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1.
Glenn S. Orton 《Icarus》1983,53(2):293-300
It is possible for large particles of NH3 ice to explain two phenomena associated with observations of thermal infrared emission from the atmosphere of Saturn: (1) the depression of thermal brightness near the equator, which is coincident with a visibly bright zone-like region, and (2) some disagreements between infrared and radio occultation results. Particles of NH3 ice can provide the requisite opacity to explain the contrast between the equatorial region and the brighter area near 15°S for Pioneer Saturn Infrared Radiometer 45-μm channel data. NH3 ice particle clouds can also reconcile the 45-μm brightness of both regions (near the equator and near 15°S) with the mean temperatures structure of the Voyager 2 radio occultation results. A cloud model with ice particles distributed in equal ratio with gas particles up to the 100-mbar pressure level best fits the equatorial data; a thinner cloud or one which does not extend higher than the 400-mbar limit of the convective region best matches data for the 15°S region. At 20 μm, however, the radio occultation temperature structure predicts brightnesses which are lower than those observed for both regions, and it could indicate the possibility that another source of opacity which is latitudinally variable exists in the stratosphere.  相似文献   

2.
A simple jovian cloud scheme has been developed for the Oxford Planetary Unified model System (OPUS). NH3-ice, NH4SH-solid, H2O-ice and H2O-liquid clouds have been modeled in Southern hemisphere limited area simulations of Jupiter. We found that either three or four of the condensates existed in the model. For a deep atmospheric water abundance close to solar composition, an NH3-ice deck above 0.7 bar, an NH4SH-solid deck above 2.5 bar and a H2O-liquid deck with a base at about 7.5 bar and frozen cloud tops formed. If a depleted deep water abundance is assumed, however, a very compact cloud structure develops, where an H2O-ice cloud forms by direct sublimation above 3 bar. The condensates constitute good tracers of atmospheric motion, and we have confirmed that zonal velocities determined from manual feature tracking in the modeled cloud layers agree reasonably well with the modeled zonal velocities. Dense and elevated clouds form over latitudes with strong atmospheric upwelling and depleted clouds exist over areas with strong downwelling. In the NH3-ice deck this leads to elevated cloud bands over the zones in the domain and thin clouds over the belts, which is consistent with the observationally deduced distribution. Due to changes in the vertical velocity pattern in the deeper atmosphere, the NH4SH-solid and water cloud decks are more uniform. This modeled cloud structure thus includes the possibility of more frequent water cloud observations in belts, as this deeper deck could be more easily detected under areas with thin NH3-ice clouds. Large scale vortices appeared spontaneously in the model and were characterized by elevated NH3-ice clouds, as expected from observations. These eddies leave the most discernible imprint on the lighter condensate particles of the uppermost layer.  相似文献   

3.
The formation, evolution and properties of noctilucent clouds are studied using a timedependent one-dimensional model of ice particles at mesospheric altitudes. The model treats ice crystals, meteoric dust, water vapor and air ionization as fully interactive cloud elements. For ice particles, the microphysical processes of nucleation, condensation, coagulation and sedimentation are included; the crystal habits of ice are also accounted for. Meteoric dust is analyzed in the manner of Hunten et al. (1980). The simulated particle sizes range from 10 Å to 2.6μm. The chemistry of water vapor and the charge balance of the mesosphere are also analyzed in detail.Based on model calculations, including numerous sensitivity tests, several conclusions are reached. Extremely cold mesopause temperatures (<140K) are necessary to form noctilucent clouds; such temperatures only exist at high latitudes in summer. A water vapor concentration of 4–5 ppmv is sufficient to form a visible cloud. However, a subvisible cloud can exist in the presence of only 1 ppmv of H2O. Ample cloud condensation nuclei are always present in the mesosphere; at very low temperatures, either meteoric dust or hydrated ions can act as cloud nuclei. To be effective, meteoric dust particles must be larger than 10–15 Å in radius. When dust is present, water vapor supersaturations may be held to such low values that ion nucleation is not possible. Ion nucleation can occur, however, in the absence of dust or at extremely low temperatures (<130K). While dust nucleation leads to a small number (<10cm?3) of large ice particles (>0.05 μm radius) and cloud optical depths (at 550 nm) ～10?4, ion nucleation generally leads to a large number (～103cm?3) of smaller particles and optical depths ～10?5). However, because calculated nucleation rates in noctilucent clouds are highly uncertain, the predominant nucleus for the clouds (i.e., dust or ions) cannot be unambiguously established. Noctilucent clouds require several hours-up to a day-to materialize. Once formed, they may persist for several days, depending on local meteorological conditions. However, the clouds can disappear suddenly if the air warms by 10–20 K. The environmental conditions which exist at the high-latitude summer mesopause, together with the microphysics of small ice crystals, dictate that particle sizes will be ? 0.1 μm radius. The ice crystals are probably cubic in structure. It is demonstrated that particles of this size and shape can explain the manifestations of noctilucent clouds. Denser clouds are favored by higher water vapor concentrations, more rapid vertical diffusion and persistent upward convection (which can occur at the summer pole). Noctilucent clouds may also condense in the cold “troughs” of gravity wave trains. Such clouds are bright when the particles remain in the troughs for several hours or more; otherwise they are weak or subvisible.Model simulations are compared with a wide variety of noctilucent cloud data. It is shown that the present physical model is consistent with most of the measurements, as well as many previous theoretical results. Ambient noctilucent clouds are found to have a negligible influence on the climate of Earth. Anthropogenic perturbations of the clouds that are forecast for the next few decades are also shown to have insignificant climatological implications.  相似文献   

