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1.
Mariner 9 ultraviolet spectra of the 1971 dust clouds were analyzed to obtain the phase function times single scattering albedo of the dust particles. The phase functions were matched with Mie scattering calculations for size distributions of spheres of homogeneous and isotropic material. The absorption index of the dust particles was found to increase with decreasing wavelenght from 350 nm down to about 210 nm, and drop off shortward of this wavelength. A structural shoulder occurs in the absorption spectrum between 240 and 250 nm. Titanium dioxide (anatase) has the correct transitions at 210 and 250 nm to match the absorption curve of Martian dust, and is proposed as a candidate constituent in Martian surface material. The spectral neutrality of TiO2 between 0.5 and 4 μm is consistent with visible and infrared observations of Mars. The high refractive index of TiO2 can explain the large refringence of Martian dust. The titanium dioxide content of the dust particles is estimated to be a few percent or less. Uncertainties in the results due to limitations in the model and data are discussed. 相似文献
2.
The Mariner 9 ultraviolet spectrometer observed the brightness of a region on the south polar cap centered at approximately ?87°S, 10°W. Measurements taken at various incidence and emission angles (i and ?) show that the brightness increased with decreasing air mass, ≈(sec i + sec ?). The observed intensity consists primarily of a component reflected from the cap and twice-attenuated by the atmosphere and a component diffusely reflected from the atmosphere. The diffusely reflected component was determined from nearby observations of non-polar regions at the same incidence and emission angles and was substrated from the total intensity. Inversion of the intensity difference using a formula analogous to the Bouger-Langley law yielded the optical thickness of the atmosphere. The dust cloud over the polar cap was moderately thick between November 26 and December 2, 1971. At this time the optical thickness was near unity, and it decreased approximately linearly with time, reaching a value close to that of a Rayleigh atmosphere by mid-February. The optical thickness showed little dependence on the wavelength during the early orbital observations. As the dust storm cleared, the atmospheric optical thickness exhibited increasingly strong inverse wavelength dependence. Particles large compared with the wavelength dominated the Martian dust storm. These particles are estimated to have a mean radius of about 2 μm. 相似文献
3.
Infrared spectra obtained from the Mariner 9 spacecraft during the 1971–1972 dust storm are used to derive information on the composition and particle size distribution of the dust and to study the time evolution of the storm. The dust is not composed of pure granite, basalt, basaltic glass, obsidian, quartz, andesite, or montmorillonite. The infrared spectra suggest that the dust is a mixture of materials, dominated by igneous silicates with >62;60% SiO2, or weathering products such as clay minerals, but the dust could possibly have a significant component of lower SiO2 materials such as basalt. Substantial quantities of carbonates, nitrates, or carbon suboxide are excluded from the mixture. All infrared, visible, and ultraviolet data on the Martian surface composition seem consistent with a mixture of basalt and clay minerals or high SiO2 igneous rocks, with a surface patina of oxides of iron. For all candidate compositions, the data are best matched with a size distribution that approximates a differential power law function of slope ?4. This size distribution is quite similar to terrestial size distributions in regions remote from sources of dust. The relative abundance of particles between 1- and 10-μm radius did not change during the Mariner 9 mission; thus suspended particles did not experience Stokes-Cunningham fallout but instead were supported by turbulence with an eddy diffusion coefficient, . The aerosol optical depth, standardized to 0.3-μm wavelength, varied from about 1.5 early in the mission to about 0.2 at Orbit 200. 相似文献
4.
A semiquantitative analysis of clearing in the 1971 great dust storm on Mars is presented as a function of time and altitude, using Mariner 9 orange- and visual-light photos. Steady settling of dust approximately accounts for the decline of the storm after December 22, 1971. Continuous settling cannot be invoked prior to that date; injection of dust into the atmosphere, i.e., a storm resurgence, occurred in mid-December 1971. Theoretical models of optical depth versus time are developed for various distributions of particles in the atmosphere. By intespreting settling in terms of Stokes' law, estimates of the maximum radii of dust particles throughout the atmosphere have been obtained. Models which best account for the dust-storm decline indicate particles ? 5μm in diameter high in the atmosphere, with a concentration of larger particles (? 10μm) near the ground in the lowest parts of Mars. Long-term thin high hazes should persist through much of the Martian year, perhaps clearing before perihelion. 相似文献
5.
Richard W. Zurek 《Icarus》1978,35(2):196-208
This paper examines the solar heating of the Martian atmosphere during the 1971 global dust storm observed by Mariner 9. Radiative scattering as well as absorption is included by utilizing the delta-Eddington approximation to the full radiative transfer equation. The necessary optical parameters are generated by a Mie program which uses a size distribution and a complex refractive index inferred from a number of sources, particularly from recent analyces of Mariner 9 UVS and TV observations. Assuming uniform mixing of the dust, the solar heating per unit mass during a Martian global dust storm is remarkably uniform with height for small solar zenith angles. Heating rates may reach 80°K day? for overhead sunlight. Overall, 20% of the direct insolation is absorbed by the dust-laden atmosphere. Even optically thin widespread dust hazes may produce heating rates of several degrees Kelvin per day. 相似文献
6.
