Brightness Temperature of Venus at 1.35 cm.

Abstract

Previous observations have indicated that the radio-wavelength spectrum of the dark hemisphere of Venus corresponds to equivalent blackbody temperatures of 560~6000K at wavelengths between 3 and 21 cm, but lower temperatures of 3500 and 4100K were found at 0.43 and 0.86 cm, respectively. In order to investigate the brightness transition between 0.86 and 3.15 cm, measurements were undertaken at 1.35 cm during the period near the lower conjunction of November 1962. This wavelength, at the center of a water vapor absorption band, was chosen with the intent of revealing any evidence of Cytherean water vapor. To ensure unity with previous millimeter results, these observations were made using the same 10-ft equatorial reflector, with north-south polarization as before. A rapid-comparison crystal-mixer superheterodyne receiver with input noise figure of 8.5 dB and 11 Mc bandwidth (at 43 Mc) yielded an output fluctuation of 0.150C (rms) for 10 sec integrati on time. Terrestrial atmospheric extinction was found to be comparable to that at 0.86 cm, probably because of lessened oxygen absorption at 1.35 cm. Measurements near conjunction yielded 5200 ~400K (probable error) for brightness temperature. This value may be increased somewhat when additional data are reduced. This temperature approaches the 3-cm value, and shows no obvious indication of high-altitude Cytherean water vapor attenuation of thermal radiation from below, in agreement with recent spectrographic analysis by Spinrad (Icarus 1, 266, 1962). The high 1.35-cm temperature compared to the 0.86-cm value does not conform to CO2 spectra computed by Barrett (Astrophys. J. 133, 281, 1961). The observations are being continued in search of brightness change with phase of illumination.