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1.
We investigated the two deepest absorption bands observed in the spectra of stars and protostars, the water-ice band with the center near 3.1 μm and the silicate band with the center near 9.7 μm, by using a core-mantle confocal spheroid model with various axial ratios and relative volumes of the core material. We considered the effect of grain size, shape, structure, chemical composition, and orientation on the central wavelengths of the two bands, their full widths at half maximum (FWHMs), the ratio of the optical depths at their centers, and the polarization. We found that the observed relationships between the FWHMs of the bands and the ratio of their optical depths at the band centers could be explained if we chose slightly oblate or prolate particles (a/b ? 2) of small sizes (rv ? 0.35 μm) with a silicate core and a thin ice mantle (Vcore/Vtotal ? 0.7).  相似文献   

2.
We investigate the linear polarization in the two deepest infrared absorption bands observed in the spectra of protostars, the water-ice band with the center near 3.1 μm and the silicate band with the center near 9.7 μm, using a core-mantle confocal spheroid model with various axial ratios a/b and relative volumes of the core material. We consider the effect of the grain shape, structure, and type (oblate, prolate) as well as the type of grain orientation and its location relative to the incident ray of light and the magnetic field direction on the central wavelengths of the two bands and the polarizability in the bands. We have found that the observed relationships between the polarizability in the bands and the ratio of their optical depths at the band centers can be explained if we choose slightly oblate or prolate particles (a/b ≲2 for the silicate band and 1.3 ≲ a/b ≲ 2 for the ice band). For any type of orientation, the core-mantle confocal spheroid model requires different axial ratios for the ice and silicate bands to account for the observed polarization. We show that picket-fence-oriented particles can explain the observed polarization in the ice band at angles α between the particle rotation axis and the incident ray ≳30° and in the silicate band at any α. Perfectly Davis-Greenstein-oriented particles can explain the observed polarization in the ice band at angles Ω between the line of sight and the magnetic field direction ≳60° and in the silicate band at any Ω. The orientation parameter ζ (imperfect Davis-Greenstein orientation) must be no more than 0.5 (oblate particles) and 0.1 (prolate particles) for the ice band and can be arbitrary for the silicate band. Original Russian Text ? T.V. Zinov’eva, 2006, published in Pis’ma v Astronomicheskiĭ Zhurnal, 2006, Vol. 32, No. 10, pp. 748–766.  相似文献   

3.
We present the results of solving the radiative transfer equation for the Stokes vector in the case of light scattering by spherical forsterite dust particles in an axisymmetric circumstellar envelope of a red giant. We have assumed that the surfaces of constant scattering-particle density are prolate or oblate spheroids, the particle density decreases with radius as N d ∝ r −2, and the dust particles at the inner boundary of the envelope are in thermal equilibrium with the stellar emission at solid-phase evaporation temperature T ev = 800 K. In the wavelength range 0.27 μm ≤ λ ≤ 1 μm, particles with radii 0.03 μm ≲ a ≲ 0.2 μm make a major contribution to the linear polarization of the stellar emission. The increase in scattering efficiency factor with decreasing wavelength λ is mainly responsible for the growth of polarization toward the short wavelengths known from observations. However, at a mean number of scatterings 1.2 ≤ N sca ≤ 1.6, the polarization ceases to grow due to depolarization effects and decreases rapidly as the wavelength decreases further. The wavelength of the polarization maximum is determined mainly by two quantities: the particle radius and the mass loss rate. The upper limits for the degree of linear polarization in the case of light scattering in circumstellar dust envelopes with the geometries of prolate and oblate spheroids are p ≈ 3 and 5%, respectively. The polarization for light scattering by enstatite particles is higher than that for light scattering by forsterite particles approximately by 0.3%. Original Russian Text ? Yu.A. Fadeyev, 2007, published in Pis’ma v Astronomicheskiĭ Zhurnal, 2007, Vol. 33, No. 2, pp. 123–133.  相似文献   