4.
We report high-spectral-resolution (λ/δλ = 800-2300) near-infrared mapping observations of Mars at Ls = 130° (April 1999), which were obtained by drift-scanning the cryogenic long-slit spectrometer at the KPNO 2.2-m telescope across the disk. Data were reformatted into calibrated spectral image cubes (x,y,λ) spanning 2.19 to 4.12 μm, which distinguish atmospheric CO2 features, solar lines, and surface and aerosol features. Maps of relative band depth between 3.0 and 3.5 μm trace water ice clouds and show the diurnal evolution of features in the persistent northern summer aphelion cloud belt, which was mapped contemporaneously but at fixed local time by the Mars Global Surveyor Thermal Emission Spectrometer (MGS/TES). Cloud optical depth, particle sizes, and ice aerosol content were estimated using a two-stream, single-layer scattering model, with Mie coefficients derived from recently published ice optical constants, followed by a linear spectral deconvolution process. A comparison of data and model spectra shows evaporating nighttime clouds in the morning followed by afternoon growth of a prominent orographic cloud feature on the west flank of Elysium Mons. Cloud optical depth at 3.2 μm evolved to 0.28 ± 0.13 and ice aerosol column abundance to 0.9 ± 0.3 pr μm in the afternoon. Column abundances as large as 0.17 pr μm were retrieved in nonorographic clouds within the aphelion cloud band around midday. These clouds exhibit a modest decline in optical depth during the afternoon. Results show that ice particle radii from <2 μm to >4 μm exist in both cloud types. However, large particles dominate the spectra, consistent with recent MGS/TES emission phase function measurements of aphelion cloud aerosol properties.  相似文献   

5.
A spectrum of the disk of Jupiter was obtained in January 1978 from the Kuiper Airborne Observatory, covering the 100- to 300-cm?1 spectral range at a resolution corresponding to 1.65 cm?1. Although taken more than a year before the Voyager 1 Jupiter encounter, this spectrum serves to extend the Voyager IRIS experiment coverage down from its lower limit of 200 cm?1. Analysis of the spectrum provides information on global mean properties of ammonia gas and an ammonia ice haze. A vertical distribution indistinguishable from saturation equilibrium, with a sharp depletion near the temperature minimum, matches the observed shape of the rotational line absorption best. Constraints on the total optical thickness of the ammonia ice haze can be made, but other properties, such as particle size or vertical scale height, cannot be distinguished clearly from our data in this spectral region. Nevertheless, all models of the haze produce a “continuum” thermal emission between the NH3 line manifolds which is much lower than that produced by the H2 collision-induced dipole opacity.  相似文献   

