The Composition, Vertical Structure and Global Variability of the Lower Cloud Deck on Venus as Determined by Radio Occultation Techniques

Citation

Cimino, Josephine Beatrice (1982) The Composition, Vertical Structure and Global Variability of the Lower Cloud Deck on Venus as Determined by Radio Occultation Techniques. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/j7et-kc02. https://resolver.caltech.edu/CaltechTHESIS:01302013-145914913

Abstract

The opportunity to determine the planetwide temperature and cloud structure of Venus using radio occultation techniques arose with Pioneer Venus. This orbiting spacecraft contained a dual frequency radio system which was used to collect radio occultation data for more than two years.

Amplitude and Doppler data provided by the radio occultation experiment provide a unique and powerful means of examining atmospheric properties in a region of Venus which may otherwise be observed only by means of a probe.

The extraction of atmospheric properties from the received occultation data is a complicated non-linear process for which careful account of all contributing uncertainties must be taken. Such atmospheric phenomena as turbulence, oblateness due to winds, perturbations in the temperature, atmospheric density or gaseous composition affect both the Doppler and the amplitude data. Uncertainties inherent in the experiment itself and the data collection and processing procedures, such as trajectory errors, fluctuations in the power profile, averaging of the data and spacecraft wobble, result in additional uncertainties which must be considered.

Absorption coefficient and temperature profiles were analyzed in detail for the effects of major sources of uncertainty. Inasmuch as the power loss due to refractive defocussing is determined from the Doppler data, uncertainties in the refractivity as well as the received power are considered. Power fluctuations are found to produce the greatest uncertainty in the S-band absorption coefficient profiles. Power fluctuations and spacecraft wobble contribute the greatest uncertainty to the X-band data. In fact, in many cases the spacecraft wobble has rendered the X-band data useless.

Absorption due to gaseous components of the atmosphere are subtracted from the measured absorption coefficient profiles before they are used to compute cloud mass contents. The H₂O mixing ratio profile used is that measured by the Venera 11 and 12 spectrophotometers. The SO₂ profile was determined from the Venera 11 and 12 and Pioneer probe gas chromatograph and mass spectrometer results. The sulfuric acid saturation vapor mixing ratio used is the equilibrium vapor pressure above a concentrated sulfuric acid-water solution at the temperatures of the lower cloud deck.

The absorption due to the gaseous components is found to represent a small part of the total absorption. In the main cloud deck, gaseous absorption contributes 10 to 20% and at the bottom of the detected absorption layer the sulfuric acid vapor contributes 60 to 100% to the absorption due to increased acid vapor pressures resulting from higher temperatures in this region. The clouds are the primary contributing absorbers in the 1 to 3 bar level of the Venus atmosphere. Below about 3 bars, absorption due to sulfuric acid vapor dominates.

As a first attempt, mass content profiles were produced from the absorption coefficient data assuming the clouds were composed of concentrated sulfuric acid-water liquid. The resulting mass contents were on the order of several grams per cubic meter. Although planetwide variability in the lower cloud deck may account for differences in these radio occultation mass contents and those determined by the Pioneer probes, a second factor discounting purely liquid clouds has arisen. The dielectric constants of liquid sulfuric acid were measured in the laboratory at S-and X-band wavelengths and the results suggest the wavelength dependence for absorption by the liquid is 1/λ². The wavelength dependence measured by the radio occultation experiment is about 1/λ^1.2. In addition, the wavelength dependence required to fit the opacity determined from the 1.35 cm microwave emission data is 1/λ^1.2 to 1/λ^1.5. Apparently the liquid sulfuric acid droplet model does not satisfy the observed wavelength dependence or mass content.

If, however, a cloud particle model consisting of a solid non-absorbing dielectric sphere with a concentric liquid sulfuric acid coating is invoked, the absorptivity of the particles increases and the mass content derived from the absorption coefficient profiles decreases. As the ratio of the core radius to the total radius (q) increases, absorption increases by more than a factor of 10 for high values of q. In the case of pure sulfuric acid droplets, the conductivity is sufficiently high that some of the field is excluded from the interior of the droplet thereby reducing the absorption. When a dielectric core of nonabsorbing material is introduced, two effects occur which contribute to the increase in the electric field which penetrates the drop. First, the relative volume filled by sulfuric acid decreases and the number of available free charges decreases. Second, the dielectric core, which is also polarized in the presence of the electric field, affects the arrangement of the free charges by attracting them to the inner surface and by acting as a barrier in the presence of the rapidly alternating electric field.

The mass contents for all orbits in the equatorial region of Venus are calculated using values of q of from 0 to 1. The resulting profiles match the probe mass content profiles at similar locations when a q of 0.98 is chosen. Values for q of 0.97 to 0.99 change the absorptivity of the cloud particles by as much as 50%.

The Wavelength dependence of the absorption for the spherical shell model varies with q from 1/λ² for pure liquid to λ^0.2 for a large core. A q of 0.98 results in a wavelength dependence of 1/λ^1.0 to 1/λ^1.4. which matches the radio occultation absorption wavelength dependence and the microwave opacity wavelength dependence.

Mass content profiles using a q of 0.98 were determined for occultations in the polar, collar, midlatitudinal and equatorial regions assuming q remains constant over the planet. The results show considerable variability in both the level and the magnitude of the lower cloud deck. The cloud layer is lowest in altitude in the polar region. This might be expected as the temperature profile is cooler in the polar region than over the rest of the planet. The mass content is greatest in the polar and collar regions; however, many of the collar profiles are cut off due to fluctuations resulting from increased turbulence in the collar region. The mass contents are least dense in the midlatitude regions.

These results support the value of using radio occultation techniques to obtain temporal and spatial coverage of the planet Venus in terms of cloud composition and mass content.

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