4.
Multiaperture K photometry and 2.0- to 2.5-μm spectrophotometry of Uranus and its ring system are presented. The photometric results are used, together with a previously published measurement, to set limits on the geometric albedos of Uranus and the rings at ～2.2 μm: (0.74 ± 0.02) × 10(su?4) ≤ pK (Uranus) ≤ (1.5 ± 0.3) × 10?4, and (2.7 ± 0.6) × 10?2 ≤ pK (rings) ≤ (3.4 ± 0.1) × 10?2. Reflectance spectra of Uranus and Uranus plus rings show features in the planet's spectrum which are attributed to gaseous CH4 absorption, and a 2.20-μm feature in the combined spectrum which may be due to the rings. This feature is tentatively identified with either the 2.26-μm absorption feature of NH3 frost, or the 2.2-μm OH band exhibited by certain silicate minerals. The results of JHK photometry of Uranus' satellite, Ariel (U1), indicate that the infrared colors of this object are very similar to those of the satellites U2, U3, and U4.  相似文献   

5.
H.M. Schmid  F. Joos  D. Gisler 《Icarus》2011,212(2):701-713
We present ground-based limb polarization measurements of Jupiter and Saturn consisting of full disk imaging polarimetry for the wavelength 7300 Å and spatially resolved (long-slit) spectropolarimetry covering the wavelength range 5200-9350 Å.For the polar region of Jupiter we find for λ = 6000 Å a very strong radial (perpendicular to the limb) fractional polarization with a seeing corrected maximum of about +11.5% in the South and +10.0% in the North. This indicates that the polarizing haze layer is thicker at the South pole. The polar haze layers extend down to 58° in latitude. The derived polarization values are much higher than reported in previous studies because of the better spatial resolution of our data and an appropriate consideration of the atmospheric seeing. Model calculations demonstrate that the high limb polarization can be explained by strongly polarizing (p ≈ 1.0), high albedo (ω ≈ 0.98) haze particles with a scattering asymmetry parameter of g ≈ 0.6 as expected for aggregate particles of the type described by West and Smith (West, R.A., Smith, P.H. [1991]. Icarus 90, 330-333). The deduced particle parameters are distinctively different when compared to lower latitude regions.The spectropolarimetry of Jupiter shows a decrease in the polar limb polarization towards longer wavelengths and a significantly enhanced polarization in strong methane bands when compared to the adjacent continuum. This is a natural outcome for a highly polarizing haze layer above an atmosphere where multiple scatterings are suppressed in absorption bands. For lower latitudes the fractional polarization is small, negative, and it depends only little on wavelength except for the strong CH4-band at 8870 Å.The South pole of Saturn shows a lower polarization (p ≈ 1.0-1.5%) than the poles of Jupiter. The spectropolarimetric signal for Saturn decrease rapidly with wavelength and shows no significant enhancements in the fractional polarization in the absorption bands. These properties can be explained by a vertically extended stratospheric haze region composed of small particles <100 nm as suggested previously by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2005]. Icarus 179, 195-221).In addition we find in the V- and R-band a previously not observed strong polarization feature (p = 1.5-2.0%) near the equator of Saturn. The origin of this polarization signal is unclear but it could be related to a seasonal effect.Finally we discuss the potential of ground-based limb polarization measurements for the investigation of the scattering particles in the atmospheres of Jupiter and Saturn.  相似文献   

6.
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   

7.
A spectrum from 1.2 to 2.5 μm of Uranus' small satellite Miranda obtained in June 1999 reveals strong water-ice signatures. It confirms the existence of a 2.0-μm water feature previously detected on Miranda and shows a strong second broad 1.5-μm water-ice absorption feature. The spectra also reveal a weak absorption band at 1.65 μm that is indicative of crystalline water ice. Reflectance models which combine the new spectra with new photometry indicate that the spectra are characteristic of a mostly water-ice surface, with a large fraction of carbonaceous or silicate contaminates, and the possible presence of ammonia hydrate, as implied by an apparent weak feature near 2.2 μm. The possible presence of other volatiles is also investigated.  相似文献   

