Infrared Photometric Mapping of Venus through the 8- to 14-Micron Atmospheric Window1

The 200-inch Hale telescope has been used to obtain high-resolution maps, on four mornings juxtaposed about the Mariner 2 encounter , of the brightness temperature of Venus in the 8to 14-micron wavelength interval. The resolution was about 1/30 of the disk of Venus. The signal-to-noise ratio was in excess of 100. The maps reveal (1) a general limb darkening, (2) a bilateral symmetry about the planet's orbital plane, (3) a very slight wedging of the contours as the only day-to-night effect, and (4) a transient temperature anomaly in the southern hemisphere . Introduction. The thermal emISSIOn from Venus was first measured by Pettit and Nichol­ son [1924]. The measurements have been re­ viewed by Pettit [1961]. Characteristic bright­ ne8S temperatures were found, and the interest­ ing discovery was made that the dark part of the disk of Venus was emitting about as strongly as the sunlit part. These observations were made on the 100-inch telescope at Mount Wilson with a vacuum thermocouple. Sinton and Strong [1960] have made observations with a spectrophotometer incorporating a Golay cell on the 200-inch telescope. They have recorded the average center-to-limb brightness profile and real departures from blackbody spectral dis­ tribution. In the present study high-resolution bright­ ness-temperature contour maps have been con­ structed on four successive nights juxtaposed about the Venus encounter of the Mariner 2 spacecraft. Preliminary results have been re­ ported briefly [Murray, Wildey, and Westphal, 1963] . The observations. The observations were collected at the east-arm Cassegrain focus of the 200-inch telescope during the morning twilights of December 14, 15, 16, and 17, 1962. The photometer and detector (mercury-doped ger­ manium photoconductor) used have been de­ scribed by Westphal et al. [1963] and more briefly by Murray and Wildey [1963a]. The 1 Contribution 1172 of the Division of Geological Sciences of the California Institute of Technology, Pasadena, CaUfomia. reduction and calibration of observations for extended sources are described by Murray and Wildey [1963b], together with a discussion of the possible systematic errors. The atmospheric extinction correction used in the present study is that adopted previously by Wildey and Mw'­ ray [1963] and is due to Westphal (private communication) . It may be an important source of systematic uncertainty. The reduced data are presented in the form of a map of brightness temperature (the temperature for which the measured specific intensity equals a Planck function weighted precisely the same over wave­ length) . The telescope's hour-angle drive was adjusted to slightly exceed the earth's rotation rate. Be­ ginning at the north limb of the Venus disk a series of right-ascension scans, separated by small southerly decrements in declination, was thus made. The scans were reproducible, and the signal-to-noise ratio near the center of the disk was about 100 to 1. This is seen in scans 1 and 2 of Figure 2, which correspond to tracks 1 and 2 of Figure 1. The scans were reduced at in­ tervals commensurate with the photometer reso­ lution (about 1.5 seconds of arc, 1/30 of the disk diameter, or 400 km at the Venus subearth point). The resulting brightness temperatures were fixed geographically by translating the scan parallel to right ascension until its length be­ came the chord of a circle whose diameter was preassigned as the length of the scan that bi­ sected the disk. This procedure was necessitate d by the lack of precision with which the tele­ scope's declination circles can be read, even