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1.
Analysis of Titan’s hemispheric brightness asymmetry from mapped Cassini images reveals an axis of symmetry that is tilted with respect to the rotational axis of the solid body. Twenty images taken from 2004 through 2007 show a mean axial offset of 3.8 ± 0.9° relative to the solid body’s pole, directed 79 ± 24° to the west of the sub-solar longitude. These values are consistent with recent measurements of an implied atmospheric spin axis determined from isothermal mapping by [Achterberg, R.K., Conrath, B.J., Gierasch, P.J., Flasar, F.M., Nixon, C.A., 2008. Icarus 197, 549-555].  相似文献   

2.
Two and a half years after Saturn orbit insertion (SOI) the Cassini composite infrared spectrometer (CIRS) has acquired an extensive set of thermal measurements (including physical temperature and filling factor) of Saturn's main rings for a number of different viewing geometries, most of which are not available from Earth. Thermal mapping of both the lit and unlit faces of the rings is being performed within a multidimensional observation space that includes solar phase angle, spacecraft elevation and solar elevation. Comprehensive thermal mapping is a key requirement for detailed modeling of ring thermal properties.To first order, the largest temperature changes on the lit face of the rings are driven by variations in phase angle while differences in temperature with changing spacecraft elevation are a secondary effect. Ring temperatures decrease with increasing phase angle suggesting a population of slowly rotating ring particles [Spilker, L.J., Pilorz, S.H., Wallis, B.D., Pearl, J.C., Cuzzi, J.N., Brooks, S.M., Altobelli, N., Edgington, S.G., Showalter, M., Michael Flasar, F., Ferrari, C., Leyrat, C. 2006. Cassini thermal observations of Saturn's main rings: implications for particle rotation and vertical mixing. Planet. Space Sci. 54, 1167-1176, doi: 10.1016/j.pss.2006.05.033]. Both lit A and B rings show that temperature decreases with decreasing rings solar elevation while temperature changes in the C ring and Cassini Division are more muted. Variations in the geometrical filling factor, β, are primarily driven by changes in spacecraft elevation. For the optically thinnest region of the C ring, β variations are found to be nearly exclusively determined by spacecraft elevation. Both a multilayer and a monolayer model provide an excellent fit to the data in this region. In both cases, a ring infrared emissivity >0.9 is required, together with a random and homogeneous distribution of the particles. The interparticle shadowing function required for the monolayer model is very well constrained by our data and matches experimental measurements performed by Froidevaux [1981a. Saturn's rings: infrared brightness variation with solar elevation. Icarus 46, 4-17].  相似文献   

3.
C. Ferrari  S. Brooks  C. Leyrat  L. Spilker 《Icarus》2009,199(1):145-153
The CIRS infrared spectrometer onboard the Cassini spacecraft has scanned Saturn's A ring azimuthally from several viewing angles since its orbit insertion in 2004. A quadrupolar asymmetry has been detected in this ring at spacecraft elevations ranging between 16° to 37°. Its fractional amplitude decreases from 22% to 8% from 20° to 37° elevations. The patterns observed in two almost complete azimuthal scans at elevations 20° and 36° strongly favor the self-gravity wakes as the origin of the asymmetry. The elliptical, infinite cylinder model of Hedman et al. [Hedman, M.M., Nicholson, P.D., Salo, H., Wallis, B.D., Buratti, B.J., Baines, K.H., Brown, R.H., Clark, R.N., 2007. Astron. J. 133, 2624-2629] can reproduce the CIRS observations well. Such wakes are found to have an average height-to-spacing ratio H/λ=0.1607±0.0002, a width-over-spacing W/λ=0.3833±0.0008. Gaps between wakes, which are filled with particles, have an optical depth τG=0.1231±0.0005. The wakes mean pitch angle ΦW is 70.70°±0.07°, relative to the radial direction. The comparison of ground-based visible data with CIRS observations constrains the A ring to be a monolayer. For a surface mass density of 40 g cm−2 [Tiscarino, M.S., Burns, J.A., Nicholson, P.D., Hedman, M.M., Porco, C.C., 2007. Icarus 189, 14-34], the expected spacing of wakes is λ≈60 m. Their height and width would then be H≈10 m and W≈24 m, values that match the maximum size of particles in this ring as determined from ground-based stellar occultations [French, R.G., Nicholson, P.D., 2000. Icarus 145, 502-523].  相似文献   