8.
We report results of telescope polarimetric imaging of the Moon with a CCD LineScan Camera at large phase angles, near 88°. This allows measurements of the polarization degree with an absolute accuracy better than 0.3% and detection of features with polarization contrast as small as 0.1%. The measurements are carried out in two spectral bands centered near 0.65 and 0.42 μm. We suggest characterizing the lunar regolith with the parameter a(Pmax)A, where Pmax,A, and a are the degree of maximum polarization, albedo, and the parameter describing the linear regression of the correlation Pmax-A. The parameter bears significant information on the particle characteristic size and packing density of the lunar regolith. We also suggest characterizing the lunar regolith with color-ratio images obtained with a polarization filter at large phase angles. We here consider the color-ratios C||(0.65/0.42 μm) and C⊥(0.65/0.42 μm). Using light scattering model calculations we show that the color-ratio images obtained with a polarization filter at large phase angles suggest a new tool to study the lunar surface. In particular, it turns out that the color-ratios C||(0.65/0.42 μm) and C⊥(0.65/0.42 μm) are sensitive to somewhat different thicknesses of the surfaces of regolith particles. We consider the applicability of the Hubble Space Telescope, the Very Large Telescope (ESO), and a spacecraft on a lunar polar orbit for polarimetric observations of the lunar surface.  相似文献   

9.
The water ice and silicate dust bands centered at about 3 and 10 μm, respectively, are simultaneously observed in the spectra of several objects. So far the wavelength dependence of the polarization in both bands has been modeled using two-layer spheroids, with the shape of the silicate core being confocal to that of the ice mantle. We show that nonconfocality of the spheroidal core and mantle boundaries changes fundamentally the wavelength dependence of the polarization within the 10-μm silicate band and affects significantly the polarization within the 3-μm water ice band, while the extinction profiles of these bands remain essentially unchanged. Since the results have been obtained for a theoretical model, we discuss their applicability and significance for cosmic dust grains. Original Russian Text ? M.S. Prokopjeva, V.B. Il’in, 2007, published in Pis’ma v Astronomicheskiĭ Zhurnal, 2007, Vol. 33, No. 10, pp. 784–791.  相似文献   

10.
E.A. Cloutis  P. Hudon  T. Hiroi  M.J. Gaffey 《Icarus》2012,217(1):389-407
Powdered samples of a suite of 14 CR and CR-like chondrites, ranging from petrologic grade 1 to 3, were spectrally characterized over the 0.3–2.5 μm interval as part of a larger study of carbonaceous chondrite reflectance spectra. Spectral analysis was complicated by absorption bands due to Fe oxyhydroxides near 0.9 μm, resulting from terrestrial weathering. This absorption feature masks expected absorption bands due to constituent silicates in this region. In spite of this interference, most of the CR spectra exhibit absorption bands attributable to silicates, in particular an absorption feature due to Fe2+-bearing phyllosilicates near 1.1 μm. Mafic silicate absorption bands are weak to nonexistent due to a number of factors, including low Fe content, low degree of silicate crystallinity in some cases, and presence of fine-grained, finely dispersed opaques. With increasing aqueous alteration, phyllosilicate: mafic silicate ratios increase, resulting in more resolvable phyllosilicate absorption bands in the 1.1 μm region. In the most phyllosilicate-rich CR chondrite, GRO 95577 (CR1), an additional possible phyllosilicate absorption band is seen at 2.38 μm. In contrast to CM spectra, CR spectra generally do not exhibit an absorption band in the 0.65–0.7 μm region, which is attributable to Fe3+–Fe2+ charge transfers, suggesting that CR phyllosilicates are not as Fe3+-rich as CM phyllosilicates. CR2 and CR3 spectra are uniformly red-sloped, likely due to the presence of abundant Fe–Ni metal. Absolute reflectance seems to decrease with increasing degree of aqueous alteration, perhaps due to the formation of fine-grained opaques from pre-existing metal. Overall, CR spectra are characterized by widely varying reflectance (4–21% maximum reflectance), weak silicate absorption bands in the 0.9–1.3 μm region, overall red slopes, and the lack of an Fe3+–Fe2+ charge transfer absorption band in the 0.65–0.7 μm region.  相似文献   

