Boris Ragent

Overview of VEGA Venus Balloon in Situ Meteorological Measurements

The VEGA balloons made in situ measurements of pressure, temperature, vertical wind velocity, ambient light, frequency of lightning, and cloud particle backscatter. Both balloons encountered highly variable atmospheric conditions, with periods of intense vertical winds occurring sporadically throughout their flights. Downward winds as large as 3.5 meters per second occasionally forced the balloons to descend as much as 2.5 kilometers below their equilibrium float altitudes. Large variations, in pressure, temperature, ambient light level, and cloud particle backscatter (VEGA-1 only) correlated well during these excursions, indicating that these properties were strong functions of altitude in those parts of the middle cloud layer sampled by the balloons.

The clouds of Jupiter: Results of the Galileo Jupiter Mission Probe Nephelometer Experiment

The results of the nephelometer experiment conducted aboard the probe of the Galileo mission to Jupiter are presented. The tenuous clouds and sparse particulate matter in the relatively particle-free 5-μm “hot spot” region of the probes descent were documented from about 0.46 bar to about 12 bars. Three regions of apparent coherent structure were noted, in addition to many indications of extremely small particle concentrations along the descent path. From the first valid measurement at about 0.46 bar down to about 0.55 bar, a feeble decaying lower portion of a cloud, corresponding with the predicted ammonia particle cloud, was encountered. A denser, but still very modest, particle structure was present in the pressure regime extending from about 0.76 bar to a distinctive base at 1.34 bars and is compatible with the expected ammonium hydrosulfide cloud. No massive water cloud was encountered, although below the second structure, a small, vertically thin layer at about 1.65 bars may be detached from the cloud above, but may also be water condensation, compatible with reported measurements of water abundance from other Galileo Mission experiments. A third small signal region, extending from about 1.9 to 4.5 bars, exhibited quite weak but still distinctive structure and, although the identification of the light scatterers in this region is uncertain, may also be a water cloud, perhaps associated with lateral atmospheric motion and/or reduced to a small mass density by atmospheric subsidence or other causes. Rough descriptions of the particle size distributions and cloud properties in these regions have been derived, although they may be imprecise because of the small signals and experimental difficulties. These descriptions document the small number densities of particles, the moderate particle sizes, generally in the slightly submicron to few micron range, and the resulting small optical depths, mass densities due to particles, column particle number loading, and column mass loading in the atmosphere encountered by the Galileo probe during its descent.

Particulate matter in the Venus atmosphere

Abstract We present a summary of the data currently available (June 1984) describing the planet-enshrouding particulate matter in the Venus atmosphere. A description and discussion of the state of knowledge of the Venus clouds and hazes precedes the tables and plots. The tabular material includes a precis of upper haze and cloud-top properties, parameters for model-size distributions for particles and particulate layers, and columnar masses and mass loadings. The plots are of experimental and derived quantities and include: (1) altitude dependences of volumetric scattering cross sections, size mode number densities, mass loadings, and optical depths, (2) wavelength dependences of asymmetry parameters and optical depths, and (3) latitude and longitude dependence of cloud top properties.

VEGA Balloon Dynamics and Vertical Winds in the Venus Middle Cloud Region.

The VEGA balloons provided a long-term record of vertical wind fluctuations in a planetary atmosphere other than Earths. The vertical winds were calculated from the observed displacement of the balloon relative to its equilibrium float altitude. The winds were intermittent; a large burst lasted several hours, and the peak velocity was 3 meters per second.

Implications of the Vega balloon results for Venus atmospheric dynamics

Both VEGA balloons encountered vertical winds with typical velocities of 1 to 2 meters per second. These values are consistent with those estimated from mixing length theory of thermal convection. However, small-scale temperature fluctuations for each balloon were sometimes larger than predicted. The approximate 6.5-kelvin difference in temperature consistently seen between VEGA-1 and VEGA-2 is probably due to synoptic or planetary-scale nonaxisymmetric disturbances that propagate westward with respect to the planet. There is also evidence from Doppler data for the existence of solar-fixed nonaxisymmetric motions that may be thermal tides. Surface topography may influence atmospheric motions experienced by the VEGA-2 balloon.

