Estimates of visibility of the surface of Venus from descent probes and balloons

One of the tasks of future missions to Venus could be imaging of the surface either from a probe during its descent or from a balloon that drifts at a definite height below the main cloud deck. The undercloud atmosphere restricts capabilities of this experiment in three ways: (1) true absorption in bands of CO2 and H2O, (2) gaseous Rayleigh scattering, and (3) scattering and absorption by the undercloud haze. The first and second are dominant, at least below 30–35 km. Wavelength 1.02 μm seems to be the most favorable as the centre of a window for imaging because the true (band) absorption and Rayleigh optical depth are minimal here. It is also important that this wavelength is within the range of silicon CCD spectral sensitivity. Wavelengths 0.85 and 0.65 μm are included in the analysis. A special number (the visibility factor) is introduced for the quantitative estimates of the atmospheric influence on the quality of surface images. Results of Venera 13 and 14 are used as key information about the optical properties of the atmosphere of Venus. Two cases are discussed: (1) imaging during the daytime in all three windows and (2) night imaging in 1 μm window using the thermal emission of the surface. It is shown that at daytime 3-color imaging with the use of all three windows would be difficult from heights more than a few kilometers, but in 1 μm the approximate upper limit is about 15 km. Visibility of highlands will be better. Night conditions are better for imaging: pictures in 1 μm window. Night images may have acceptable quality just after the passage of the lower boundary of the main cloud deck (48–50 km). However, interpretation may meet difficulties due to mixing effects of temperatures and emissivity surface fields. NIR surface mapping from orbiters is possible, but it will not provide space resolution better than about 50–100 km. This mapping will deliver information about surface temperature (linked with topography). Constraints on the mineral surface composition would be difficult to derive from orbital observations due to multiple reflections between the surface and atmosphere.