4.
L.A. Sromovsky  P.M. Fry  J.H. Kim 《Icarus》2011,215(1):292-312
Lindal et al. (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987]. J. Geophys. Res. 92 (11), 14987-15001) presented a range of temperature and methane profiles for Uranus that were consistent with 1986 Voyager radio occultation measurements of refractivity versus altitude. A localized refractivity slope variation near 1.2 bars was interpreted to be the result of a condensed methane cloud layer. However, models fit to near-IR spectra found particle concentrations much deeper in the atmosphere, in the 1.5-3 bar range (Sromovsky, L.A., Irwin, P.G.J., Fry, P.M. [2006]. Icarus 182, 577-593; Sromovsky, L.A., Fry, P.M. [2010]. Icarus 210, 211-229; Irwin, P.G.J., Teanby, N.A., Davis, G.R. [2010]. Icarus 208, 913-926), and a recent analysis of STIS spectra argued for a model in which aerosol particles formed diffusely distributed hazes, with no compact condensation layer (Karkoschka, E., Tomasko, M. [2009]. Icarus 202, 287-309). To try to reconcile these results, we reanalyzed the occultation observations with the He volume mixing ratio reduced from 0.15 to 0.116, which is near the edge of the 0.033 uncertainty range given by Conrath et al. (Conrath, B., Hanel, R., Gautier, D., Marten, A., Lindal, G. [1987]. J. Geophys. Res. 92 (11), 15003-15010). This allowed us to obtain saturated mixing ratios within the putative cloud layer and to reach above-cloud and deep methane mixing ratios compatible with STIS spectral constraints. Using a 5-layer vertical aerosol model with two compact cloud layers in the 1-3 bar region, we find that the best fit pressure for the upper compact layer is virtually identical to the pressure range inferred from the occultation analysis for a methane mixing ratio near 4% at 5°S. This strongly argues that Uranus does indeed have a compact methane cloud layer. In addition, our cloud model can fit the latitudinal variations in spectra between 30°S and 20°N, using the same profiles of temperature and methane mixing ratio. But closer to the pole, the model fails to provide accurate fits without introducing an increasingly strong upper tropospheric depletion of methane at increased latitudes, in rough agreement with the trend identified by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009]. Icarus 202, 287-309).  相似文献   

5.
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   

6.
The Cassini Imaging Science Subsystem (ISS) and Composite Infrared Spectrometer (CIRS) reported a North Equatorial Belt (NEB) wave in Jupiter's atmosphere from optical images [Porco, C.C., and 23 colleagues, 2003. Science 299, 1541-1547] and thermal maps [Flasar, F.M., and 39 colleagues, 2004. Nature 427, 132-135], respectively. The connection between the two waves remained uncertain because the two observations were not simultaneous. Here we report on simultaneous ISS images and CIRS thermal maps that confirm that the NEB wave shown in the ISS ultraviolet (UV1) and strong methane band (MT3) images is correlated with the thermal wave in the CIRS temperature maps, with low temperatures in the CIRS maps (upwelling) corresponding to dark regions in the UV1 images (UV-absorbing particles) and bright regions in the MT3 images (high clouds and haze). The long period of the NEB wave suggests that it is a planetary (Rossby) wave. The combined observations from the ISS and CIRS are utilized to discuss the vertical and meridional propagation of the NEB wave, which offers a possible explanation for why the NEB wave is confined to specific latitudes and altitudes. Further, the ISS UV1 images reveal a circumpolar wave centered at 48.5° S (planetocentric) and probably located in the stratosphere, as suggested by the ISS and CIRS observations. The simultaneous comparison between the ISS and CIRS also implies that the large dark oval in the polar stratosphere of Jupiter discovered in the ISS UV1 images [Porco, C.C., and 23 colleagues, 2003. Science 299, 1541-1547] is the same feature as the warm regions at high northern latitudes in the CIRS 1-mbar temperature maps [Flasar, F.M., and 39 colleagues, 2004. Nature 427, 132-135]. This comparison supports a previous suggestion that the dark oval in the ISS UV1 images is linked to auroral precipitation and heating [Porco, C.C., and 23 colleagues, 2003. Science 299, 1541-1547].  相似文献   

