Attempts to study the radiowave reflection properties of Venus by radar at 3.8 cm wavelength are reviewed. Measurements made by the Lincoln Laboratory during the last three inferior conjunctions place the scattering cross section at 1.7% of the projected area of the planetary disk. At 12.5 cm wavelength, a cross section of 11.5% has been reported and at wavelengths of 23 cm or longer the cross section appears to be ≥15%. No comparable wavelength dependence is found in the radar cross sections of the moon, Mercury or Mars, and it is believed that in the case of Venus, absorption of the waves by the atmosphere is responsible for the low cross section observed at the shortest wavelength.
Support for this conclusion has been obtained by comparing the scattering behavior observed at 3.8 and 12.5 cm. For Venus the reflectivity of the limbs compared to that of the disk center is lower at 3.8 cm than at 12.5 cm wavelength, while in the case of the moon the reverse is true. If the additional limb darkening is attributed to the attenuation of the rays that pass through the atmosphere obliquely, the difference in the two-way absorption can be established as 5±1 db. The radar cross section observed at 3.8 cm is lower than that at 12.5 cm by 8±3 db. Thus, it appears that the one-way absorption of 3.8 cm microwaves by the atmosphere of Venus is at least 2.5 db and possibly more. This is significantly greater than can be accounted for by an atmosphere consisting of CO2 with a pressure of 19±2 atm as implied by the recent Soviet probe. Either the pressure is considerably greater than this, or other gases that are more effective microwave absorbers are present.