11.
Results are given for polarization measurements of both the entire Jupiter disk and its centre for seven wavelength regions in the 0.373–0.800 μm range. Interpretation of these observations is based on two model atmospheres: (A) The cloud layer particles and molecules are mixed with a constant ratio. (B) A gas layer with small optical thickness, τ0, is situated above the cloud layer which consists of aerosol particles. The aerosol particles are considered to be non-absorbing spheres, their size distribution being normal Gaussian. The index of refraction for the particles is considered to be independent of wavelength in the above spectral range. An approximate method is used for the determination of parameters of the Jovian atmosphere. This method was tested by evaluation of the parameters for the Venus cloud layer: The refractive index was found to be n = 1.435 ± 0.015, the square of the logarithmic dispersion of the radius of particles σ2 = 0.12 and the mean geometrical radius of particles r0 = 0.74 μm which agree well with exact values given by Hansen and Arking (1971). For the atmosphere of Jupiter it was found: n = 1.36 ± 0.01, σ2 ? 0.3, r0 ? 0.2 μm. This refractive index for the particles agrees well with the ammonia cloud layer hypothesis.  相似文献   

12.
Spectra of Asteroid 9969 Braille in the 1.25-2.6 μm region returned by the Deep Space 1 (DS1) Mission show a ∼10% absorption band centered at 2 μm, and a reflectance peak at 1.6 μm. Analysis of these features suggest that the composition of Braille is roughly equal parts pyroxene and olivine. Its spectrum between 0.4 and 2.5 μm suggests that it is most closely related to the Q taxonomic type of asteroid. The spectrum also closely matches that of the ordinary chondrites, the most common type of terrestrial meteorite. The geometric albedo of Braille is unusually high (pv=0.34), which is also consistent with its placement within the rarer classes of stony asteroids, and which suggests it has a relatively fresh, unweathered surface, perhaps due to a recent collision.  相似文献   

13.
The upper atmospheric layer of Venus, Mars, Jupiter, Saturn, and earth contains an aerosol layer. The meteorites, rings, and removal of small planetary particles may be responsible for its appearance. The observations from 1979–1992 have shown that the optical aerosol thickness over the earth’s polar regions varies from τ ≈ 0.0002 to 0.1 to λ = 1 μm. The highest τ value was in 1984 and 1992 and was preceded by intense activity of the El Chichon (1982) and Pinatubo (1991) volcanoes. We have shown that increase in τ of the stratospheric aerosol may lead to decrease in ozone layer registered in the 1970s. The nature of the stratospheric aerosol (a real part of the refraction index), effective size particles r, and latitudinal variation τ remain unknown. The analysis of phase dependence of the degree of polarization is effective among the distal methods of determination of n r and r. The observation value of intensity and degree of polarization in the visible light are caused by the optical surface properties and optical atmospheric thickness, whose values varied with latitude, longitude, and in time. Thus, it is impossible to correctly distinguish the contribution of the stratospheric aerosol. In UV-rays (λ < 300 nm), the ozone layer stops the influence of the surface and earth’s atmosphere up to height of 20–25 km. In this spectrum area, the negative factors are emission of various depolarizating gases, horizontal heterogeneity of the effective optical height of the ozone layer, and oriented particles indicated by variation of the polarization plane.  相似文献   