6.
V.G. Teifel 《Icarus》1977,30(1):138-154
Results of photoelectric measurements of the intensity in CH4 5430, 6190, and 7250 Å absorption bands, CH4 absorption lines in the 3ν3 band, and the NH3 6457.1 Å line are examined from the point of view of a model which takes into account the role of multiple scattering inside a homogeneous semi-infinite cloud layer in the formation of absorption components in the Jovian spectrum. Introduced are a number of simple ratios between depths of lines and bands and the parameters which characterize the properties of the cloud layer and the atmosphere above the clouds for occurrence of the Henyey-Greenstein scattering phase function at various degrees of asymmetry in g. The CH4 content inside the cloud layer is determined as an equivalent thickness on the mean free path between scattering events. The latter was found to be equal to AL ? 10 ± 2 m-amagat at g = 0.75 or AL ? 20 ± 3 m-amagat at g = 0.5 along all the above-mentioned CH4 absorption bands. For NH3 it is AL ? 31 ± 4 cm-amagat at g = 0.75 and AL ? 62 ± 8 cm-amagat at g = 0.5.The weakening of the CH4 absorption bands toward the edges of the Jovian disc requires a volume scattering coefficient in the cloud layer of σa ～ 10?6 cm?1. The mean specific abundance of NH3 obtained within the cloud layer does not contradict the calculated abundance of saturated gaseous ammonia.  相似文献   

7.
Observations of water ice clouds and dust are among the main scientific goals of the Planetary Fourier Spectrometer (PFS), a payload instrument of the European Mars Express mission. We report some results, obtained in three orbits: 37, 41 and 68. The temperature profile, and dust and water ice cloud opacities are retrieved from the thermal infrared (long-wavelength channel of PFS) in a self-consistent way using the same spectrum. Orographic ice clouds are identified above Olympus (orbit 37) and Ascraeus Mons (orbit 68). Both volcanoes were observed near noon at Ls=337° and 342°, respectively. The effective radius of ice particles is preliminary estimated as 1-3 μm, changing along the flanks. The corresponding visual opacity changes in the interval 0.2-0.4 above Olympus and 0.1-0.6 above Ascraeus Mons. In the case of Ascraeus Mons, the ice clouds were observed mainly above the Southern flank of the volcano with maximum opacity near the summit. In the case of Olympus, the clouds were found above both sides of the top. A different type of ice cloud is observed at latitudes above 50°N (orbit 68) in the polar hood: the effective particle radius is estimated to be 4 μm. Below the 1 mb level an inversion in the temperature profiles is found with maximum temperature at around 0.6 mb. Along orbit 68 it appears above Alba Patera, then it increases to the north and decreases above the CO2 polar cap. Beginning from latitude 20°S above Tharsis (orbit 68), the ice clouds and dust contribute equally to the spectral shape. Further on, the ice clouds are found everywhere along orbit 68 up to the Northern polar cap, except the areas between the Northern flank of Ascraeus Mons (below 10 km) and the edge of Alba Patera. Orbit 41 is shifted from the orbit 68 by roughly 180° longitude and passes through Hellas. Ice clouds are not visible in this orbit at latitudes below 80°S. The dust opacity is anticorrelated with the surface altitude. From 70°S to 25°N latitude the vertical dust distribution follows an exponential law with a scale height of 11.5±0.5 km, which corresponds to the gaseous scale height near noon and indicates a well-mixed condition. The 9 μm dust opacity, reduced to zero surface altitude, is found to be 0.25±0.05, which corresponds to a visual opacity of 0.5-0.7 (depending on the particle size).  相似文献   

