Strong refraction near the Venus surface: Effects observed by descent probes


  • Thomas A. Croft


The high-pressure atmosphere of Venus causes strong downward refraction of radio rays, with consequences that were evident in the telemetry signals from Pioneer Venus probes. As the probes descended, the strength of the direct signal to the earth gradually diminished because of the combined actions of absorption and refractive defocusing. Both phenomena became increasingly pronounced as penetration proceeded. Like occultation recordings of signal strength, these records too can be analyzed to yield a profile of atmospheric temperature versus altitude. During the final 30 km of descent, there was in addition to the direct signal a second measured component that traveled to the surface of Venus and from there scattered toward the earth. The strong atmospheric refraction played a key role leading to the strengthening (and hence the detectability) of these echoes. Surprisingly, it is found that such surface-reflected signals provide a good indication of the horizontal winds existing at each altitude during the descent of the probes. Since the refraction is roughly the same in the visible band as it is at radio frequencies, there was some speculation that refractive effects might be visible in the images of the surface returned by the Soviet landers. No such effects were apparent. I demonstrate that the absence is attributable to limited visibility.