14.
New image-tube spectra of Triton are analyzed for a determination of the reflectance of the satellite between 0.32 and 0.74 μm. Comparison of the violet reflectance of Triton with that of terrestrial minerals, lunar samples, and meteorites, gives evidence that the satellite surface is composed largely of rocky material having the same sources of violet opacity (mineral charge transfer and crystal field transitions). New radiometric observations set a stringent upper limit to the satellite radius (r ? 2600 km) and a lower limit to the geometric albedo (pv ? 0.19). The albedo can be somewhat higher and still within the range allowed by a rocky surface. No useful constraints can be put on the mean density of Triton because of remaining uncertainties in the radius and the mass. The image-tube spectra show no evidence of gaseous absorption in the methane bands, though a stronger band has been found in the infrared at 2.3 μm (Cruikshank and Silvaggio, 1979, in press; the near-infrared photometric colors may be affected by the CH4 band. Rayleigh scattering computations of a potential inert atmospheric component of Triton appear to preclude the presence of large quantities of nitrogen and the noble gases.  相似文献   

15.
L.A. Sromovsky 《Icarus》2005,173(1):254-283
Raman scattering by H2 in Neptune's atmosphere has significant effects on its reflectivity for λ<0.5 μm, producing baseline decreases of ∼20% in a clear atmosphere and ∼10% in a hazy atmosphere. However, few accurate Raman calculations are carried out because of their complexity and computational costs. Here we present the first radiation transfer algorithm that includes both polarization and Raman scattering and facilitates computation of spatially resolved spectra. New calculations show that Cochran and Trafton's (1978, Astrophys. J. 219, 756-762) suggestion that light reflected in the deep CH4 bands is mainly Raman scattered is not valid for current estimates of the CH4 vertical distribution, which implies only a 4% Raman contribution. Comparisons with IUE, HST, and groundbased observations confirm that high altitude haze absorption is reducing Neptune's geometric albedo by ∼6% in the 0.22-0.26 μm range and by ∼13% in the 0.35-0.45 μm range. A sample haze model with 0.2 optical depths of 0.2-μm radius particles between 0.1 and 0.8 bars fits reasonably well, but is not a unique solution. We used accurate calculations to evaluate several approximations of Raman scattering. The Karkoschka (1994, Icarus 111, 174-192) method of applying Raman corrections to calculated spectra and removing Raman effects from observed spectra is shown to have limited applicability and to undercorrect the depths of weak CH4 absorption bands. The relatively large Q-branch contribution observed by Karkoschka is shown to be consistent with current estimates of Raman cross-sections. The Wallace (1972, Astrophys. J. 176, 249-257) approximation, produces geometric albedo ∼5% low as originally proposed, but can be made much more accurate by including a scattering contribution from the vibrational transition. The original Pollack et al. (1986, Icarus 65, 442-466) approximation is inaccurate and unstable, but can be greatly improved by several simple modifications. A new approximation based on spectral tuning of the effective molecular single scattering albedo provides low errors for zenith angles below 70° in a clear atmosphere, although intermediate clouds present problems at longer wavelengths.  相似文献   

16.
Joseph Masiero 《Icarus》2010,207(2):795-799
We have investigated the effect of rotation on the polarization of scattered light for the near-Earth asteroid (1943) Anteros using the Dual Beam Imaging Polarimeter on the University of Hawaii’s 2.2 m telescope. Anteros is an L-type asteroid that has not been previously observed polarimetrically. We find weak but significant variations in the polarization of Anteros as a function of rotation, indicating albedo changes across the surface. Specifically, we find that Anteros has a background albedo of pv = 0.18 ± 0.02 with a dark spot of pv < 0.09 covering <2% of the surface.  相似文献   

17.
To provide material for interpretations of forthcoming zodiacal light measurements the characteristics of 468 single-component, in-ecliptic models are summarized in two survey diagrams. The models are based on Mie theory and on a power law dn ∞ r?γα?kdα for the dependence of the particle number density n on solar distance r and on the size parameter α (circumference/wavelength). The size range involves particles with αmin ≤ α ≤ 120; (αmin = 1,2,4,10,60), flat (k = 2·5) and steep (k = 4) size spectra, and complex refractive indices m = m1 ? m2i with m1 = 1·33; 1·5; 1·7 and m2 = 0; 0·01; 0·05; 0·1.The models suggest that the spatial variation of dust particle number densities should be less than about ∞ r?0·5 in the ecliptic plane. Either dielectric particles of tenth-micron size or absorbing particles of half-micron size or very slightly absorbing particles of some tens of microns in size are able to produce polarization that agrees in sign and location of the maximum with the observations. Ambiguities can only be removed by considering intensity and polarization over a wide range of wavelengths.  相似文献   

