere of Jupiter and their correlation with 5-μm opacity, Icarus 149, 397-415; Irwin and Dyudina, 2002, The retrieval of cloud structure maps in the equatorial region of Jupiter using a principal component analysis of Galileo/NIMS data, Icarus 156, 52-63). Although significant differences in methane opacity are found at cooler temperatures, the difference in the optical depth of the atmosphere due to methane is found to diminish rapidly with increasing pressure and temperature and thus has negligible effect on the cloud structure inferred at deeper levels. Hence the main cloud opacity variation is still found to peak at around 1-2 bar using our previous analytical approach, and is thus still in disagreement with Galileo Solid State Imager (SSI) determinations (Banfield et al., 1998, Jupiter's cloud structure from Galileo imaging data, Icarus 135, 230-250; Simon-Miller et al., 2001, Color and the vertical structure in Jupiter's belts, zones and weather systems, Icarus 154, 459-474) which place the main cloud deck near 0.9 bar. Further analysis of our retrievals reveals that this discrepancy is probably due to the different assumptions of the two analyses. Our retrievals use a smooth vertically extended cloud profile while the SSI determinations assume a thin NH3 cloud below an extended haze. When the main opacity in our model is similarly assumed to be due to a thin cloud below an extended haze, we find the main level of cloud opacity variation to be near the 1 bar level—close to that determined by SSI and moderately close to the expected condensation level of ammonia ice of 0.85 bar, assuming that the abundance of ammonia on Jupiter is (7±1)×10−4 (Folkner et al., 1998, Ammonia abundance in Jupiter's atmosphere derived from the attenuation of the Galileo probe's radio signal, J. Geophys. Res. 103, 22,847-22,855; Atreya et al., 1999, A comparison of the atmospheres of Jupiter and Saturn: deep atmospheric composition, cloud structure, vertical mixing, and origin, Planet. Space Sci. 47, 1243-1262). However our data in the 1-2.5 μm range have good height discrimination and our lowest estimate of the cloud base pressure of 1 bar is still too great to be consistent with the most recent estimates of the ammonia abundance of 3.5 × solar. Furthermore the observed limited spatial distribution of ammonia ice absorption features on Jupiter suggests that pure ammonia ice is only present in regions of localised vigorous uplift (Baines et al., 2002, Fresh ammonia ice clouds in Jupiter: spectroscopic identification, spatial distribution, and dynamical implications, Icarus 159, 74-94) and is subsequently rapidly modified in some way which masks its pure absorption features. Hence we conclude that the main cloud deck on Jupiter is unlikely to be composed of pure ammonia ice and instead find that it must be composed of either NH4SH or some other unknown combination of ammonia, water, and hydrogen sulphide and exists at pressures of between 1 and 2 bar.  相似文献   

9.
I. Kulyk  K. Jockers 《Icarus》2004,170(1):24-34
We present the results of photometric measurements of the inner jovian satellites Thebe, Amalthea and Metis based on extensive optical observations taken from October 1999 to January 2002. The observations were made in the phase angle range from 8.1° to 0.3°. The Two-Channel Focal Reducer of the Max-Planck Institute for Aeronomy attached to the 2-m RCC telescope at Terskol Observatory (Pik Terskol, Northern Caucasus) was used in coronagraph mode. The observations were performed at a wavelength of 0.887 μm. Mean observational uncertainties corresponding to 1σ rms errors were 3% for the leading and trailing sides of Amalthea, 7 and 9% for the leading and trailing sides of Thebe and 9% for the leading side of Metis after taking into account the longitude brightness variations. Photometric data calibrated on an absolute scale were used to evaluate the near-opposition behavior of satellite brightness. All three satellites exhibit significant opposition brightening, but the strength of this effect, measured as the ratios of intensities at α1=1.6° and α2=6.7° does not vary significantly among these satellites. In order to measure the opposition surge parameters the empirical law proposed by Karkoschka and Hapke's model were used. The parameters of the satellite opposition effects are presented and discussed. The values of geometric albedos calculated with best-fit Hapke parameters are 0.096, 0.157, and 0.24 for Thebe, Amalthea, and Metis respectively. We found that the average leading/trailing ratios of surface reflectance at the measured phase angles are 1.53±0.05, 1.25±0.04, 1.04±0.08 for Amalthea, Thebe, and Metis.  相似文献   

