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Aviation History
1963
1963 - 0402.PDF
382 FLIGHT International, 14 March 1963 Missiles and Spaceflight dioxide, but probably has little free oxygen or water vapour. Other Earth-based measurements have been made in the microwave and infra-red regions of the spectrum to try to detect surface temperatures. The terrestrial microwave measurements showed brightness temperatures of approximately 600°F near the surface averaged over the entire planet, and the infra-red showed readings of —40°F in the upper atmosphere. But because of the distances, and other factors, the scientists could not be sure of the significance of their measurements. Thus, Mariner's mission during the fly-by was to take measure ments on two channels in the microwave region and two measure ments in the infra-red region. The microwave radiometer and infra-red radiometer were turned on by radio command by the Goldstone station of the Deep Space Instrumentation Facility on the morning of December 14 and immediately started to work, even though the planet encounter was still 6hr 30min away. The scan started at 1959hr GMT and lasted until 2034hr GMT. The first scan was on the dark side, the second across the terminator and the third was on the light side. The instruments scanned in a nodding motion, up and down the planet, and the lateral motion was provided by the velocity of the spacecraft. The instruments continued to scan, even though the planet had been left behind, until the command was sent an hour later to return the spacecraft to its cruise mode. The microwave radiometer scanned the surface of Venus at 13.5 and 19mm. The former is a water-absorption band, while the 19mm wavelength is not affected by water vapour and was capable of penetrating the cloud cover and deeper atmosphere down to very close to the surface. Thus, the larger the temperature differences between the two radiometer measurements, the more water vapour is present in the atmosphere. In addition, the 19mm wavelength was able to show that Venus does not have an intense ionosphere. It did this by detecting the condition called limb-darkening. NASA charts based on the readings telemetered back to Earth by Mariner II. Although Venus is covered by cold, dense cloud in its upper atmosphere, the planet's surface is at some 800°F MARINER 2 MICROWAVE TEMPERATURE STUDY OF THE VENUS ATMOSPHERE AND SURFACE STRAIGHT DOWN PATH SENSES MORE SURFACE LESS ATMOSPHERE SLANT PATH SENSES MORE ATMOSPHERE AND LESS SURFACE MAMNSt PASSH5 SHOW VENUS AT CLOSEST APPROACH < SI^MS Mi. NASA diagram of the fly-by of Venus by Mariner II last December As the radiometer scanned the planet, it looked through the least amount of atmosphere when it was pointed straight down at the planet, and the most amount when it was pointing at the limb, or edge, of the planet. If a high-electron-density ionosphere had existed, the radiometer would have detected the condition called limb-brightening as it looked through the greater concentration of electrons in the atmosphere at the edges of the planet. The theory which had been assumed by most scientists before the Mariner measurements is that Earth-based radio measurements are explained by a hot surface on the planet. Looking straight down at the planet from space, one would see the hot surface, but looking toward the edge one would be looking through a thicker concen tration of the cooler clouds. It was this condition, limb darkening, which was detected for the first time by Mariner II. The microwave had a scan of 123.5° and scanned at the rate of O.r/sec:— Angular extent Altitude at midscan Scan location Brightness Scan I About 10° 25,000 miles Dark side 370°F Scan 2 Scan 3 About 15° 23,500 miles Near terminator 570°F About 10° 22,500 miles Light side 260°F Brightness temperature is calculated from a formula using the amount of light, or radio energy, emitted by an object. If the object is not a perfect light emitter, it is really hotter than the brightness measurement indicates. It will take some time to reduce the bright ness numbers to temperature numbers, but a preliminary estimate is that Venus has a surface temperature of approximately 800°F at the terminator, which is probably closest to the average. The infra-red radiometer was designed to detect emissions from Venus at 8.4 and 10.4 microns. Measurements from Earth at these wavelengths indicated temperatures below zero, but it was not clear from the data whether some of this radiation came from the surface through cloud breaks. For many years, some astronomers have been able to observe some kind of variable markings in the Venus cloud cover. If these markings are indeed cloud breaks, they will stand out with greater contrast in the infra-red, and a substantial difference will be detected between measurements at the two wavelengths. In the eight-micron region, the atmosphere is transparent except for clouds; in the ten-micron region, the lower atmosphere is hidden by the presence of carbon dioxide. Preliminary analysis of the data from both IR wavelengths gives approximately the same temperature at all points. This suggests that both channels saw down to the same depth in the Venus atmosphere, indicating that both channels were looking at a thick, dense cloud layer opaque to IR radiation. The IR instruments observed limb darkening, as did the microwave radiometer, and for a similar reason. This observation indicates that the cloud layer is thick and somewhat translucent to infra-red—like a thin fog is translucent to ordinary light. The IR measurements produced a curious result. Toward the south end of the terminator, the temperatures on both channels showed a cold spot on Venus, about 20°F cooler than the rest of the cloud layer. This means that the clouds in this region are higher, or more opaque, or both. An interesting possibility is that this cooler section of the cloud layer is associated with some hidden surface feature. This cold spot was the only anomaly observed. Temperature in the cloud layer was the same on the dark side as on the light. At the centre of the planet, it was measured at about - 30°F.
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