2. Scentific background

The principal mode of the atmospheric circulation of Venus is a zonal retrograde super-rotation of the entire atmosphere (e.g., Schubert, 1983). The wind speed increases with height and reaches ~100 m s^-1 near the cloud top (~65 km altitude), although the solid planet rotates very slowly with a period of 243 Earth days corresponding to an equatorial rotation speed of 1.6 m s^-1. Since eddy viscosity should transport angular momentum downward and pass it to the solid planet, a mechanism which extracts angular momentum from the solid planet and transports it upward is required to maintain the vertical shear.

Various mechanisms explaining the super-rotation have been proposed so far; among them are the combination of meridional circulation and large-scale eddies which transport angular momentum equatorward (Giearsch, 1975; Yamamoto and Takahashi, 2003; Iga and Matsuda, 2005), thermal tides which are excited in the cloud layer and propagate vertically (Fels and Lindzen, 1974; Takagi and Matsuda, 2005), and gravity or Kelvin waves which are excited in the lower atmosphere and propagate upward (Del Genio and Rossow, 1990; Yamamoto and Tanaka 1997). Some of the eddies so far observed, such as the equatorial 4-day wave and the mid-latitude 5-day wave seen in cloud-top albedo contrasts (e.g., Del Genio and Rossow, 1990), the diurnal and semidiurnal tides observed in albedo contrasts and the temperature structure above clouds (e.g., Taylor et al., 1980; Zasova et al., 2002), and gravity waves encountered by Vega balloons (Sagdeev et al., 1986), might play crucial roles in maintaining the vertical shear via mechanisms mentioned above. However, insufficient information on the three-dimensional structures of those eddies prevents clear identification of their sources and the evaluation of the angular momentum across deep atmospheric layers. Evaluation of eddy momentum transport using cloud-tracked wind vectors indicated the deceleration of equatorial atmosphere and the acceleration of the polar atmosphere (Rossow et al., 1990). This result, however, might be influenced by the absence of wind vectors on the nightside and the insufficient sampling frequency. The structure of meridional circulation is also highly uncertain as described later. The identification of the mechanism requires more detailed information on atmospheric waves and meridional circulation above and below the cloud layer over wide solar longitudes.

Meridional circulation is an important issue not only in the momentum balance of the super-rotating atmosphere but also from the viewpoint of the thermal balance and the chemical cycle of the atmosphere. Vertically-stacked cells have been proposed based on the wind profiles obtained by entry probes (Schubert et al., 1980), while recent numerical models predict the occurrence of one direct cell in each hemisphere (Yamamoto and Takahashi, 2003). Although strong poleward circulation was observed at the dayside cloud top by cloud tracking (e.g., Rossow et al., 1990), the circulation pattern is still highly uncertain due to the lack of nightside data and the insufficient measurement below the cloud top. The cloud-top meridional circulation might supply angular momentum to the polar region to maintain the polar vortex and bring about the polar dipole by initiating some instability (e.g., Taylor et al., 1980); the polar dipole is a prominent feature in the polar region and might play key roles in the momentum balance of the middle atmosphere and the exchange of air across the cloud layer. In the thermosphere, on the other hand, subsolar-to-antisolar circulation is expected to take the place of meridional circulation (Bougher et al., 1997). Gravity waves are believed to play important roles in determining the circulation structure. The investigation of the thermospheric circulation will offer clues to the question why zonal circulation dominates in the lower atmosphere in such a slowly-rotating planet.

The thermal balance and chemical cycle of the Venusian atmosphere is strongly influenced by the H₂SO₄-H₂O cloud layer at 45-65 km altitudes (Esposito et al., 1983). The H₂SO₄ is thought to be produced photochemically near the cloud top via the oxidation of SO₂: the clouds basically have the characteristics of photochemical aerosols. On the other hand, there must be a strong dynamical coupling between clouds and atmospheric motions in the lower part of the cloud layer, where the static stability is weak and rapid condensation and evaporation of cloud droplets will occur in the course of vertical convection (e.g., Imamura and Hashimoto, 2001). Remote sounding of the spatial distribution and the microphysical properties of clouds, combined with other meteorological observations, might provide key information on such dynamical coupling in the cloud layer. The cloud dynamics is further related to the ultraviolet markings at the cloud top, such as large cell-like structures, bow shape, and circum-equatorial belts, whose dynamical origins are mostly unknown (Rossow et al., 1980).

Lightning discharge is closely related to cloud formation and can be an indicator of strong convective activity. The occurrence of lightning in the Venusian atmosphere has been suggested by various observations (Grebowsky et al., 1997); however, the occurrence is still under debate since the standard mechanism of lightning requires H₂O ice particles, which will not be formed in the Venusian atmosphere. Large solid particles of unknown composition suggested by the Pioneer Venus probe (Knollenberg and Hunten, 1980) might play some roles. New observation techniques are required to confirm the occurrence of lightning and reveal the thundercloud distribution.

The composition and abundance of the Venusian atmosphere might be controlled by the chemical coupling with the crust (Fegley et al., 1997; Wood, 1997; Hashimoto and Abe, 2005). The low-emissivity areas at Venusian highlands observed at radio wavelengths will have been produced by some temperature-dependent thermodynamical reactions between atmospheric constituents and surface minerals. One of the possible atmosphere-surface reactions may influence the atmospheric SO₂ abundance, which controls the cloud albedo, thereby changing the surface temperature; via such a feedback the Venusian climate can be stabilized (Hashimoto and Abe, 2000). The nature of the surface minerals and the related reactions are, however, highly controversial. The knowledge on the current status of volcanism is also an important key to understand the sulfur cycle in the current Venusian environment as well as the climate evolution and the internal structure of the planet(Hashimoto and Imamura, 2001).

Observation of the zodiacal light, i.e. the interplanetary dust (IPD) cloud, is an important issue in the cruising phase of the mission. Big problem on the zodiacal dust cloud is its origin, since the lifetime of the IPD particles under the Poynting-Robertson drag is absolutely shorter than the age of the solar system. We have several approaches to study the IPD cloud, and imaging observations are one of the very effective techniques to reveal its origin because the IPD particles keep the morphological structures of their sources. Observations of zodiacal light took the first step by ground-based measurements at visible wavelength, and the early photoelectric observations suffered from calibration uncertainty and poor spatial resolution. The all-sky map of zodiacal light brightness at visible band now reaches the relative accuracy better than 10% and the spatial resolution of 2 arc-degree, and is being improved drastically by the WIZARD project (Ishiguro et al., 2002). Infrared Astronomical Satellite (IRAS) dramatically changed the smooth featureless picture of the zodiacal dust cloud by revealing numerous bands of asteroidal debris, several narrow trails of cometary dust, and a clumpy dust ring. The success of IRAS was largely due to the improvements in relative accuracy and spatial resolution. The ring clumps comprise always the same configuration in the frame rotating at the rate of the Earth's mean motion. The most important thing is that the IRAS observations of zodiacal emission are free from the Earth's scattering atmosphere. COBE/DIRBE also surveyed almost entire sky with a 0.7 arc-degree size beam and with much better calibration (Kelsall et al., 1998). One of the most important results of the DIRBE/COBE mission is a confirmation of the mean motion resonance dust ring, and an isolation of the leading and trailing blobs in the mean motion resonance feature.