VEGA Balloon System and Instrumentation.

The VEGA Venus balloon radio transmissions received on Earth were used to measure the motion of the balloons and to obtain the data recorded by onboard sensors measuring atmospheric characteristics. Thus the balloons themselves, the gondolas, the onboard sensors, and the radio transmission system were all components of the experiment. A description of these elements is given, and a few details of data sampling and formatting are discussed.

Preliminary Results of the Pioneer Venus Nephelometer Experiment

Preliminary results of the nephelometer experiments conducted aboard the large sounder, day, north, and night probes of the Pioneer Venus mission are presented. The vertical structures of the Venus clouds observed simultaneously at each of the four locations from altitudes of from 63 kilometers to the surface are compared, and similarities and differences are noted. Tentative results from attempting to use the data from the nephelometer and cloud particle size spectrometer on the sounder probe to identify the indices of refraction of cloud particles in various regions of the Venus clouds are reported. Finally the nephelometer readings for the day probe during impact on the surface of Venus are presented.

Thermal structure of the venus atmosphere in the middle cloud layer.

Thermal structure measurements obtained by the two VEGA balloons show the Venus middle cloud layer to be generally adiabatic. Temperatures measured by the two balloons at locations roughly symmetric about the equator differed by about 6.5 kelvins at a given pressure. The VEGA-2 temperatures were about 2.5 kelvins cooler and those of VEGA-1 about 4 kelvins warmer than temperatures measured by the Pioneer Venus Large Probe at these levels. Data taken by the VEGA-2 lander as it passed through the middle cloud agreed with those of the VEGA-2 balloon. Study of individual frames of the balloon data suggests the presence of multiple discrete air masses that are internally adiabatic but lie on slightly different adiabats. These adiabats, for a given balloon, can differ in temperature by as much as 1 kelvin at a given pressure.

Large, solid particles in the clouds of Venus: Do they exist?

Abstract The discovery of large, solid particles in the clouds of Venus is one of the most significant findings of Pioneer Venus because it means that a substantial mass of the clouds is composed of a material other than sulfuric acid. The evidence which suggests that solid particles form a distinctive size mode is reexamined. The mode is defined by a discontinuity between two size ranges of the Pioneer Venus particle size spectrometer. This discontinuity could represent a real size mode. However, it could also be an artifact of the measurement technique. R. G. Knollenberg (1984) discusses several possible instrumental effects which might have caused this discontinuity. It is hypothesized herein that such effects did occur and that the large particles are really the tail of the mode 2 sulfuric acid particle size distribution and are not a separate mode of solid particles. Using such a revised size distribution, it is shown that all of the Pioneer Venus and Venera optical data from the lower clouds can be explained with sulfuric acid droplets without introducing any solid particles. As a by-product of this analysis, it is also found that the upper clouds of Venus must contain a material with a higher refractive index than sulfuric acid. A small quantity of sulfur could account for this observation.

Nature of the ultraviolet absorber in the venus clouds: inferences based on pioneer venus data.

Several photometric measurements of Venus made from the Pioneer Venus orbiter and probes indicate that solar near-ultraviolet radiation is being absorbed throughout much of the main cloud region, but little above the clouds or within the first one or two optical depths. Radiative transfer calculations were carried out to simulate both Pioneer Venus and ground-based data for a number of proposed cloud compositions. This comparison rules out models invoking nitrogen dioxide, meteoritic material, and volatile metals as the source of the ultraviolet absorption. Models involving either small (∼1 micrometer) or large (∼10 micrometers) sulfur particles have some serious difficulties, while ones invoking sulfur dioxide gas appear to be promising.