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A Determination of the Composition of the Venus Clouds from Aircraft Observations in the Near Infrared
Abstract
Summary of the evidence showing that the first optical depth of the Venus cloud layer is composed of a water solution of sulfuric acid, including earlier aircraft observations of Venus' reflectivity in the region from 1 to 4 microns obtained at a phase angle of 120 deg. Analyses of these aircraft results indicated that of all the proposed cloud candidates only a sulfuric acid solution with a concentration of 75% or more H2SO4 by weight was consistent with the observed 3-micron cloud feature. Aircraft observations of Venus are presented which were obtained in the same spectral region at a phase angle of 40 deg and in the region from 3 to 6 microns at a phase angle of 136 deg. Comparing the two sets of observations in the region from 1 to 4 microns, a striking phase effect is found: the reflectivity is much lower in the 3-micron region and there is a much more marked decline between 1.3 and 2.5 microns for the data obtained at the smaller phase angle. The observations made at the 40-deg phase angle are consistent with the theoretical behavior of a sulfuric acid cloud and imply that the sulfuric acid is present to at least many tens of optical depth below the cloud tops. Arguments concerning the concentration of the solution are reviewed, and it is concluded that the best current estimate is about 85% H2SO4 by weight.
On January 17, 2003, novelist Michael Crichton gave Caltech’s Michelin Lecture. Its title was Aliens Cause Global Warming. “I am going to argue that extraterrestrials lie behind global warming. Or to speak more precisely, I will argue that a belief in extraterrestrials has paved the way, in a progression of steps, to a belief in global warming.”1 His paper begins with an attack on the Drake equation and SETI, then on Carl Sagan and nuclear winter, and then finally expands to an attack on all scientific modeling. Since one can choose any set of values to base a model calculation on (according to Crichton), any result is possible. Thus modeling is a religion, and global warming theology, not science.
This publication is an attempt to systematically apply numerical simulations to investigations into cosmochemical and planetological systems. The approaches formulated during the development of stable techniques of numerical simulation of physicochemical processes on the basis of equilibrium thermodynamics, newly obtained estimates of the physicochemical parameters of the preplanetary nebula and the interiors of planets, as well as newly developed approaches to the modeling of stratification processes in self-gravitating systems can be regarded as the first systematic steps toward the creation of a comprehensive model for the physicochemical evolution of matter from the stage of interstellar dust to the distribution of this material in the outer shells of planetary bodies in natural geologic bodies, for example, layered intrusions.
New data about the top clouds of Venus were obtained during the radiometric experiment on-board the Venera 9 and Venera 10 orbiters. A diurnal component of the ir thermal radiation was determined for the latitude range −40, +50°. The brightness temperature of radiation referred to the normal was measured; it was 244°K at night and 239°K at the subsolar point for the 7- to 13-, 17- to 30-μm bands. Minimum temperatures correspond to the meridian of local time 16.00h and are 232°K. There is also a zone of lower temperatures in the region of local time 7.5h. Absolute temperatures were measured with an accuracy of −1.9°+1.2°. Thermal radiation has no distinct latitudinal dependence but has a day-night asymmetry, with the night radiation flux exceeding that on the day side by 17%. The limb-darkening law for thermal radiation is rather complicated, depending on the time of day. There are at least two states of the radiating cloud cover: day and night. The extinction coefficient is close to 0.24 km−1. The analysis shows that the source function of the medium is close to Planck's function. During the day the flux of thermal radiation is assumed to be weakened by an aerosol medium forming by photochemical processes. Comparison of experimental and calculated data yields a particle concentration in the radiating cloud cover of about 95 cm−3. Experimental data and the results of ground-based measurements were used to determine the radiometric albedo of Venus, 0.79−0.01+0.02.
The spatial and temporal variations of the polarization of light
scattered by Venus, as observed by the Pioneer Venus Orbiter between
1978 and 1990, is analyzed in terms of spatial and temporal variations
of Venus upper haze properties. Special attention is given to choosing
maps with sufficiently accurate geometrical information. This selection
involves multiple-scattering calculations for a number of reasonable
models for the Venus atmosphere. For a set of observations in four
wavelength bands of the Venus disk as a whole or a particular region
thereof, observed values of Stokes parameters I and Q were used to
obtain the average observed relative Stokes parameter qobs.
