studies, including planetary scale waves, thermal tides, and vertically propagating gravity waves. These are probably of greatest importance above the lowest one to two scale heights (~20 to 30 km), but gravity waves could have their dominant source at the ground as a result of winds blowing over surface topography.

It is likely that a Hadley circulation also exists in the lower atmosphere, with mean flow toward the equator at lower levels and toward the pole at somewhat higher levels. Such a circulation would tend to transport angular momentum upward and poleward and could be important in governing the zonal wind structure.

The circulation of the atmosphere in the lowest one to two scale heights and the maintenance of the atmospheric superrotation are among the outstanding problems in planetary atmospheric science.6 Venus is a very slowly rotating body, with a thick atmosphere in which considerable heating occurs at relatively high altitudes. Titan is the other example of such an atmosphere, and there is indirect evidence that its atmosphere may also superrotate.7,8

Recent modeling9 provides a suggestion that superrotation might be a general feature of slowly rotating bodies with thick atmospheres. Thus far, however, modeling has not been able to reproduce the very strong superrotation of Venus's atmosphere. By comparison, the atmospheres of Earth and Mars exhibit only very weak, if any, superrotations. Both are rapidly rotating bodies with relatively thin atmospheres in which the bulk of the solar heating occurs at the ground.

Even in the case of Venus, where only a small fraction of the incident sunlight reaches the ground, the transfer of heat from the surface to the atmosphere contributes significantly to the forcing of lower atmospheric circulation. Similarly the transfer of momentum between the ground and atmosphere is very important. Both of these processes take place through a planetary boundary layer, of which essentially nothing is known at present for Venus. Observations of very high vertical resolution are crucial to resolving the boundary layer and the transfer processes that occur within it. The boundary layer can be strongly affected by the nature of the surface, and this may vary considerably on Venus from place to place.

Necessary Observations

The following measurements are needed to achieve a better understanding of the circulation in Venus's lower atmosphere:

  • Multiple, geographically dispersed, simultaneous, and temporally extended measurements of pressure and temperature as a function of altitude and horizontal location to determine the basic structure of the lower atmosphere;
  • Multiple, geographically dispersed, simultaneous, and temporally extended measurements of the winds in the lowest one to two scale heights, with sufficient accuracy and sampling rate to enable the definition of the circulation itself and the determination of horizontal-and vertical-momentum fluxes as a function of altitude and horizontal location; and
  • Multiple measurements of radiative fluxes, both solar and infrared, as a function of altitude and at different locations to determine the radiative forcing of the atmospheric circulation.

Need for Mobility

A geographically dispersed series of entry probes could obtain valuable measurements, but would provide only a snapshot of the atmospheric circulation. Temporally extended measurements are essential because the atmospheric eddies involved in the superrotation are expected to vary on time scales of hours to days and longer. Remote-sensing techniques appear to be feasible for higher atmospheric levels, but probably would not reveal the atmospheric structure within one to two scale heights of the ground because of the extremely high density of the atmosphere. Mobility within the atmosphere provides an efficient approach, and the relevant measurements could, potentially, be made using the following modes of mobility:

  • A number of balloons, capable of semi-autonomous flight for an extended period, to obtain simultaneous

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