Barney J. Conrath 《Icarus》1975,24(1):36-46
The secular variation of the thermal structure of the Martian atmosphere during the dissipation phase of the 1971 dust storm is examined, using temperatures obtained by the infrared spectroscopy investigation on Mariner 9. For the latitude range ?20° to ?30°, the mean temperature at the 2mbar level is found to decrease from approximately 220 K in mid-December 1971 to about 190 K by June 1972 while for the 0.3mbar level a decrease from 203 K to 160 K is observed. Over the same period, the amplitude of the diurnal temperature wave also decreased. Assuming a simplified radiative heating model, the dust optical depth is found to decrease approximately exponentially with an e-folding time of about 60 days at both the 0.3 and 2mbar levels. Stokes-Cunningham settling alone cannot account for this behavior. Sedimentation models which include both gravitational settling and vertical mixing are developed in an effort to explain the time evolution of the dust. Within the framework of a model which assumes an effective vertical diffusivity K independent of height, a mean dust particle diameter of ～2 μm is inferred. To provide the necessary vertical mixing, K ? 107 cm2sec?1 is required in the lower atmosphere. 相似文献
7.
Surface materials exposed throughout the equatorial region of Mars have been classified and mapped on the basis of spectral reflectance properties determined by the Viking II Orbiter vidicon cameras. Frames acquired at each of three wavelengths (0.45 ± 0.03 μm, 0.53 ± 0.05 μm, and 0.59 ± 0.05 μm) during the approach of Viking Orbiter II in Martian summer (Ls = 105°) were mosaicked by computer. The mosaics cover latitudes 30°N to 63°S for 360° of longitude and have resolutions between 10 and 20 km per line pair. Image processing included Mercator transformation and removal of an average Martian photometric function to produce albedo maps at three wavelengths. The classical dark region between the equator and ～30°S in the Martian highlands is composed of two units: (i) and ancient unit consisting of topographic highs (ridges, crater rims, and rugged plateaus riddled with small dendritic channels) which is among the reddest on the planet ; and (ii) intermediate age, smooth, intercrater volcanic plains displaying numerous mare ridges which are among the least red on Mars . The relatively young shield volcanoes are, like the oldest unit, dark and very red. Two probable eolian deposits are recognized in the intermediate and high albedo regions. The stratigraphically lower unit is intermediate in both color and albedo. The upper unit has the highest albedo, is very red , and is apparently the major constituent of the annual dust storms as its areal extent changes from year to year. The south polar ice cap and condensate clouds dominate the southernmost part of the mosaics. 相似文献
8.
Models of Saturn's B ring have been investigated which include the shadowing mechanism, realistic phase functions for the ring particles, and the effects of multiple scattering and a particle size dispersion. These models are based on the assumption that the rings form a layer many particles thick. A power law relation dn ∝ ??s is used for the size dispersion law of the ring particles, where dn is the number of particles with radii between ? and ? + d?. In the calculation of the infrared brightness temperature of the rings, the effect of mutual heating among the ring particles is considered quantitatively for the first time. The parameters of the polydisperse s = 2 model can be chosen to satisfy both optical (λ ? 1.1 μ) and infrared data, but the situation could be much clarified if a good phase curve for the rings were available in the red, if the ring brightness were known accurately for λ > 1 μ, and if it could be established whether the ring particles are rotating synchronously. 相似文献
9.
Sherman S.C. Wu 《Icarus》1978,33(3):417-440
Topographic contour maps of Mars are compiled by the synthesis of data acquired from various scientific experiments of the Mariner 9 mission, including S-band radio-occulation, the ultraviolet spectrometer (UVS), the infrared radiometer (IRR), the infrared interferometer spectrometer (IRIS) and television imagery, as well as Earth-based radar information collected at Goldstone, Haystack, and Arecibo Observatories. The entire planet is mapped at scales of 1:25,000,000 and 1:25,000,000 using Mercator, Lambert, and polar stereographic map projections. For the computation of map projections, a biaxial spheroid figure is adopted. The semimajor and semiminor axes are 3393.4 and 3375.7 km, respectively, with a polar flattening of 0.0052. For the computation of elevations, a topographic datum is defined by a gravity field described in terms of spherical harmonics of fourth order and fourth degree combined with a 6.1-mbar occulation pressure surface. This areoid can be approximated by a triaxial ellipsoid with semimajor axes of A = 3394.6 km and B = 3393.3 km and a semiminor axis of C = 3376.3 km. The semimajor axis A intersects the Martian surface at longitude 105°W. The dynamic flattening of Mars is 0.00525. The contour intercal of the maps is 1 km. For some prominent features where overlapping pictures from Mariner 9 are available, local contour maps at relatively larger scales were also compiled by photogrammetric methods on stereo plotters. 相似文献
10.
