The atmosphere of Venus has continued to provide a unique laboratory for research by atmospheric dynamists, chemists, and climatologists. Venus is our closest planetary neighbor, and while there are some similarities between Venus and the Earth, there are many more differences. The orbital, physical, and atmospheric properties of both Venus and the Earth are listed for comparison in Table 1.1. A summary of the structure and properties of the Venus clouds (composed of sulfuric acid) is given in Table 1.2. The greatest similarity is the size of Venus which is about 5% smaller than the Earth. The climate of Venus, though, is dominated by a strong greenhouse effect. A very high surface temperature (735 K) results from the trapping of solar energy by the large thermal opacity of the atmosphere (composed of 96.5% CO2 and 3.5% N2) and the thick clouds.

In contrast to the Earth, about 80% of the solar radiation incident on Venus (2620 W m-2) is reflected to space, about 10% is absorbed by the clouds, and only about 2.5% is absorbed by the surface (Tomasko et al., 1980). The large reflectivity of solar radiation from Venus is due in part to the planet's global cloud coverage, as opposed to only 40% mean cloud coverage over the Earth. It has also been shown (see e.g. Pollack et al., 1980) that the small percentage of surface solar absorption is sufficient to account for the high Venus surface temperature given the greenhouse mechanism.

Table 1.1: The orbital, physical, and atmospheric properties of Venus and the Earth.

Orbital and Rotational Data

Mean distance from sun (AU) Venus 0.72 / Earth 1.00

Eccentricity Venus 0.0068 / Earth 0.0167

Inclination of equator to orbital plane Venus 2.60˚ / Earth 23.45˚

Orbital period (days) Venus 224.701 / Earth 365.256

Rotation period Venus 243 days retrograde / Earth 23.93 hr prograde

Solar constant (W m-2) Venus 2620 / Earth 1380

Solid Body Data

Mass (kg) Venus 4.87 x 1024 / Earth 5.98 x 1024

Mean radius (km) Venus 6052 / Earth 6370

Density (kg m3) Venus 5250 / Earth 5515

Atmospheric Data

Composition (percentage by volume) Venus 96.5 CO2, 3.5 N2 / Earth 78 N2, 21 O2, 1 Ar

Mean surface temperature (K) Venus 735 / Earth 280

Troposphere lapse rate (K km-1) Venus 8 (adiabatic) / Earth 6.5 (sub-adiabatic)

Surface pressure (atm) Venus 92.0 / Earth 1.0

Albedo Venus 0.8 / Earth 0.3

Cloud cover Venus 100% / Earth 40%

Cloud composition Venus sulfuric acid / Earth water

Table 1.2: The structure and physical properties of the Venus clouds.

Cloud

Layer

Designation Altitude

Range

(km) Mean Particle

Number Densitya

(N cm-3) Mean Particle

Diametera

(m) Peak Particle

Diameterb

(m)

Upper layer 57 - 63 200 - 350 2.0 - 2.8 4.0

Middle layer 51 - 57 250 - 350 3.4 - 5.0 25.0

Lower layer 47 - 51c 50 - 150 1.8 - 3.4 25.0

aData from Marov et al. (1980)

bData from Knollenberg and Hunten (1980)

cThe altitude of the lower cloud layer base is variable (dependent on latitude)

Much of the latitudinal, zonal, and vertical atmospheric motion in the Venus atmosphere is driven by variations in the solar energy absorption by the clouds. The most pronounced global dynamical feature on Venus is the zonal retrograde superrotation of the atmosphere. While the planet rotates very slowly (243 days per rotation), the atmosphere rotates westward at a much faster rate (5 days per rotation). While this feature has been readily characterized in ultraviolet photographs of Venus by spacecraft such as Mariner 10 and Pioneer-Venus, it has not been fully understood. It has been suggested (see e.g. Rossow et al., 1980 or Limaye et al., 1982) that the atmospheric superrotation could be maintained in part by a mean meridional flow such as a Hadley cell, which transports both heat and westward zonal angular momentum to the poles at the cloud tops. While there is certainly evidence for the existence of a cloud-level Hadley cell, specific characteristics such as the degree of its polar extension are not well known.

One way to identify important features of such a Hadley cell is to examine its impact on the abundance and distribution of sulfuric acid (H2SO4) vapor below the cloud layer. The characterization of the vertical, zonal, and latitudinal variations in gaseous H2SO4 abundance can give much insight into the dynamical processes of the Hadley cell. Such a characterization can also aid in the understanding of the atmospheric processes which affect cloud formation, as well as the thermochemical processes which constrain the abundances of other trace gases in the Venus atmosphere such as SO2, SO3, CO, OCS, and H2O. Venus entry probes, such as the Venera landers and the Pioneer-Venus large probe, could not measure the abundance of H2SO4 vapor, though, due to significant condensation of the gas in the inlet leaks of their mass spectrometers. Therefore, remote sensing techniques are required for the accurate characterization of gaseous H2SO4 abundance.

For more information on this topic, please see the following references:

[1] Jenkins, J. M., M. A. Kolodner, B. J. Butler, S. H. Suleiman, and P.G. Steffes, "Microwave Remote Sensing of the Temperature and Distribution of Sulfur Compounds in the Lower Atmosphere of Venus," Icarus, 158, pp. 312-328.

[2] Butler, B. J., P. G. Steffes, S. H. Suleiman, M. A. Kolodner, and J. M. Jenkins, "Accurate and Consistent Microwave Observations of Venus and Their Implications," Icarus, 154, pp. 226-238.

[3] Kolodner, M. A. and P. G. Steffes, "The Microwave Absorption and Abundance of Sulfuric Acid Vapor in the Venus Atmosphere based on New Laboratory Measurements," Icarus, 132, pp. 151-169.

[4] Suleiman, S. H., M. A. Kolodner, and P. G. Steffes, "Laboratory Measurement of the Temperature Dependence of Gaseous Sulfur Dioxide Microwave Absorption with Application to the Venus Atmosphere," J. Geophys. Res., 101, pp. 4623-4635.