Clouds are the main absorbers contributing to the 1 to 3 bar level of Venus' atmosphere. The mixing ratio of the gaseous components of the atmosphere for the orbits. a) Intensity as a function of frequency for sulfuric lines.
The refractive index of the lower cloud layer mode 3 particles was determined to be 1.2 to 1.3. A final contribution to the chemistry of the lower cloud cover was made by Surkov (1980) on Venera 11 and 12.
3 SBN
BAND
MARINER 10 S-BAND
A radio signal with a given amplitude and frequency is sent from the spacecraft. to Earth via a path which is bent as it passes through the atmosphere due to refraction of the beam. On Earth, the received signal is Doppler shifted due to a change in the projected speed of the beam as it bends.
EARTH
SPACECRAFT
Temperature-pressure profiles are calculated from the refractivity profiles derived from the Doppler data as described in the previous section. To calculate refractivity from the Doppler or diffraction angle data, an Abel integral transform must be used.
Overview of the procedure for determining the error due to a disturbance in the refraction profile. Pressure and temperature profiles are then calculated from n, and 1'lpp to reveal the final effect of the disturbance. The kind of perturbations in the refractive index profile studied here are signatures in the refractive index profile that are not correctly modeled with an assumed C02, N2 atmosphere.
These perturbations can result from a number of atmospheric phenomena, including variations in gas composition, contributions from liquid or solid particles, and effects due to atmospheric flattening. The change in the refractive index of the atmosphere due to the presence of a cloud can be calculated using any of several theoretical models (Van Beek, 1967). The disturbance c!Icct extends above and below the actual disturbance level for an additional kilometer in each direction.
ORBIT 9N S-BAND
The Abel transform apparently has little effect on the perturbed refractive index profile for perturbations of 1% or less. Disturbances in the refraction profile are best observed by plotting the slope of the refraction profile. The refractive index profile also affects the refractive defoeussing used in the calculation of the absorption coefficient.
A comparison of the shape of the disturbed region of the refracting dc focusing curve with the fluctuation in the actual data at 6112.4 km suggests that there may indeed be an actual disturbance in the atmosphere at this level. In the area of the lower cloud deck, however, there is no such turbulent layer or. Refractive index profiles including the original refractive index (n), the perturbed index (n,) and the refractive index calculated from the perturbed bending angle (n,).
TEMPERATURE (K)
ORBIT 9N
S BAND
Smaller swings in the refractive defocus curve can be of really small consequence. The results of the investigation of errors resulting from variations in the refractive index profile show that perturbations of the order of 1% or more can affect both the pressure-temperature profile and the refractive defocus profile. These curves are used to determine the existence of any artificially produced fluctuations in the refractive defocus profile that may be confused with cloud layers.
In several profiles, particularly in the collar region, small fluctuations (much smaller than those that would be caused by a 1% perturbation in the refractive index profile) resulted in small fluctuations in the final absorption coefficient profile. The uncertainties are 5 a values determined from the uncertainty in the x,y,z position of the spacecraft (Kliore, private communication). The effect of the flattened n.tmosphere on the refractive defocusing profile is negligible in the lower cloud region, as shown in Figure 23.
ORBIT 9N S-BAND
The difference in temperature at the lower levels of the atmosphere is less than 1%, but it corresponds to a temperature difference of up to 6 K, which is significant. Variations in the temperature profiles are important when calculating the concentration of the sulfuric acid cloud particles from the measured water vapor profiles and sulfuric acid-water saturates vapor pressure data (see section V.4). The dry adiabatic lapse rate was estimated from the heat capacity of the atmosphere, assuming an ideal atmosphere.
In all cases, 6 r is greater than zero in the lower cloud deck region. The drift rate for the orbit of 358N fluctuates to a much greater extent than in other parts of the atmosphere. In addition, deviations of the spacecraft's spin axis from its nominal position due to unbalance and other factors result in gain axis variability.
ANTENNA POWER PATTERN
BAND ANTENNA POWER PATTERN
This simulated rotation period contained an offset that was estimated to cause some degree of uncertainty in the positions of the spacecraft and antennas toward the end of the occultation. Since one degree of uncertainty in antenna position results in a small change in S-band power, the occultation from orbit 69X was analyzed without power corrections. The gain tables for the Pioneer Venus antenna are measured in the laboratory for each wavelength, for the elevation and azimuth directions, and for nine different dish heights.