8.
Vertical distributions and spectral characteristics of Titan’s photochemical aerosol and stratospheric ices are determined between 20 and 560 cm?1 (500–18 μm) from the Cassini Composite Infrared Spectrometer (CIRS). Results are obtained for latitudes of 15°N, 15°S, and 58°S, where accurate temperature profiles can be independently determined.In addition, estimates of aerosol and ice abundances at 62°N relative to those at 15°S are derived. Aerosol abundances are comparable at the two latitudes, but stratospheric ices are ～3 times more abundant at 62°N than at 15°S. Generally, nitrile ice clouds (probably HCN and HC3N), as inferred from a composite emission feature at ～160 cm?1, appear to be located over a narrow altitude range in the stratosphere centered at ～90 km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58°S.There is some evidence of a second ice cloud layer at ～60 km altitude at 58°S associated with an emission feature at ～80 cm?1. We speculate that the identify of this cloud may be due to C2H6 ice, which in the vapor phase is the most abundant hydrocarbon (next to CH4) in the stratosphere of Titan.Unlike the highly restricted range of altitudes (50–100 km) associated with organic condensate clouds, Titan’s photochemical aerosol appears to be well-mixed from the surface to the top of the stratosphere near an altitude of 300 km, and the spectral shape does not appear to change between 15°N and 58°S latitude. The ratio of aerosol-to-gas scale heights range from 1.3–2.4 at about 160 km to 1.1–1.4 at 300 km, although there is considerable variability with latitude. The aerosol exhibits a very broad emission feature peaking at ～140 cm?1. Due to its extreme breadth and low wavenumber, we speculate that this feature may be caused by low-energy vibrations of two-dimensional lattice structures of large molecules. Examples of such molecules include polycyclic aromatic hydrocarbons (PAHs) and nitrogenated aromatics.Finally, volume extinction coefficients NχE derived from 15°S CIRS data at a wavelength of λ = 62.5 μm are compared with those derived from the 10°S Huygens Descent Imager/Spectral Radiometer (DISR) data at 1.583 μm. This comparison yields volume extinction coefficient ratios NχE(1.583 μm)/NχE(62.5 μm) of roughly 70 and 20, respectively, for Titan’s aerosol and stratospheric ices. The inferred particle cross-section ratios χE(1.583 μm)/χE(62.5 μm) appear to be consistent with sub-micron size aerosol particles, and effective radii of only a few microns for stratospheric ice cloud particles.  相似文献   

9.
We observed the H2CO(110–111) absorption lines and H110α radio recombination lines (RRL) toward 180 NH3 sources using the Nanshan 25-m radio telescope. In our observation, 138 sources were found to have H2CO lines and 36 have H110α RRLs. Among the 138 detected H2CO sources, 38 sources were first detected. The detection rates of H2CO have a better correlation with extinction than with background continuum radiation. Line center velocities of H2CO and NH3 agree well. The line width ratios of H2CO and NH3 are generally larger than unity and are similar to that of 13CO. The correlation between column densities of H2CO and extinction is better than that between NH3 and extinction. These line width relation and column density relation indicate H2CO is distributed on a larger scale than that of NH3, being similar to the regions of 13CO. The abundance ratios between NH3 and H2CO were found to be different in local clouds and other clouds.  相似文献   

10.
Data processing and interpretation of the nephelometer measurements made in the Venus atmosphere aboard the Venera 9, 10 and 11 landers in the sunlit hemisphere near the equator are discussed. These results were used to obtain the aerosol distribution and its microphysical properties from 62 km to the surface. The main aerosol content is found in the altitude range between 62 km (where measurements began) and 48 km, the location of the cloud region. Three prominent layers labeled as I (between 62 and 57 km), II (between 57 and 51 km) and III (between 51 and 48 km), each with different particle characteristics are discovered within the clouds. The measured light-scattering patterns can be intrepreted as having been produced by particles with effective radii from 1 to 2 μm depending on height and indices of refractivity from 1.45 in layer I to 1.42 in layer III. These values do not contradict the idea that the droplets are made of sulfuric acid. In layers II and III the particle size distribution is at least bimodal rather than uni-modal. The index of refraction is found to decrease to 1.33 in the lower part of layer II, suggesting a predominant abundance of larger particles of different chemical origin, and chlorine compounds are assumed to be relevant to this effect. In the entire heightrange of the Venera 9–11 craft descents, the clouds are rather rarefied and are characterized by a mean volume scattering coefficient σ ～ 2 × 10?5 cm?1 that corresponds to the mean meteorological range of visibility of about 2 km. The average mass content of condensate is estimated to be equal to 4 × 10?9 g/cm3, and the total optical depth of clouds to τ ～ 35. Near the bottom of layer III clouds are strongly variable. In the subcloud atmosphere a haze was observed between 48 and 32 km; that haze is mainly made of submicron particles, reff ～ 0.1μm. The atmosphere below that is totally transparent but separate (sometimes possibly disappearing) layers may be present up to a height of 8 km above the surface. A model of this region with a very low particle density (N ? 2–3 cm?3) strongly refractive large particles (reff ? 2.5 μm; 1.7 < n < 2.0) provided satisfactory agreement. The optical depth of aerosol in the atmosphere below the subcloud haze does not exceed 2.5.  相似文献   