7.
Measuring the spatial distribution of chemical compounds in Saturn’s stratosphere is critical to better understand the planet’s photochemistry and dynamics. Here we present an analysis of infrared spectra in the range 600-1400 cm−1 acquired in limb geometry by the Cassini spacecraft between March 2005 and January 2008. We first determine the vertical temperature profiles from 3 to 0.01 hPa, at latitudes ranging from 70°N to 80°S. We infer a similar meridional temperature gradient at 1-2 hPa as in recent previous studies [Fletcher, L.N., Irwin, P.G.J., Teanby, N.A., Orton, G.S., Parrish, P.D., de Kok, R., Howett, C., Calcutt, S.B., Bowles, N., Taylor, F.W., 2007. Icarus 189, 457-478; Howett, C.J.A., Irwin, P.G.J., Teanby, N.A., Simon-Miller, A., Calcutt, S.B., Fletcher, L.N., de Kok, R., 2007. Icarus 190, 556-572]. We then retrieve the vertical profiles of C2H6 and C2H2 from 3 to 0.01 hPa and of C3H8 around 1 hPa. At 1 hPa, the meridional variation of C2H2 is found to follow the yearly averaged solar insolation, except for a strong equatorial mole fraction of 8×10-7, nearly two times higher than expected. This enhancement in abundance can be explained by the descent of hydrocarbon-rich air, with a vertical wind speed at the equator of 0.25±0.1 mm/s at 1 hPa and 0.4±0.15 mm/s at 0.1 hPa. The ethane distribution is relatively uniform at 1 hPa, with only a moderate 25% increase from 35°S to 80°S. Propane is found to increase from north to south by a factor of 1.9, suggesting that its lifetime may be shorter than Saturn’s year at 1 hPa. At high altitudes (1 Pa), C2H2 and C2H6 abundances depart significantly from the photochemical model predictions of Moses and Greathouse [Moses, J.I., Greathouse, T.K., 2005. J. Geophys. Res. 110, 9007], except at high southern latitudes (62, 70 and 80°S) and near the equator. The observed abundances are found strongly depleted in the 20-40°S region and enhanced in the 20-30°N region, the latter coinciding with the ring’s shadow. We favor a dynamical explanation for these anomalies.  相似文献   

8.
Retrievals performed on Cassini Composite Infrared Spectrometer data obtained during the distant Jupiter flyby in 2000/2001 have been used to generate global temperature maps of the planet in the troposphere and stratosphere, but to higher latitudes than were shown previously by Flasar et al. [Flasar, F.M., 39 colleagues, 2004a. Nature 427, 132-135; Flasar, F.M., 44 colleagues, 2004b. Space Sci. Rev. 115, 169-297]. Similar retrievals were performed on Voyager 1 IRIS data to provide the first detailed IRIS map of the stratosphere, and high latitudes in the troposphere. Thermal winds were calculated for each data set and show strong vertical shears in the zonal winds at low latitudes, and meridional temperature gradients indicate the presence of circumpolar jets, as well. The temperatures retrieved from the two spacecraft were also compared with yearly ground-based data obtained over the intervening two decades. Tropospheric temperatures reveal gradual changes at low latitudes, with little obvious seasonal or short-term variation [Orton et al., 1994. Science 265, 625-631]. Stratospheric temperatures show much more complicated behavior over short timescales, consistent with quasi-quadrennial oscillations at low latitudes, as suggested in prior analyses of shorter intervals of ground-based data [Orton et al., 1991. Science 252, 537-542; Friedson, A.J., 1999. Icarus 137, 34-55]. A scaling analysis indicates that meridional motions, mechanically forced by wave or eddy convergence, play an important role in modulating the temperatures and winds in the upper troposphere and stratosphere on seasonal and shorter timescales. At latitudes away from the equator, the mechanical forcing can be derived simply from a temporal record of temperature and its vertical derivative. Ground-based observations with improved vertical resolution and/or long-term monitoring from spacecraft are required for this purpose, though the Voyager and Cassini data given indications that the magnitude of the forcing is very small.  相似文献   