A constant gain contour, as shown in the inset in Figure 31, can be described as an ellipse with axes a. G(r) is determined for 1/IAZ and 61/JeL as a function of time for each of the nine in dish heights measured in the laboratory. A sharp deviation in the internal frequency indicates loss of lock, as shown in the insets in Figures 32 and 33.
RADIUS
ORBIT 18 S-BAND
ORBIT IBN X- BAND
There is no evidence of any similar deviation in the lower part of the curve at the heights of the lower cloud cover. This is only the lowest part of the lower cloud cover and is probably responsible for the fluctuations in the absorption coefficient profile in this region. 18 in the Arctic) the excess attenuation constitutes a significant fraction of the total attenuation.
Above 6102.5 km, the excess attenuation is almost zero with the refractive defocus accounting for almost all of the total attenuation. In the lower part of the X-band profiles (Fig. 36b), the excess attenuation is equivalent to the refractive defocusing. These comparisons are made to demonstrate that the excess attenuation is of the same magnitude as the total attenuation and the refractive defocus (and not the result of subtracting two large numbers!).
ORBIT 9N S-BAND I
ORBIT 18N S-BAND
ORBIT IBN X-BAND
Errors Due to Power Fluctuations and Averaging of Power 'ft. Radius Data
The dashed and dashed lines in the power profiles for orbits 9N (S·band) and 18N (S and X-band) shown in Figures 38 and 39 (a and b), respectively, represent the maximum and minimum possible errors expected is due to received power fluctuations. The maximum and minimum curves are extreme and are intended to represent very conservative error estimates. Also represented in the figures are curves derived by 0.1, 0.2 and 0.3 km average of the power vs.
Power fluctuations cause a significantly greater uncertainty in the total and overdamping than average uncertainties. The uncertainty due to power fluctuations is much smaller for the X-band data than for the S·band data because the total power loss at X-band is greater. Again, the power fluctuations contribute the most to the absorption coefficient uncertainty, and the S·band power fluctuations produce greater uncertainty than the X-band power fluctuations.
ORBIT IBN S-BANQ
The error due to wobbles in the S-band power profile can be estimated from Figures 38 and 39. The corresponding error in the X-band power profile due to spacecraft wobbles is shown in Figure 44. The error is of the same magnitude (in fact approximately equal to ) the error calculated due to power fluctuations.
The maximum error due to antenna gain calculations was determined previously for all circuits in Table Vll. These errors are significantly smaller than errors due to current fluctuations and, in the case of X-band gain, spacecraft yaw. The effect of the antenna angle variation due to the spacecraft wobble on the antenna gain is shown by the maximum off-axis angle due to the antenna angle.
BAND
ORBIT ISN X- BAND
Refractive Defocussing Errors
Errors arising from the calculation of refractive defocusing are mainly due to the calculation of d"f!db (the slope of the bending angle compared to the slope at point i is determined by differentiating the equation at i determined by the quadratic fit of points i + 1. The slope at point i is determined by calculating the linear slope for points i + 1 and i - 1. The slope at i is calculated by differentiating the Lagrange polynomials. Table VIII also lists the mean and standard deviations calculated for each radius for d "//I db and the resulting standard deviation in the refractive defocus equation.
Above this radius, the standard deviation is less than 0.6 dB and in most cases less than 0.2 dB. Although small fluctuations in the absorption coefficient profiles cannot be attributed to cloud layers within the lower cloud deck, the estimated uncertainties by no means exclude the existence of a strongly absorbing lower cloud layer. Absorption coefficient profile for circuits 9N and lBN (S· and X·bands) showing error bars due to all miscalculations.
ORBIT 9 S-BAND
ORBIT 18
S AND X-BAND
The primary objective of this thesis is to determine the composition and global variability of the lower cloud deck on Venus using absorption coefficient profiles obtained from Pioneer's Venus Radio Occultation Experiment. The H2S04 profile is the equilibrium vapor pressure of sulfuric acid above the liquid sulfuric acid-water ratio at the temperature and composition of the lower cloud (see section on cloud concentration calculations). The Van Vlcek-Weiskopf function was used to determine the pressure propagation effects that are important in the lower cloud region.
Gaseous absorption contributes significantly to total atmospheric absorption only in the lower regions of the lower cloud deck where SO2 and H2S04 absorption is significant. H2S04 absorption contributes the most to the total absorption coefficient; however, it is significantly less than the absorption due to cloud material except at the lowest levels of the lower cloud deck. Now that the absorption coefficient profiles due to absorption by the cloud material alone have been derived, the cloud liquid content can be calculated d.