11.
Observations of Uranus were made in September 2009 with the Gemini-North telescope in Hawaii, using both the NIFS and NIRI instruments. Observations were acquired in Adaptive Optics mode and have a spatial resolution of approximately 0.1″.NIRI images were recorded with three spectral filters to constrain the overall appearance of the planet: J, H-continuum and CH4(long), and long slit spectroscopy measurements were also made (1.49-1.79 μm) with the entrance slit aligned on Uranus’ central meridian. To acquire spectra from other points on the planet, the NIFS instrument was used and its 3″ × 3″ field of view stepped across Uranus’ disc. These observations were combined to yield complete images of Uranus at 2040 wavelengths between 1.476 and 1.803 μm.The observed spectra along Uranus central meridian were analysed with the NEMESIS retrieval tool and used to infer the vertical/latitudinal variation in cloud optical depth. We find that the 2009 Gemini data perfectly complement our observations/conclusions from UKIRT/UIST observations made in 2006-2008 and show that the north polar zone at 45°N has continued to steadily brighten while that at 45°S has continued to fade. The improved spatial resolution of the Gemini observations compared with the non-AO UKIRT/UIST data removes some of the earlier ambiguities with our previous analyses and shows that the opacity of clouds deeper than the 2-bar level does indeed diminish towards the poles and also reveals a darkening of the deeper cloud deck near the equator, perhaps coinciding with a region of subduction. We find that the clouds at 45°N,S lie at slightly lower pressures than the clouds at more equatorial latitudes, which suggests that they might possibly be composed of a different condensate, presumably CH4 ice, rather than H2S or NH3 ice, which is assumed for the deeper cloud. In addition, analysis of the centre-to-limb curves of both the Gemini/NIFS and earlier UKIRT/UIST IFU observations shows that the main cloud deck has a well-defined top, and also allows us to better constrain the particle scattering properties.Overall, Uranus appeared to be less convectively active in 2009 than in the previous 3 years, which suggests that now the northern spring equinox (which occurred in 2007) is passed the atmosphere is settling back into the quiescent state seen by Voyager 2 in 1986. However, a number of discrete clouds were still observed, with one at 15°N found to lie near the 500 mb level, while another at 30°N, was seen to be much higher at near the 200 mb level. Such high clouds are assumed to be composed of CH4 ice.  相似文献   

12.
The infrared AOTF spectrometer is a part of the SPICAM experiment onboard the Mars-Express ESA mission. The instrument has a capability of solar occultations and operates in the spectral range of 1-1.7 μm with a spectral resolution of ∼3.5 cm−1. We report results from 24 orbits obtained during MY28 at Ls 130°-160°, and the latitude range of 40°-55° N. For these orbits the atmospheric density from 1.43 μm CO2 band, water vapor mixing ratio based on 1.38 μm absorption, and aerosol opacities were retrieved simultaneously. The vertical resolution of measurements is better than 3.5 km. Aerosol vertical extinction profiles were obtained at 10 wavelengths in the altitude range from 10 to 60 km. The interpretation using Mie scattering theory with adopted refraction indices of dust and H2O ice allows to retrieve particle size (reff∼0.5-1 μm) and number density (∼1 cm−3 at 15-30 km) profiles. The haze top is generally below 40 km, except the longitude range of 320°-50° E, where high-altitude clouds at 50-60 km were detected. Optical properties of these clouds are compatible with ice particles (effective radius reff=0.1-0.3 μm, number density N∼10 cm−3) distributed with variance νeff=0.1-0.2 μm. The vertical optical depth of the clouds is below 0.001 at 1 μm. The atmospheric density profiles are retrieved from CO2 band in the altitude range of 10-90 km, and H2O mixing ratio is determined at 15-50 km. Unless a supersaturation of the water vapor occurs in the martian atmosphere, the H2O mixing ratio indicates ∼5 K warmer atmosphere at 25-45 km than predicted by models.  相似文献   