David Pieri 《Icarus》1976,27(1):25-50
The distribution of small channels on Mars has been mapped from Mariner 9 images, at the 1:5 000 000 scale, by the author. The small channels referred to here are small valleys ranging in width from the resolution limit of the Mariner 9 wide-angle images (～1 km) to about 10 km. The greatest density of small band occurs in dark cratered terrain. This dark zone forms a broad subequatorial band around the planet. The observed distribution may be the result of decreased small-channel visibility in bright areas due to obscuration by a high albedo dust or sediment mantle. Crater densities within two small-channel segments show crater size-frequency distributions consistent with those of the oldest of the heavily cratered plains units. Such crater densities coupled with the almost exclusive occurrence of small channels in old cratered terrain and the generally degraded appearance of small channels in the high-resolution images (～100 m) imply a major episode of small-channel formation early in Martian geologic history. 相似文献
11.
An experimental study was undertaken to determine how the spectral and photometric properties of representative Martian areas are affected by fallout of atmospheric dust suspended during dust forms. A laboratory apparatus was used to simulate the uniform fallout and deposition of particles 1 to 5 μm in diameter. Spectral measurements from 0.4- to 1.2-μm wavelengths and photometric measurements at several wavelengths were made for a number of Mars-analog materials before and after deposition of 6 × 10?5 to 1.5 × 10?3 g/cm2 of simulated fallout. These results indicate that the spectral and photometric properties of Martian regions can be affected significantly even by minute amounts of fallout. For instance, the reflectance at 0.56 μm of an average dark area will increase by 35% after deposition of only 9 × 10?5 g/cm2, and by 70% after deposition of 1.5 × 10?4 g/cm2. Thus the fallout from even one dust storm season (～2 × 10?3 g/cm2) is sufficient to change significantly the spectral and photometric characteristics of the substrate material, if the fallout were ubiquitous over the surface and if no competing processes of dust removal from surface grains occured. 相似文献
12.
Four surveys in which the geometrical parameters were suitable for observations on weak scattering objects were carried out by the Venera 9, 10 orbiters using 3000–8000 Å spectrometers. The results of one survey can be explained by a dust layer at the height of sighting h = 100–700 km. Its absence in other sessions suggests a ring structure. The spectrum of dust scattering is a power function of the wavelength with the index varying from ?2.1 at 100km to ?1.3 at 500km. A method is proposed for obtaining the optical thickness, density and size distribution of dust particles from the scattering spectra. For m > 10?14 g the number of dust particles with a mass higher than m is proportional to m?1.3. The radial optical thickness τ is 0.7 × 10?5 at 5000 Å assuming the geometric thickness δ to be 100 km. The maximum optical thickness along the normal to the plane of the ring is τn = 4 × 10?6. The mass of the ring is 20 tons or 5 × 10?3 g cm?1 per unit circumference length; the maximum mass in a column normal to the ring plane is 10?10g cm?2; the maximum density (for δ = 100 km) is 10?17 g cm?3. A satellite of Venus gradually destroyed by temperature effects and by meteorite streams and plasma fluxes is suggested as the source of dust in the ring. One of 1 km radius could sustain such a ring for a billion years. The zodiacal light intensity near Venus is estimated. 相似文献
13.
William K. Hartmann 《Icarus》1978,33(2):380-387
Mariner 9 high-resolution photos and topographic information were used to make a topographic analysis of “blue” and “red” cloud positions reported over a 66-year period in Lowell Observatory records. A sample of 77 “blue” cloud sites lay preferentially at the highest Martian elevations; 60% centered precisely on the seven major volcanic mountain peaks (unknown when the clouds were observed); another 16% lay on substantial slopes or contacts between cratered terrain and lower plains. The median altitude of blue cloud sites was 2.1 km above the global topographic median. These results agree with other evidence that most Earth-detected blue clouds are orographic uplift clouds, composed of condensates. Over half of 131 sporadic yellowish or red clouds were associated with blue clouds or volcanoes, and thus probably did not represent dust storm phenomena, contrary to a commonly held belief. Of 88 “possible dust clouds” (chosen by additional criteria), about two-thirds occur at borders between light and dark areas, in the light regions. These sites may have thin veneers of dust, and current depositional or denudational activity. Median altitude of “possible dust cloud” sites was 0.5 km below the global topographic median. Major dust storms begin in a few “core areas,” two of which associate with major basins Hellas and Isidis, probable reservoirs of mobile dust; but exact topographic control and causes of dust storms are unclear. 相似文献
14.