9.
M. Seiß  F. Spahn  Jürgen Schmidt 《Icarus》2010,210(1):298-317
Saturn’s rings host two known moons, Pan and Daphnis, which are massive enough to clear circumferential gaps in the ring around their orbits. Both moons create wake patterns at the gap edges by gravitational deflection of the ring material (Cuzzi, J.N., Scargle, J.D. [1985]. Astrophys. J. 292, 276-290; Showalter, M.R., Cuzzi, J.N., Marouf, E.A., Esposito, L.W. [1986]. Icarus 66, 297-323). New Cassini observations revealed that these wavy edges deviate from the sinusoidal waveform, which one would expect from a theory that assumes a circular orbit of the perturbing moon and neglects particle interactions. Resonant perturbations of the edges by moons outside the ring system, as well as an eccentric orbit of the embedded moon, may partly explain this behavior (Porco, C.C., and 34 colleagues [2005]. Science 307, 1226-1236; Tiscareno, M.S., Burns, J.A., Hedman, M.M., Spitale, J.N., Porco, C.C., Murray, C.D., and the Cassini Imaging team [2005]. Bull. Am. Astron. Soc. 37, 767; Weiss, J.W., Porco, C.C., Tiscareno, M.S., Burns, J.A., Dones, L. [2005]. Bull. Am. Astron. Soc. 37, 767; Weiss, J.W., Porco, C.C., Tiscareno, M.S. [2009]. Astron. J. 138, 272-286). Here we present an extended non-collisional streamline model which accounts for both effects. We describe the resulting variations of the density structure and the modification of the nonlinearity parameter q. Furthermore, an estimate is given for the applicability of the model. We use the streamwire model introduced by Stewart (Stewart, G.R. [1991]. Icarus 94, 436-450) to plot the perturbed ring density at the gap edges.We apply our model to the Keeler gap edges undulated by Daphnis and to a faint ringlet in the Encke gap close to the orbit of Pan. The modulations of the latter ringlet, induced by the perturbations of Pan (Burns, J.A., Hedman, M.M., Tiscareno, M.S., Nicholson, P.D., Streetman, B.J., Colwell, J.E., Showalter, M.R., Murray, C.D., Cuzzi, J.N., Porco, C.C., and the Cassini ISS team [2005]. Bull. Am. Astron. Soc. 37, 766), can be well described by our analytical model. Our analysis yields a Hill radius of Pan of 17.5 km, which is 9% smaller than the value presented by Porco (Porco, C.C., and 34 colleagues [2005]. Science 307, 1226-1236), but fits well to the radial semi-axis of Pan of 17.4 km. This supports the idea that Pan has filled its Hill sphere with accreted material (Porco, C.C., Thomas, P.C., Weiss, J.W., Richardson, D.C. [2007]. Science 318, 1602-1607). A numerical solution of a streamline is used to estimate the parameters of the Daphnis-Keeler gap system, since the close proximity of the gap edge to the moon induces strong perturbations, not allowing an application of the analytic streamline model. We obtain a Hill radius of 5.1 km for Daphnis, an inner edge variation of 8 km, and an eccentricity for Daphnis of 1.5 × 10−5. The latter two quantities deviate by a factor of two from values gained by direct observations (Jacobson, R.A., Spitale, J., Porco, C.C., Beurle, K., Cooper, N.J., Evans, M.W., Murray, C.D. [2008]. Astron. J. 135, 261-263; Tiscareno, M.S., Burns, J.A., Hedman, M.M., Spitale, J.N., Porco, C.C., Murray, C.D., and the Cassini Imaging team [2005]. Bull. Am. Astron. Soc. 37, 767), which might be attributed to the neglect of particle interactions and vertical motion in our model.  相似文献   

10.
The dynamics of Titan's stratosphere is discussed in this study, based on a comparison between observations by the CIRS instrument on board the Cassini spacecraft, and results of the 2-dimensional circulation model developed at the Institute Pierre-Simon Laplace, available at http://www.lmd.jussieu.fr/titanDbase [Rannou, P., Lebonnois, S., Hourdin, F., Luz, D., 2005. Adv. Space Res. 36, 2194-2198]. The comparison aims at both evaluating the model's capabilities and interpreting the observations concerning: (1) dynamical and thermal structure using temperature retrievals from Cassini/CIRS and the vertical profile of zonal wind at the Huygens landing site obtained by Huygens/DWE; and (2) vertical and latitudinal profiles of stratospheric gases deduced from Cassini/CIRS data. The modeled thermal structure is similar to that inferred from observations (Cassini/CIRS and Earth-based observations). However, the upper stratosphere (above 0.05 mbar) is systematically too hot in the 2D-CM, and therefore the stratopause region is not well represented. This bias may be related to the haze structure and to misrepresented radiative effects in this region, such as the cooling effect of hydrogen cyanide (HCN). The 2D-CM produces a strong atmospheric superrotation, with zonal winds reaching 200 m s−1 at high winter latitudes between 200 and 300 km altitude (0.1-1 mbar). The modeled zonal winds are in good agreement with retrieved wind fields from occultation observations, Cassini/CIRS and Huygens/DWE. Changes to the thermal structure are coupled to changes in the meridional circulation and polar vortex extension, and therefore affect chemical distributions, especially in winter polar regions. When a higher altitude haze production source is used, the resulting modeled meridional circulation is weaker and the vertical and horizontal mixing due to the polar vortex is less extended in latitude. There is an overall good agreement between modeled chemical distributions and observations in equatorial regions. The difference in observed vertical gradients of C2H2 and HCN may be an indicator of the relative strength of circulation and chemical loss of HCN. The negative vertical gradient of ethylene in the low stratosphere at 15° S, cannot be modeled with simple 1-dimensional models, where a strong photochemical sink in the middle stratosphere would be necessary. It is explained here by dynamical advection from the winter pole towards the equator in the low stratosphere and by the fact that ethylene does not condense. Near the winter pole (80° N), some compounds (C4H2, C3H4) exhibit an (interior) minimum in the observed abundance vertical profiles, whereas 2D-CM profiles are well mixed all along the atmospheric column. This minimum can be a diagnostic of the strength of the meridional circulation, and of the spatial extension of the winter polar vortex where strong descending motions are present. In the summer hemisphere, observed stratospheric abundances are uniform in latitude, whereas the model maintains a residual enrichment over the summer pole from the spring cell due to a secondary meridional overturning between 1 and 50 mbar, at latitudes south of 40-50° S. The strength, as well as spatial and temporal extensions of this structure are a difficulty, that may be linked to possible misrepresentation of horizontally mixing processes, due to the restricted 2-dimensional nature of the model. This restriction should also be kept in mind as a possible source of other discrepancies.  相似文献   