10.
We report on dust measurements obtained during the seventh orbit of the Galileo spacecraft about Jupiter. The most prominent features observed are highly time variable dust streams recorded throughout the Jovian system. The impact rate varied by more than an order of magnitude with a 5 and 10 hour periodicity, which shows a correlation with Galileo's position relative to the Jovian magnetic field. This behavior can be qualitatively explained by strong coupling of nanometer-sized dust to the Jovian magnetic field. In addition to the 5 and 10 h periodicities, a longer period which is compatible with Io's orbital period is evident in the dust impact rate. This feature indicates that Io most likely is the source of the dust streams. During a close (3,095 km altitude) flyby at Ganymede on 5 April 1997 an enhanced rate of dust impacts has been observed, which suggests that Ganymede is a source of ejecta particles. Within a distance of about 25 RJ(Jupiter radius, RJ= 71,492 km) from Jupiter impacts of micrometer-sized particles have been recorded which could be particles on bound orbits about Jupiter. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
David E. Dunn  Imke de Pater 《Icarus》2010,208(2):927-937
We present a Monte Carlo model of the uranian rings, and compare this model to images of the system obtained with the Keck adaptive optics system in July 2004, at a wavelength of 2.2 μm (from de Pater et al. (de Pater, I., Gibbard, S.G., Hammel, H.B. [2006a]. Icarus 180, 186-200)). We confirm the presence of the ζ ring, but show that this ring must extend inwards much further than previously thought, although with an optical depth much lower than that in the main ζ ring component. We further confirm dust rings between rings α-4 and β-α, as well as near the λ ring. In addition, we show that a broad sheet of faint material (τ0 ∼ 10−3) must be present through most of the ring region, from the α ring through the λ ring.  相似文献   

12.
The backscattered reflectivity of Jupiter's ring has been previously measured over distinct visible and near infrared wavelength bands by a number of ground-based and spaceborne instruments. We present spectra of Jupiter's main ring from 2.21-2.46 μm taken with the NIRSPEC spectrometer at the W.M. Keck observatory. At these wavelengths, scattered light from Jupiter is minimal due to the strong absorption of methane in the planet's atmosphere. We find an overall flat spectral slope over this wavelength interval, except for a possible red slope shortward of 2.25 μm. We extended the spectral coverage of the ring to shorter wavelengths by adding a narrow-band image at 1.64 μm, and show results from 2.27-μm images over phase angles of 1.2°-11.0°. Our images at 1.64 and 2.27 μm reveal that the halo contribution is stronger at the shorter wavelength, possibly due to the redder spectrum of the ring parent bodies as compared with the halo dust component. We find no variation in main ring reflectivity over the 1.2°-11.0° phase angle range at 2.27 μm. We use adaptive optics imaging at the longer wavelength L′ band (3.4-4.1 μm) to determine a 2-σ upper limit of 22 m of vertically-integrated I/F. Our observing campaign also produced an L′ image of Callisto, showing a darker leading hemisphere, and a spectrum of Amalthea over the 2.2-2.5 and 2.85-3.03 μm ranges, showing deep 3-μm absorption.  相似文献   

13.
Tenuous dust clouds of Jupiter's Galilean moons Io, Europa, Ganymede and Callisto have been detected with the in-situ dust detector on board the Galileo spacecraft. The majority of the dust particles have been sensed at altitudes below five radii of these lunar-sized satellites. We identify the particles in the duut clouds surrounding the moons by their impact direction, impact velocity, and mass distribution. Average particle sizes are between 0.5 and 1 μm, just above the detector threshold, indicating a size distribution with decreasing numbers towards bigger particles. Our results imply that the particles have been kicked up by hypervelocity impacts of micrometeoroids onto the satellites' surfaces. The measured radial dust density profiles are consistent with predictions by dynamical modeling for satellite ejecta produced by interplanetary impactors (Krivov et al., 2003, Planet. Space Sci. 51, 251-269), assuming yield, mass and velocity distributions of the ejecta from laboratory measurements. A comparison of all four Galilean moons (data for Ganymede published earlier; Krüger et al., 2000, Planet. Space Sci. 48, 1457-1471) shows that the dust clouds of the three outer Galilean moons have very similar properties and are in good agreement with the model predictions for solid ice-silicate surfaces. The dust density in the vicinity of Io, however, is more than an order of magnitude lower than expected from theory. This may be due to a softer, fluffier surface of Io (volcanic deposits) as compared to the other moons. The log-log slope of the dust number density in the clouds vs. distance from the satellite center ranges between −1.6 and −2.8. Appreciable variations of number densities obtained from individual flybys with varying geometry, especially at Callisto, are found. These might be indicative of leading-trailing asymmetries of the clouds due to the motion of the moons with respect to the field of impactors.  相似文献   