Earth-based UBV photometry, high-quality photographs from the Lowell Observatory collection, and Mariner 9 data have been combined with a new radiative transfer theory to derive physical parameters for the Martian surface and atmosphere, both before and during the 1971 dust storm. We find that the dust particles of the storm had a single-scattering albedo of 0.84 ± 0.02 and an asymmetry factor of 0.35 ± 0.10 in green (V) light. The geometric albedo of Mars was 0.15 and the phase integral 1.83, which yield 0.27 for the Bond albedo. The mean optical thickness of the “clear” atmosphere averaged over the whole planet was 0.15 ± 0.05 and was not detectably dependent on wavelength. Geometric albedos for the surface are 0.25 (light areas) and 0.17 (dark areas) in V, 0.095 in B (both areas), and 0.060 in U (both areas). The soil particles are moderately backward scattering with an asymmetry factor of ?0.20, indicating them to be rather opaque. The mean surface roughness, on a scale larger than that of individual dust particles and therefore large compared with the wavelength, is 0.57. This represents the depth/radius ratio of an average hole and it is only one-half as large as values typical for the Moon and asteroids. 相似文献
15.
R. O. Kuzmin has proposed that all crater-associated wind streaks on Mars are depositional and consist of unresolved barchan-like dunes. He claims that any streak can appear either bright or dark relative to its surroundings depending on the azimuth of the Sun relative to the streak axis and on the elevation of the Sun above the horizon. Our studies of the entire Mariner 9 picture collection as well as of available Viking data lend no support to these ideas. We find that the conditions for visibility of bright and dark streaks are identical. In Mariner 9 images both types of streaks are visible for viewing angles ? ? 60°, illumination angles of 15° ? i ? 75°, and over the whole range of phase angles covered (about 15 to 85°). There are numerous examples of dark and light streaks visible at the same azimuth angle of the Sun, contrary to Kuzmin's claim. There is much evidence to indicate that bright and dark streaks differ both in morphology and in character. The common ragged dark streaks are probably erosion scars, while most bright streaks probably represent accumulations of bright dust-storm fallout. There is no evidence at present that these accumulations have a barchan-like texture. 相似文献
16.
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
17.
Medium spectral resolution (20 cm?1) infrared measurements of the Martian disk made between 2900 and 5600 cm?1 from the NASA Lear Airborne Observatory have been successfully compared with predictions derived from a model of the Martian soil and atmosphere. Modeling of the Martian atmosphere permitted the extraction of Martian soil reflectance in the CO2 bands centered at 3657 cm?1. Three previously considered acceptable Martian soil analogs, limonite, montmorillonite, and basalt, were analyzed to determine the optical complex indices of refraction in the same range as the airborne observations, for mathematical modeling. A characteristic surface particle size ～1 to 3 μm diameter is indicated. It is concluded that the Martian soil surface near-infrared optical properties are consistent with a soil composition similar to montmorillonite or limonite, mixed with a basalt. 相似文献
18.
The photometric properties of local areas on Mars are studied using Minnaert's rule of surface scattering to analyze Mariner 6 and 7 Infrared Spectrometer data. Several bright deserts, Hellas, and the south polar cap are found to obey Minnaert's function well. The coefficients B0(α, λ) and k(α, λ) are obtained at α = 39, 48, 56, 84° and λ = 1.85, 2.23, 3.50 μm. Observed bright regions all have similar values of k, except for Hellas and the south polar cap. The lower k of Hellas is apparently caused by microscopic effects rather than by large-scale roughness due to cratering. The higher k of the cap is similar to terrestrial snows in the visual at the same phase angle. Using existing Earth-based observations, at smaller α and λ, a bolometric Bond albedo of is calculated. 相似文献
19.
The computed variation of the infrared flux and polarization of Venus as a function of phase angle, based upon multiple-scattering calculations for the cloud model of Kawabata et al. (1980) with an internal heat source, precludes the possibility of sulfuric acid as the composition of the haze particles located above the main cloud. Furthermore, our calculations reveal that the hazticle should have a large absorption coefficient at these wavelengths, i.e, . The optical thickness of the haze layer must be about 0.15 at λ = 3.4 μm. 相似文献
20.
Marcel Nicolet 《Planetary and Space Science》1984,32(11):1467-1468
A recent determination by D. R. Bates of the Rayleigh scattering cross section (σRS) for air from 0.2 to 1 μm leads to a simple empirical formula (λ in μm) where for the spectral region 0.2 μm < λ < 0.55 μm ; the accuracy is within ±0.5%. From the visible at 0.55 μm to the infrared (i.r.) at 1 μm, the same accuracy can be obtained using a constant value, x = 0.04. The formula accounts for the degree of depolarization which varies with the wavelength according to the latest determination by Bates. 相似文献