11.
Up to now, there has been no corroboration from Cassini CIRS of the Voyager IRIS-discovery of cyanoacetylene (HC3N) ice in Titan’s thermal infrared spectrum. We report the first compelling spectral evidence from CIRS for the ν6 HC3N ice feature at 506 cm−1 at latitudes 62°N and 70°N, from which we derive particle sizes and column abundances in Titan’s lower stratosphere. We find mean particle radii of 3.0 μm and 2.3 μm for condensed HC3N at 62°N and 70°N, respectively, and corresponding ice phase molecular column abundances in the range 1-10 × 1016 mol cm−2. Only upper limits for cloud abundances can be established at latitudes of 85°N, 55°N, 30°N, 10°N, and 15°S. Under the assumption that cloud tops coincide with the uppermost levels at which HC3N vapor saturates, we infer geometric thicknesses for the clouds equivalent to 10-20 km or so, with tops at 165 km and 150 km at 70°N and 62°N, respectively.  相似文献   

12.
We have measured the vertical shear of the zonal winds in the cloud-haze upper layer of Saturn using Cassini ISS images obtained in the filters MT2 (753 nm methane absorption band, sensitive to the upper haze) and CB2 (adjacent continuum, sensitive to the lower cloud). Our radiative transfer models indicate that at the eastward jet peaks these filters are sensing clouds at the respective ∼100 mbar and ∼350 mbar levels. We have found a systematic velocity difference between those filters of 15 to 20 m s−1 only in the eastward jets peaks (27° S, 42° S, 55° S and 70° S) which implies a vertical shear of ∼10-20 m s−1 H−1. Our overall results agree with those derived from the thermal-wind relationship using CIRS thermal data [Fletcher, L.N., and 13 colleagues, 2008. Science 319, 79-81] and with previous equatorial measurements [Sánchez-Lavega, A., Hueso, R., Pérez-Hoyos, S., 2007. Icarus 187, 510-519].  相似文献   

13.
The recent measurements of the vertical distribution and optical properties of haze aerosols as well as of the absorption coefficients for methane at long paths and cold temperatures by the Huygens entry probe of Titan permit the computation of the solar heating rate on Titan with greater certainty than heretofore. We use the haze model derived from the Descent Imager/Spectral Radiometer (DISR) instrument on the Huygens probe [Tomasko, M.G., Doose, L., Engel, S., Dafoe, L.E., West, R., Lemmon, M., Karkoschka, E., See, C., 2008a. A model of Titan's aerosols based on measurements made inside the atmosphere. Planet. Space Sci., this issue, doi:10.1016/j.pss.2007.11.019] to evaluate the variation in solar heating rate with altitude and solar zenith angle in Titan's atmosphere. We find the disk-averaged solar energy deposition profile to be in remarkably good agreement with earlier estimates using very different aerosol distributions and optical properties. We also evaluated the radiative cooling rate using measurements of the thermal emission spectrum by the Cassini Composite Infrared Spectrometer (CIRS) around the latitude of the Huygens site. The thermal flux was calculated as a function of altitude using temperature, gas, and haze profiles derived from Huygens and Cassini/CIRS data. We find that the cooling rate profile is in good agreement with the solar heating profile averaged over the planet if the haze structure is assumed the same at all latitudes. We also computed the solar energy deposition profile at the 10°S latitude of the probe-landing site averaged over one Titan day. We find that some 80% of the sunlight that strikes the top of the atmosphere at this latitude is absorbed in all, with 60% of the incident solar energy absorbed below 150 km, 40% below 80 km, and 11% at the surface at the time of the Huygens landing near the beginning of summer in the southern hemisphere. We compare the radiative cooling rate with the solar heating rate near the Huygens landing site averaging over all longitudes. At this location, we find that the solar heating rate exceeds the radiative cooling rate by a maximum of 0.5 K/Titan day near 120 km altitude and decreases strongly above and below this altitude. Since there is no evidence that the temperature structure at this latitude is changing, the general circulation must redistribute this heat to higher latitudes.  相似文献   