13.
G Tobie  F Forget  F Lott 《Icarus》2003,164(1):33-49
In 1998, the Mars Orbiter Laser Altimeter revealed the presence of isolated or quasi-periodic thick clouds during the martian polar night. They are believed to be composed of CO2 ice particles and to be tilted against the wind direction, a feature characteristic of vertically propagating orographic gravity waves. To support that interpretation, we present here numerical simulations with a two-dimensional anelastic model of stratified shear flow that includes simple CO2 ice microphysics. In some of the simulations presented, the orography is an idealized trough, with dimensions characteristic of the many troughs that shape the Mars polar cap. In others, it is near the real orography. In the polar night conditions, our model shows that gravity waves over the north polar cap are strong enough to induce adiabatic cooling below the CO2 frost point. From this cooling, airborne heterogeneous nucleation of CO2 ice particles occurs from the ground up to the altitude of the polar thermal inversion. Although the model predicts that clouds can be present above 15 km, only low altitude clouds can backscatter the Laser beams of MOLA at a detectable level. Accordingly, the shape of the Laser echoes is related to the shape of the clouds at low level, but do not necessarily coincide with the top of the clouds. The model helps to interpret the cloud patterns observed by MOLA. Above an isolated orographic trough, an isolated extended sloping cloud tilted against the wind is obtained. The model shows that the observed quasi-periodic clouds are due to the succession of small-scale topographic features, rather than to the presence of resonant trapped lee waves. Indeed, the CO2 condensation greatly damps the buoyancy force, essential for the maintenance of gravity waves far from their sources. Simulations with realistic topography profiles show the cloud response is sensitive to the wind direction. When the wind is directed upslope of the polar cap, on the one hand, a large scale cloud, modulated by small-scale waves, forms just above the ground. On the other hand, when the wind is directed downslope, air is globally warmed, and periodic ice clouds induced by small-scale orography form at altitudes higher than 3-5 km above the ground. In both cases, a good agreement between the simulated echoes and the observed one is obtained. According to our model, we conclude that the observed clouds are quasi-stationary clouds made of moving ice particles that successively grow and sublimate by crossing cold and warm phases of orographic gravity waves generated by the successive polar troughs. We also find that the rate of ice precipitation is relatively weak, except when there is a large scale air dynamical cooling.  相似文献   

14.
The atmospheric transmission window at 2.7 μm in Jupiter's atmosphere was observed at a spectral resolution of 0.1 cm?1 from the Kuiper Airborne Observatory. From analysis of the CH4 abundance (～80m-am) and the H2O abundance (<0.0125cm-am) it was determined that the penetration depth of solar flux at 2.7 μm is near the base of the NH3 cloud layer. The upper limit to H2O at 2.7 μm and other recent results suggest that photolytic reactions in Jupiter's lower troposphere may not be as significant as was previously thought. The search for H2S in Jupiter's atmosphere yielded an upper limit of ～0.1cm-am. The corresponding limit to the elemental abundance ratio [S]/[H] was ～1.7 × 10?8, about 10?3 times the solar value. Upon modeling the abundance and distribution of H2S in Jupiter's atmosphere it was concluded that, contrary to expectations, sulfur-bearing chromophores are not present in significant amounts in Jupiter's visible clouds. Rather, it appears that most of Jupiter's sulfur is locked up as NH4SH in a lower cloud layer. Alternatively, the global abundance of sulfur in Jupiter may be significantly depleted.  相似文献   

15.
We have measured the shape and absolute value of Venus' reflectivity spectrum in the 1.2-to 4.0-μm spectral region with a circular variable filter wheel spectrometer having a spectral resolution of 1.5%. The instrument package was mounted on the 91-cm telescope of NASA Ames Kuiper Airborne Observatory, and the measurements were obtained at an altitude of about 41,000 feet, when Venus had a phase angle of 86°. Comparing these spectra with synthetic spectra generated with a multiple-scattering computer code, we infer a number of properties of the Venus clouds. We obtain strong confirmatory evidence that the clouds are made of a water solution of sulfuric acid in their top unit optical depth and find that the clouds are made of this material down to an optical depth of at least 25. In addition, we determine that the acid concentration is 84 ± 2% H2SO4 by weight in the top unit optical depth, that the total optical depth of the clouds is 37.5 ± 12.5, and that the cross-sectional weighted mean particle radius lies between 0.5 and 1.4 μm in the top unit optical depth of the clouds. These results have been combined with a recent determination of the location of the clouds' bottom boundary [Marov et al., Cosmic Res.14, 637–642 (1976)] to infer additional properties about Venus' atmosphere. We find that the average volume mixing ratio of H2SO4 and H2O contained in the cloud material both equal approximately 2× 10?6. Employing vapor pressure arguments, we show that the acid concentration equals 84 ± 6% at the cloud bottom and that the water vapor mixing ratio beneath the clouds lies between 6 × 10?4 and 10?2.  相似文献   