14.
We present observations of the synchrotron radiation from Jupiter obtained in July 2004 with the Very Large Array at a frequency of 15 GHz. The array was in its most compact (D) configuration and the distance to Jupiter was 6.077 AU, making the apparent size of Jupiter relatively small, and favorable for such observations. We measured a total synchrotron radiation flux density of 1.26±0.12 Jy, scaled to a distance of 4.04 AU, which was 2.4% of the total flux density from the planet. Our results agree, within the uncertainties, with a previous VLA measurement (1.5±0.15 Jy in March 1991 [de Pater, I., Dunn, D., 2003. Icarus 163, 449-455]); both values are a factor of 3-3.5 higher than the flux density reported from Cassini data in 2001 [Bolton, S.J., and 20 colleagues, 2002. Nature 415, 987-991].  相似文献   

15.
Detection and measurement of atmospheric water vapor in the deep jovian atmosphere using microwave radiometry has been discussed extensively by Janssen et al. (Janssen, M.A., Hofstadter, M.D., Gulkis, S., Ingersoll, A.P., Allison, M., Bolton, S.J., Levin, S.M., Kamp, L.W. [2005]. Icarus 173 (2), 447-453.) and de Pater et al. (de Pater, I., Deboer, D., Marley, M., Freedman, R., Young, R. [2005]. Icarus 173 (2), 425-447). The NASA Juno mission will include a six-channel microwave radiometer system (MWR) operating in the 1.3-50 cm wavelength range in order to retrieve water vapor abundances from the microwave signature of Jupiter (see, e.g., Matousek, S. [2005]. The Juno new frontiers mission. Tech. Rep. IAC-05-A3.2.A.04, California Institute of Technology). In order to accurately interpret data from such observations, nearly 2000 laboratory measurements of the microwave opacity of H2O vapor in a H2/He atmosphere have been conducted in the 5-21 cm wavelength range (1.4-6 GHz) at pressures from 30 mbars to 101 bars and at temperatures from 330 to 525 K. The mole fraction of H2O (at maximum pressure) ranged from 0.19% to 3.6% with some additional measurements of pure H2O. These results have enabled development of the first model for the opacity of gaseous H2O in a H2/He atmosphere under jovian conditions developed from actual laboratory data. The new model is based on a terrestrial model of Rosenkranz et al. (Rosenkranz, P.W. [1998]. Radio Science 33, 919-928), with substantial modifications to reflect the effects of jovian conditions. The new model for water vapor opacity dramatically outperforms previous models and will provide reliable results for temperatures from 300 to 525 K, at pressures up to 100 bars and at frequencies up to 6 GHz. These results will significantly reduce the uncertainties in the retrieval of jovian atmospheric water vapor abundances from the microwave radiometric measurements from the upcoming NASA Juno mission, as well as provide a clearer understanding of the role deep atmospheric water vapor may play in the decimeter-wavelength spectrum of Saturn.  相似文献   

16.
We present observations at near-infrared wavelengths (1-5 μm) of Jupiter’s north polar region and Northern Red Oval (NN-LRS-1). The observations were taken with the near-infrared camera NIRC2 coupled to the adaptive optics system on the 10-m W.M. Keck Telescope on UT 21 August 2010. At 5-μm Jupiter’s disk reveals considerable structure, including small bright rings which appear to surround all small vortices. It is striking, though, that no such ring is seen around the Northern Red Oval. In de Pater et al. [2010a. Icarus 210, 742-762], we showed that such rings also exist around all small vortices in Jupiter’s southern hemisphere, and are absent around the Great Red Spot and Red Oval BA. We show here that the vertical structure and extent of the Northern Red Oval is very similar to that of Jupiter’s Red Oval BA. These new observations of the Northern Red Oval, therefore, support the idea of a dichotomy between small and large anticyclones, in which ovals larger than about two Rossby deformation radii do not have 5-μm bright rings. In de Pater et al. [2010a. Icarus 210, 742-762], we explained this difference in terms of the secondary circulations within the vortices. We further compare the brightness distribution of our new 5-μm images with previously published radio observations of Jupiter, highlighting the depletion of NH3 gas over areas that are bright at 5 μm.  相似文献   

17.
《Planetary and Space Science》2006,54(9-10):1014-1023
Faint rings of micrometre-sized dust particles embrace many planets in the Solar system. As a rule, they are replenished by ejecta from embedded atmosphereless moons. On a number of occasions, the ejecta are generated by hypervelocity meteoroid impacts into the moons. Small ejecta fragments are then swiftly shifted into rings by an array of non-gravitational forces, e.g. radiation pressure or plasma drag. A significant fraction of ejecta mass, however, is contained in relatively big, multi-micrometre fragments which are subject to gravity only. Having escaped from the satellite, they stay close to its orbit and form a belt around planet. This belt is itself a source of ring dust through collisional disruption of its particles. Here the contributions of belts to the respective rings are estimated for selected satellites of Jupiter and Saturn. The belts under review could supply substantially more dust to rings than the direct ejecta from satellites and should be taken into account when estimating ring dust budgets. The belts are very difficult to observe, however, and some of them remain a theoretical proposition. We find an appealing evidence for the belts due to Amalthea and Thebe around Jupiter, and for the belt due to Enceladus around Saturn.  相似文献   