14.
We report new radar observations of E-class Asteroid 64 Angelina and M-class Asteroid 69 Hesperia obtained with the Arecibo Observatory S-band radar (2480 MHz, 12.6 cm). Our measurements of Angelina’s radar bandwidth are consistent with reported diameters and poles. We find Angelina’s circular polarization ratio to be 0.8 ± 0.1, tied with 434 Hungaria for the highest value observed for main-belt asteroids and consistent with the high values observed for all E-class asteroids (Benner, L.A.M., Ostro, S.J., Magri, C., Nolan, M.C., Howell, E.S., Giorgini, J.D., Jurgens, R.F., Margot, J.L., Taylor, P.A., Busch, M.W., Shepard, M.K. [2008]. Icarus 198, 294-304; Shepard, M.K., Kressler, K.M., Clark, B.E., Ockert-Bell, M.E., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M., Ostro, S.J. [2008b]. Icarus 195, 220-225). Our radar observations of 69 Hesperia, combined with lightcurve-based shape models, lead to a diameter estimate, Deff = 110 ± 15 km, approximately 20% smaller than the reported IRAS value. We estimate Hesperia to have a radar albedo of , consistent with a high-metal content. We therefore add 69 Hesperia to the Mm-class (high metal M) (Shepard, M.K., Clark, B.E., Ockert-Bell, M., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M., Ostro, S.J., Harris, A.W., Warner, B.D., Stephens, R.D., Mueller, M. [2010]. Icarus 208, 221-237), bringing the total number of Mm-class objects to eight; this is 40% of all M-class asteroids observed by radar to date.  相似文献   

15.
P.G.J. Irwin  N.A. Teanby 《Icarus》2009,203(1):287-302
Long-slit spectroscopy observations of Uranus by the United Kingdom Infrared Telescope UIST instrument in 2006, 2007 and 2008 have been used to monitor the change in Uranus’ vertical and latitudinal cloud structure through the planet’s northern spring equinox in December 2007.The observed reflectance spectra in the Long J (1.17-1.31 μm) and H (1.45-1.65 μm) bands, obtained with the slit aligned along Uranus’ central meridian, have been fitted with an optimal estimation retrieval model to determine the vertical cloud profile from 0.1 to 6-8 bar over a wide range of latitudes. Context images in a number of spectral bands were used to discriminate general zonal cloud structural changes from passing discrete clouds. From 2006 to 2007 reflection from deep clouds at pressures between 2 and 6-8 bar increased at all latitudes, although there is some systematic uncertainty in the absolute pressure levels resulting from extrapolating the methane coefficients of Irwin et al. (Irwin, P.G.J., Sromovsky, L.A., Strong, E.K., Sihra, K., Teanby, N.A., Bowles, N., Calcutt, S.B., Remedios, J.J. [2006] Icarus, 181, 309-319) at pressures greater than 1 bar, as noted by Tomasko et al. and Karkoschka and Tomasko (Tomasko, M.G., Bezard, B., Doose, L., Engel, S., Karkoschka, E. [2008] Planet. Space Sci., 56, 624-647; Karkoschka, E., Tomasko, M. [2009] Icarus). However, from 2007 to 2008 reflection from these clouds throughout the southern hemisphere and from both northern and southern mid-latitudes (30° N,S) diminished. As a result, the southern polar collar at 45°S has diminished in brightness relative to mid-latitudes, a similar collar at 45°N has become more prominent (e.g. Rages, K.A., Hammel, H.B., Sromovsky, L. [2007] Bull. Am. Astron. Soc., 39, 425; Sromovsky, L.A., Fry, P.M., Ahue, W.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R., van Dam, M.A. [2008] vol. 40 of AAS/Division for Planetary Sciences Meeting Abstracts, pp. 488-489; Sromovsky, L.A., Ahue, W.K.M., Fry, P.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R. [2009] Icarus), and the lowering reflectivity from mid-latitudes has left a noticeable brighter cloud zone at the equator (e.g. Sromovsky, L.A., Fry, P.M. [2007] Icarus, 192, 527-557;Karkoschka, E., Tomasko, M. [2009] Icarus). For such substantial cloud changes to have occurred in just two years suggests that the circulation of Uranus’ atmosphere is much more vigorous and/or efficient than is commonly thought. The composition of the main observed cloud decks between 2 and 6-8 bar is unclear, but the absence of the expected methane cloud at 1.2-1.3 bar (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987] J. Geophys. Res., 92, 14987-15001) is striking (as previously noted by, among others, Sromovsky, L.A., Irwin, P.G.J., Fry, P.M. [2006] Icarus, 182, 577-593; Sromovsky, L.A., Fry, P.M. [2007] Icarus, 192, 527-557; Sromovsky, L.A., Fry, P.M. [2008] Icarus, 193, 252-266; Karkoschka, E., Tomasko, M. [2009] Icarus) and suggests that cloud particles may be considerably different from pure condensates and may be linked with stratospheric haze particles drizzling down from above, or that tropospheric hazes are generated near the methane condensation level and then drizzle down to deep pressures as suggested by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009] Icarus).The retrieved cloud structures were also tested for different assumptions of the deep methane mole fraction, which Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009] Icarus) find may vary from ∼1-2% in polar regions to perhaps as much as 4% equatorwards of 45°N,S. We found that such variations did not significantly affect our conclusions.  相似文献   