Subsequently, the haze particle column density and the cloud top
pressure were iteratively adjusted until values of qobs at
two wavelengths were reproduced. For the planet as a whole, it was found
that the haze particle column density decreased gradually during the
Pioneer Venus Orbiter mission, whereas little significant temporal
variability was found for the cloud top pressure. Similar long-term
behavior was derived for selected regions of Venus. Regions at higher
latitudes exhibited higher values of both the haze particle column
density and the cloud top pressure than regions at lower latitudes.
Something similar holds for regions with low solar elevations as
compared to regions in which the Sun was close to the zenith.
The results of the particle size spectrometer experiment on the Pioneer Venus sounder probe are presented. The vertical cloud structure is found to consist of three primary cloud regions of approximately 20 km total thickness suspended within an ubiquitous aerosol haze which extends more than 10 km above and below it. The three cloud regions are separated by sharp transition regions where both particle chemistry and microphysics exhibit change. The size distributions are multimodal in all cloud regions. Three size modes are observed in the middle and lower cloud region which are composed of aerosol, H2SO4 droplets, and crystals. The crystals likely could be either sulphates or chlorides. We provide interpretations of the sources, growth characteristics, and fate of the particle species through a partitioning analysis of the LCPS size distribution data. The cloud particle size spectrometer (hereafter LCPS) was a primary experiment aboard the sounder probe during the Pioneer Venus (PV) mission. This single particle device individually sized particles traversing its illuminated field during descent from 66 km to the planet's surface. Covering a size range of 0.5-500/m, it detailed the cloud microstructure from medium-sized aerosol nuclei to small precipitation elements. The relatively higher number density of small aerosol particles observed proved sufficient in population to realize the LCPS's maximum vertical resolution of 100 m in the denser cloud re
The instruments employed in the investigation were modified for aircraft use primarily by automating the repetitive measurements and incorporating image intensifier tubes to reduce exposure times. The instruments include a 35-mm camera, an absolutely calibrated photometer, and a 16-mm movie camera. The data obtained will be used in a study of the variation of parallax emission heights with geographical location, and the relationship of OH patch or stripe size to emission height.
A contradiction in the sulfuric acid cloud hypothesis of Venus, viz., nondetection of 4.8-micron polarization by Landau (1975), is examined on the basis of multiple-scattering calculations for the cloud model of Hansen and Hovenier (1974) including an internal heat source. Results show that the polarized thermal component cannot depolarize the scattered sunlight and, therefore, a large polarization of about 13% is expected at a phase angle of 110 deg and wavelength of 4.8 microns, in contrast to Landau's measurements. The present computations are, however, in agreement with the measurements by Sato et al. (1977).
Narrowband filter (3.6-micron wavelength, 0.08-micron bandwidth) observations of the infrared flux of Venus are reported. The observations over the period from 1982 through 1984 covered a range of phase angle from 27 to 94 degrees. Normalized values of flux at the Venus-earth distance of 1 AU were (4.0-5.4) x 10 to the -17th watt per square centimeter per inverse centimeter, and the phase angle dependence of the data is rather weak. Furthermore, when the evening terminator of Venus was seen, lower values of flux were obtained, in contrast with higher values at the morning terminator. The phase angle dependence is quite different from that of Martonchik and Beer (1975), the difference possibly suggesting an intrinsic time variation of haze particles over ten years in the upper haze layer of the Venus cloudcover.
A comprehensive study was performed to determine the major scientific
unknowns about the planet Venus to be expected in the post-Pioneer Venus
1978 time frame. Based on those results the desirability of future
orbiters, atmospheric entry probes, balloons, and landers as vehicles to
address the remaining scientific questions were studied. The recommended
mission scenario includes a high resolution surface mapping radar
orbiter mission for the 1981 launch opportunity, a multiple-lander
mission for 1985 and either an atmospheric entry probe or balloon
mission in 1988. All the proposed missions can be performed using
proposed space shuttle upper stage boosters. Significant amounts of
long-lead time supporting research and technology developments are
required to be initiated in the near future to permit the recommended
launch dates.
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