16.
Spectra from the Voyager 1 infrared interferometer spectrometer (IRIS) obtained near the time of closest approach to Jupiter were analyzed for the purpose of inferring ammonia cloud properties associated with the Equatorial Region. Comparisons of observed spectra with synthetic spectra computed from a radiative transfer formulation, that includes multiple scattering, yielded the following conclusions: (1) very few NH3 ice particles with radii less than 3 μm contribute to the cloud opacity; (2) the major source of cloud opacity arises from particles with radii in excess of 30 μm; (3) column particle densities are between 1 and 2 orders of magnitude smaller than those derived from thermochemical considerations alone, implying the presence of important atmospheric motion; and (4) another cloud system is confirmed to exist deeper in the Jovian troposphere.  相似文献   

17.
In March 1979, the spectrum of Venus was recorded in the far infrared from the G.P. Kuiper Airborne Observatory when the planet subtended a phase angle of 62°. The brightness temperature was observed to be 275°K near 110 cm?1, dropping to 230°K near 270 cm?1. Radiance calculations, using temperature and cloud structure formation from the Pioneer Venus mission and including gaseous absorption by the collision-induced dipole of CO2, yield results consistently brighter than the observations. Supplementing the spectral data, Pioneer Venus OIR data at similar phase angles provide the constraint that any additional infrared opacity must be contained in the upper cloud, H2SO4 to the Pioneer-measured upper cloud structure serves to reconcile the model spectrum and the observations, but cloud microphysics strongly indicates that such a high particle density haze (N ? 1.6 × 107cm?3) is implausible. The atmospheric environment is reviewed with regard to the far infrared opacity and possible particle distribution modifications are discussed. We conclude that the most likely possibility for supplementing the far-infrared opacity is a population of large particles (r ? 1 μm) in the upper cloud with number densities less than 1 particle cm?3 which has remained undetected by in situ measurements.  相似文献   

18.
We have constructed a model of the physical processes controlling Titan's clouds. Our model produces clouds that qualitatively match the present observational constraints in a wide variety of model atmospheres, including those with low atmospheric pressures (25 mbar) and high atmospheric pressures. We find the following: (1) high atmospheric temperatures (160°K) are important so that there is a large scale height in the first few optical depths of cloud; (2) the aerosol mass production occurs at very low aerosol optical depth so that the cloud particles do not directly affect the photochemistry producing them; (3) the production rate of aerosol mass by chemical processes is probably greater than 3.5 × 10?14 g cm?2 sec?1; (4) and the eddy diffusion coefficient is less than 5 × 106 cm2 sec?1 except perhaps in the top optical depth of the cloud. Our model is not extremely sensitive to particle shape, but it is sensitive to particle density. Higher particle densities require larger aerosol mass production rates to produce satisfactory clouds. Particle densities of unity require a mass production rate on the order of 3.5 × 10?13 g cm?2 sec?1. We also show that an increase in mass input causes a decrease in the mean particle size, as required by J. B. Pollack et al. (1980, Geophys. Res. Lett. 7, 829–832), to explain the observed correlation between the solar cycle and Titan's albedo; that coagulation need not be extremely inefficient in order to obtain realistic clouds as proposed by M. Podolak and E. Podolak (1980, Icarus43, 73–83); that coagulation could be inefficient due to photoelectric charging of the particles; and, that the lifetime of particles near the altitude of unit optical depth is a few months, as required to explain the temporal variability observed by S. T. Suess and G. W. Lockwood and D. P. Cruikshank and J. S. Morgan (1979, Bull. Amer. Astron. Soc.11, 564). Although Titan's aerosols are ottically thick in the vertical direction, the atmosphere is so extended that the horizontal visibility is greater than that found anywhere at Earth's surface.  相似文献   