16.
Thermal infrared spectra of Saturn from 10-1400 cm−1 at 15 cm−1 spectral resolution and a spatial resolution of 1°-2° latitude have been obtained by the Cassini Composite Infrared Spectrometer [Flasar, F.M., and 44 colleagues, 2004. Space Sci. Rev. 115, 169-297]. Many thousands of spectra, acquired over eighteen-months of observations, are analysed using an optimal estimation retrieval code [Irwin, P.G.J., Parrish, P., Fouchet, T., Calcutt, S.B., Taylor, F.W., Simon-Miller, A.A., Nixon, C.A., 2004. Icarus 172, 37-49] to retrieve the temperature structure and para-hydrogen distribution over Saturn's northern (winter) and southern (summer) hemispheres. The vertical temperature structure is analysed in detail to study seasonal asymmetries in the tropopause height (65-90 mbar), the location of the radiative-convective boundary (350-500 mbar), and the variation with latitude of a temperature knee (between 150 and 300 mbar) which was first observed in inversions of Voyager/IRIS spectra [Hanel, R., and 15 colleagues, 1981. Science 212, 192-200; Hanel, R., Conrath, B., Flasar, F.M., Kunde, V., Maguire, W., Pearl, J.C., Pirraglia, J., Samuelson, R., Cruikshank, D.P., Gautier, D., Gierasch, P.J., Horn, L., Ponnamperuma, C., 1982. Science 215, 544-548]. Uncertainties due to both the modelling of spectral absorptions (collision-induced absorption coefficients, tropospheric hazes, helium abundance) and the nature of our retrieval algorithm are quantified.Temperatures in the stratosphere near 1 mbar show a 25-30 K temperature difference between the north pole and south pole. This asymmetry becomes less pronounced with depth as the radiative time constant for the atmospheric response increases at deeper pressure levels. Hemispherically-symmetric small-scale temperature structures associated with zonal winds are superimposed onto the temperature asymmetry for pressures greater than 100 mbar. The para-hydrogen fraction in the 100-400 mbar range is greater than equilibrium predictions for the southern hemisphere and parts of the northern hemisphere, and less than equilibrium predictions polewards of 40° N.The temperature knee between 150-300 mbar is larger in the summer hemisphere than in the winter, smaller and higher at the equator, deeper and larger in the equatorial belts and small at the poles. Solar heating on tropospheric haze is proposed as a possible mechanism for this effect; the increased efficiency of ortho- to para-hydrogen conversion in the southern hemisphere is consistent with the presence of larger aerosols in the summer hemisphere, which we demonstrate to be qualitatively consistent with previous studies of Saturn's tropospheric aerosol distribution.  相似文献   