18.
Dale P. Cruikshank 《Icarus》1979,37(1):307-309
The radius and surface geometric albedo of Hyperion are calculated using the photometric/ radiometric method and a new measurement of the 20-μm thermal flux of the satellite. The results are R = 112 ± 15 km and pv = 0.47 ± 0.11.  相似文献   

19.
We present the results of a study which uses a sample of 1822 Sloan Digital Sky Survey (SDSS) quasars with reliable Wide-field Infrared Survey Explorer (WISE) detections in the redshift range 1.7≤z≤4.38 to investigate the mid-infrared fraction of broad absorption line (BAL) quasars. The BAL quasars in the sample include both high-ionization BAL (HiBAL) quasars that show broad absorption from C?iv and low-ionization BAL (LoBAL) quasars that show additional broad absorption from Mg?ii. The fraction of C?iv BAL quasars with nonzero absorption index (AI) is found to be 38.7±1.2 %, in good agreement with that derived for the Two Micron All Sky Survey (2MASS) sample. The C?iv BAL quasar fractions remain constant with magnitude in the WISE 3.4 μm (W1) and 4.6 μm (W2) bands, and increase rapidly with decreasing magnitude in the WISE 12 μm (W3) and 22 μm (W4) bands. The nonzero AI fraction of 44.5±2.1 % determined in the WISE W4 band is more likely to represent the intrinsic BAL quasar fraction. No evidence that the fraction is a strong function of redshift is found. At 1.7≤z≤2.15, the overall mid-infrared LoBAL fraction is $3.3^{+0.6}_{-0.5}~\%$ and the fractions increase significantly with decreasing magnitude in all four of WISE bands. Moreover, it is found that the mean optical-to-WISE colors of BAL quasars are ?0.2 mag redder than that of non-BAL quasars, while the traditional (nonzero balnicity) BAL quasars are redder than the nontraditional BAL quasars by ?0.15 mag, which suggest a continuum of more reddening from non-BAL to nontraditional BAL to traditional BAL. No evidence that nontraditional BALs are a distinct class from traditional BALs is found. Finally, it is shown that the mean optical-to-WISE colors of LoBALs are ?0.4 mag redder than that of HiBALs at 1.7≤z≤2.15.  相似文献   

20.
The chemistry leading to the formation of solid aerosols (tholins) in Titan's atmosphere is simulated by a capacitively coupled plasma in a N2-CH4 gas mixture. The solid grains are produced in volume directly in the gas phase and studied ex-situ by SEM imaging and by light scattering on clouds of particles. The scattered light properties depend on the physical properties of the particles (morphologies, size distribution), as well as on the phase angle and the wavelength of the light. The particles may be aggregated or agglomerated grains. The grains size distribution is studied as a function of plasma parameters such as initial methane concentration introduced into the discharge, gas flow, absorbed RF power and plasma duration. The average grain size increases when the amount of CH4 increases, when the gas flow decreases, and when the plasma duration increases up to a limit for each production condition.For all the samples, the absorption decreases with increasing wavelength in the visible domain. As usually found for irregular particles, the polarization phase curves have a bell-shaped positive branch and a shallow negative branch. The maximum of polarization (Pmax) increases when the average grain size decreases (sub-μm-sized grains). To obtain Pmax values within the range of those measured in Titan's atmosphere; the average grains diameter has to be smaller than 100 nm, in agreement with the space observations results. In the light-scattering experiment, the size of the agglomerates in the clouds is in the 40-80 μm range in equivalent diameter. As a consequence Pmax increases with decreasing wavelength due to the increasing absorption, in agreement with observations of Titan from outside the atmosphere.  相似文献   

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