19.
Results of the scattered solar radiation spectrum measurements made deep in the Venus atmosphere by the Venera 11 and 12 descent probes are presented. The instrument had two channels: spectrometric (to measure downward radiation in the range 0.45 < γ < 1.17 μm) and photometric (four filters and circular angle scanning in an almost vertical plane). Spectra and angular scans were made in the height range from 63 km above the planet surface. The integral flux of solar radiation is 90 ± 12 W m?2 measured on the surface at the subsolar point. The mean value of surface absorbed radiation flux per planetary unit area is 17.5 ± 2.3 W m?2. For Venera 11 and 12 landing sites the atmospheric absorbed radiation flux is ～15 W m?2 for H >; 43 km and ～45 W m?2 for H < 48 km in the range 0.45 to 1.55 μm. At the landing sites of the two probes the investigated portion of the cloud layer has almost the same structure: it consists of three parts with boundaries between them at about 51 and 57 km. The base of clouds is near 48 km above the surface. The optical depth of the cloud layer (below 63 km) in the range 0.5 to 1 μm does not depend on the wavelength and is ～29 and ～38 for the Venera 11 and 12 landing sites, respectively. The single-scattering albedo, ω0, in the clouds is very close to 1 outside the absorption bands. Below 58 km the parameter (1 ? ω0) is <10?3 for 0.49 and 0.7 μm. The parameter (1 ? ω0) obviously increases above 60 km. Below 48 km some aerosol is present. The optical depth here is a strong function of wavelength. It varies from 1.5 to 3 at λ = 0.49 μm and from 0.13 to 0.4 at 1.0 μm. The mean size of particles below the cloud deck is about 0.1 μm. Below 35 km true absorption was found at λ < 0.55 μm with the (1 ? ω0) maximum at H ≈ 15 km. The wavelength and height dependence of the absorption coefficient are compatible with the assumption that sulfur with a mixing ratio ～2 × 10?8 normalized to S2 molecules is the absorber. The upper limits of the mixing ratio for Cl2, Br2, and NO2 are 4 × 10?8, 2 × 10?11, and 4 × 10?10, respectively. The CO2 and H2O bands are confidently identified in the observed spectra. The mean value of the H2O mixing ratio is 3 × 10?5 < FH2O < 10?4 in the undercloud atmosphere. The H2O mixing ratio evidently varies with height. The most probable profile is characterized by a gradual increase from FH2O = 2 × 10?5 near the surface to a 10 to 20 times higher value in the clouds.  相似文献   

20.
From an analysis of the Galileo Near Infrared Imaging Spectrometer (NIMS) data, Baines et al. (Icarus 159 (2002) 74) have reported that spectrally identifiable ammonia clouds (SIACs) cover less than 1% of Jupiter. Localized ammonia clouds have been identified also in the Cassini Composite Infrared Spectrometer (CIRS) observations (Planet. Space Sci. 52 (2004a) 385). Yet, ground-based, satellite and spacecraft observations show that clouds exist everywhere on Jupiter. Thermochemical models also predict that Jupiter must be covered with clouds, with the top layer made up of ammonia ice. For a solar composition atmosphere, models predict the base of the ammonia clouds to be at 720 mb, at 1000 mb if N/H were 4×solar, and at 0.5 bar for depleted ammonia of 10−2×solar (Planet. Space Sci. 47 (1999) 1243). Thus, the above NIMS and CIRS findings are seemingly at odds with other observations and cloud physics models. We suggest that the clouds of ammonia ice are ubiquitous on Jupiter, but that spectral identification of all but the freshest of the ammonia clouds and high altitude ammonia haze is inhibited by a combination of (i) dusting, starting with hydrocarbon haze particles falling from Jupiter's stratosphere and combining with an even much larger source—the hydrazine haze; (ii) cloud properties, including ammonia aerosol particle size effects. In this paper, we investigate the role of photochemical haze and find that a substantial amount of haze material can deposit on the upper cloud layer of Jupiter, possibly enough to mask its spectral signature. The stratospheric haze particles result from condensation of polycyclic aromatic hydrocarbons (PAHs), whereas hydrazine ice is formed from ammonia photochemistry. We anticipate similar conditions to prevail on Saturn.  相似文献   

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