17.
We present a model for the general circulation and dynamical transport in Saturn’s upper troposphere and stratosphere and derive the effective advective circulation and eddy transport coefficients required for use in two-dimensional (latitude–altitude) photochemistry–transport models. A three-dimensional Outer-Planet General Circulation Model (OPGCM) is used to generate the transport data. We find that the OPGCM adequately captures the global-scale, pole-to pole temperature contrast, but overestimates mid- and high-latitude temperatures in the summer hemisphere by ～5 K. In addition, the model reproduces the local temperature minimum seen at the equator in Cassini Composite Infrared Spectrometer (CIRS) 0.1-mbar data but not the local maximum in 1-mbar temperatures, suggesting that it is capturing the phase of Saturn’s Semiannual Oscillation associated with a temperature minimum at the equator but not the opposite phase. The meridional circulation at low latitudes is found to be dominated by a seasonally reversing Hadley circulation, characterized by upwelling near the equator, cross-equatorial flow from summer to winter hemisphere, and strong subsidence centered near 25° latitude in the winter hemisphere. The cross-equatorial flow induces an asymmetry in which the equatorial jet is found to be stronger in the winter than in the summer stratosphere. The location of the subsidence near 25°N for Ls ～ 310° coincides with local maxima in acetylene, diacetylene, and methylacetylene mixing ratios measured by Cassini/CIRS (Guerlet, S., Fouchet, T., Bézard, B., Moses, J.I., Fletcher, L.N., Simon-Miller, A.A., Flasar, F.M. [2010]. Icarus 209, 682–695). This result supports the suggestion by Guerlet et al. (2010) that the hydrocarbon abundances are enhanced at this latitude by pronounced downward transport of hydrocarbon-rich air from above. The lateral eddy diffusion coefficient is found to typically be ～105–106 m2 s?1 at mid-latitudes, implying meridional eddy transport time scales of order 100–1000 years.  相似文献   

18.
High-resolution Cassini stereo images of Saturn's moon Phoebe have been used to derive a regional digital terrain model (DTM) and an orthoimage mosaic of the surface. For DTM-control a network of 130 points measured in 14 images (70-390 m/pixel resolution) was established which was simultaneously used to determine the orientation of the spin-axis. The J2000 spin-axis was found at Dec=78.0°±0.1° and RA=356.6°±0.3°, substantially different from the former Voyager solution. The control points yield a mean figure radius of 107.2 km with RMS residuals of 6.2 km demonstrating the irregular shape of this body. The DTM was computed from densely spaced conjugate image points determined by methods of digital image correlation. It has a horizontal resolution of 1-2 km and vertical accuracies in the range 50-100 m. It is limited in coverage, but higher in resolution than the previously derived global shape model of Phoebe [Porco et al., 2005. Cassini imaging science: initial results on Phoebe and Iapetus. Science 307, 1237-1242] and allows us to study the morphology of the surface in more detail. There is evidence for unconsolidated material from a steep and smooth slope at the rim of a 100 km impact feature. There are several conically shaped craters on Phoebe, which may hint at highly porous and low compaction material on the surface.  相似文献   

19.
Cassini/VIMS limb observations have been used to retrieve vertical profiles of hydrogen cyanide (HCN) from its 3 μm emission in the region from 600 to 1100 km altitude at daytime. While the daytime emission is large up to about 1100 km, it vanishes at nighttime at very low altitudes, suggesting that the daytime emission originates under non-LTE conditions. The spectrally integrated radiances around 3.0 μm shows a monotonically decrease with tangent altitude, and a slight increase with solar zenith angle in the 40-80° interval around 800 km.A sophisticated non-LTE model of HCN energy levels has been developed in order to retrieve the HCN abundance. The population of the HCN 0 00 1 energy level, that contributes mostly to the 3.0 μm limb radiance, has been shown to change significantly with the solar zenith angle (SZA) and HCN abundance. Also its population varies with the collisional rate coefficients, whose uncertainties induced errors in the retrieved HCN of about 10% at 600-800 km and about 5% above. HCN concentrations have been retrieved from a set of spectra profiles, covering a wide range of latitudes and solar zenith angles, by applying a line-by-line inversion code. The results show a significant atmospheric variability above ∼800 km with larger values for weaker solar illumination. The HCN shows a very good correlation with solar zenith angles, irrespective of latitude and local time, suggesting that HCN at these high altitudes is in or close to photochemical equilibrium. A comparison with UVS and UVIS measurements show that these are close to the lower limit (smaller SZAs) of the VIMS observations above 750 km. However, they are in reasonable agreement when combining the rather large UV measurement errors and the atmospheric variability observed in VIMS. A comparison of the mean profile derived here with the widely used profile reported by Yelle and Griffith (Yelle R.V., Griffith, C.A. [2003]. Icarus 166, 107-115) shows a good agreement for altitudes ranging from 850 to 1050 km, while below these altitudes our result exhibits higher concentrations.  相似文献   

20.
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   

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