18.
Both deep zonal winds, if they exist, and the basic rotational distortion of Jupiter contribute to its zonal gravity coefficients Jn for n ? 2. In order to capture the gravitational signature of Jupiter that is caused solely by its deep zonal winds, one must take into account the full effect of rotational distortion by computing the coefficients Jn in non-spherical geometry. This represents a difficult and challenging problem because the widely-used spherical-harmonic-expansion method becomes no longer suitable. Based on the model of a polytropic Jupiter with index unity, we compute Jupiter’s gravity coefficients J2, J4, J6, … , J12 taking into account the full effect of rotational distortion of the gaseous planet using a finite element method. For the model of deep zonal winds on cylinders parallel to the rotation axis, we also compute the variation of the gravity coefficients ΔJ2, ΔJ4, ΔJ6, … , ΔJ12 caused solely by the effect of the winds in non-spherical geometry. It is found that the effect of the zonal winds on lower-order coefficients is weak, ∣ΔJn/Jn∣ < 1%, for n = 2, 4, 6, but it is substantial for the high-degree coefficients with n ? 8.  相似文献   

19.
L.A. Sromovsky  P.M. Fry 《Icarus》2010,210(1):230-257
The Cassini flyby of Jupiter in 2000 provided spatially resolved spectra of Jupiter’s atmosphere using the Visual and Infrared Mapping Spectrometer (VIMS). A prominent characteristic of these spectra is the presence of a strong absorption at wavelengths from about 2.9 μm to 3.1 μm, previously noticed in a 3-μm spectrum obtained by the Infrared Space Observatory (ISO) in 1996. While Brooke et al. (Brooke, T.Y., Knacke, R.F., Encrenaz, T., Drossart, P., Crisp, D., Feuchtgruber, H. [1998]. Icarus 136, 1-13) were able to fit the ISO spectrum very well using ammonia ice as the sole source of particulate absorption, Sromovsky and Fry (Sromovsky, L.A., Fry, P.M. [2010]. Icarus 210, 211-229), using significantly revised NH3 gas absorption models, showed that ammonium hydrosulfide (NH4SH) provided a better fit to the ISO spectrum than NH3, but that the best fit was obtained when both NH3 and NH4SH were present in the clouds. Although the large FOV of the ISO instrument precluded identification of the spatial distribution of these two components, the VIMS spectra at low and intermediate phase angles show that 3-μm absorption is present in zones and belts, in every region investigated, and both low- and high-opacity samples are best fit with a combination of NH4SH and NH3 particles at all locations. The best fits are obtained with a layer of small ammonia-coated particles (r ∼ 0.3 μm) overlying but often close to an optically thicker but still modest layer of much larger NH4SH particles (r ∼ 10 μm), with a deeper optically thicker layer, which might also be composed of NH4SH. Although these fits put NH3 ice at pressures less than 500 mb, this is not inconsistent with the lack of prominent NH3 features in Jupiter’s longwave spectrum because the reflectivity of the core particles strongly suppresses the NH3 absorption features, at both near-IR and thermal wavelengths. Unlike Jupiter, Saturn lacks the broad 3-μm absorption feature, but does exhibit a small absorption near 2.965 μm, which resembles a similar jovian feature and suggests that both planets contain upper tropospheric clouds of sub-micron particles containing ammonia as a minor fraction.  相似文献   

20.
High speed dust streams emanating from near Jupiter were first discovered by the Ulysses spacecraft in 1992. Since then the phenomenon has been re-observed by Galileo in 1995, Cassini in 2000, and Ulysses in 2004. The dust grains are expected to be charged to a potential of , which is sufficient to allow the planet's magnetic field to accelerate them away from the planet, where they are subsequently influenced by the interplanetary magnetic field (IMF). A similar phenomenon was observed near Saturn by Cassini. Here, we report and analyze simultaneous dust, IMF and solar wind data for all dust streams from the two Ulysses Jupiter flybys. We find that compression regions (CRs) in the IMF – regions of enhanced magnetic field – precede most dust streams. Furthermore, the duration of a dust stream is roughly comparable with that of the precedent CR, and the occurrence of a dust stream and the occurrence of the previous CR are separated by a time interval that depends on the distance to Jupiter. The intensity of the dust streams and their precedent CRs are also correlated, but this correlation is only evident at distances from the planet no greater than 2 AU. Combining these observations, we argue that CRs strongly affect dust streams, probably by deflecting dust grain trajectories, so that they can reach the spacecraft and be detected by its dust sensor.  相似文献   

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