Mariner Mission to Venus

by Jet Propulsion Laboratory

CLOUD TEMPERATURES: THE INFRARED READINGS

Mariner II took a close look at Venus’ clouds with its infrared radiometer during its 35-minute encounter with the planet. This instrument was firmly attached to the microwave radiometer so the two devices would scan the same areas of Venus at the same rate and the data would be closely correlated. This arrangement was necessary to produce in effect a stereoscopic view of the planet from two different regions of the spectrum.

Because astronomers have long conjectured about the irregular dark spots discernible on the surface of Venus’ atmosphere, data to resolve these questions would be of great scientific interest. If the spots were indeed breaks in the clouds, they would stand out with much better definition in the infrared spectrum. If the radiation came from the cloud tops, there would be no breaks and the temperatures at both frequencies measured by the infrared radiometer would follow essentially the same pattern.

The Venusian atmosphere is transparent to the 8-micron region of the spectrum except for clouds. In the 10-micron range, the lower atmosphere would be hidden by carbon dioxide. If cloud breaks existed, the 8-micron emissions would come from a much lower point, since the lower atmosphere is fairly transparent at this wavelength. If increasing temperatures were shown in this region, it might mean that some radiation was coming up from the surface.

As a result of the Mariner II mission, scientists have hypothecated that the cold cloud cover could be about 15 miles thick, with the lower base beginning about 45 miles above the surface, and the top occurring at 60 miles. In this case, the bottom of the cloud layer could be approximately 200 degrees F; at the top, the readings vary from about minus 30 degrees F in the center of the planet to temperatures of perhaps minus 60 degrees to minus 70 degrees F along the edges. This temperature gradient would verify the limb-darkening effect seen by the microwave radiometer.

At the center of Venus, the radiometer saw a thicker, brighter, hotter part of the cloud layer; at the limbs, it could not see so deeply and the colder upper layers were visible. Furthermore, the temperatures along the cloud tops were approximately equally distributed, indicating that both 8- and 10-micron “channels” penetrated to the same depth and that both were looking at thick, dense clouds quite opaque to infrared radiation.

Both channels detected a curious feature along the lower portion of the terminator, or the center line between the night and day sides of the planet. In that region, a spot was shown that was apparently about 20 degrees F colder than the rest of the cloud layer. Such an anomaly could result from higher or more opaque clouds, or from such an irregularity as a hidden surface feature. A mountain could force the clouds upward, thus cooling them further, but it would have to be extremely high.

The data allow scientists to deduce that not enough carbon dioxide was present above the clouds for appreciable absorption in the 10-micron region. This effect would seem to indicate that the clouds are thick and that there is little radiation coming up from the surface. And, if present, water vapor content might be 1/1,000 of that in the Earth’s atmosphere.

Since the cloud base is apparently at a very high temperature, neither carbon dioxide nor water is likely to be present in quantity. Rather, the base of the clouds must contain some component that will condense in small quantities and not be spectroscopically detected.

As a result of the two radiometer experiments, the region below the clouds and the surface itself take on better definition. Certainly, heat-trapping of infrared radiation, or a “greenhouse” effect, must be expected to support the 800 degree F surface temperature estimated from the microwave radiometer data. Thus, a considerable amount of energy-blanketing carbon dioxide must be present below the cloud base. It is thought that some of the near-infrared sunlight might filter through the clouds in small amounts, so that the sky would not be entirely black, at least to human eyes, on the sunlit side of Venus. There also may be some very small content of oxygen below the clouds, and perhaps considerable amounts of nitrogen.

The atmospheric pressure on the surface might be very high, about 20 times the Earth’s atmosphere or more (equivalent to about 600 inches of mercury, compared with our 30 inches). The surface, despite the high temperature, is not likely to be molten because of the roughness index seen in the earlier radar experiments, and other indicators. However, the possibility of small molten metal lakes cannot be totally ignored.

The dense, high-pressure atmosphere and the heat-capturing greenhouse effect could combine over long periods of time to carry the extremely high temperature around to the dark side of Venus, despite the slow rate of rotation, possibly accounting for the relatively uniform surface temperatures apparently found by Mariner II.