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Is Venus' Heat Decreasing?
Not quite six years ago, on a summer day in 1966, 1 had an unannounced visit by two young men, in their teens. They brought me the news that I was selected the first honorary member of the Celestial Observer Society, composed of high school and college amateur astronomer-observers of New York City, with quarters at the Brooklyn Technological Institute. The Society publishes a mimeographed monthly magazine. The two delegates made their way from Brooklyn to Princeton, N.J., on bicycles—a distance of 60 miles. It was my first (and still my only) honorary distinction; I reciprocated by publishing in the December, 1966, issue of Celestial Observer the paper reprinted here.
The purpose of this paper is to suggest repeated measurements of the infrared radiation emitted by the cloud surface of Venus. It is expected by the author of this communication that a slow drop in the temperature will be detected; it is suggested that the measurements should be made at synodical intervals. It should be possible to determine the phenomenon in about five synodical periods of Venus, or eight terrestrial years at the most. The measurements need to be taken of the night and day sides of the planetary envelope and also of the terminator.
The basis for this expected detection is in my maintaining that Venus is a newcomer to the solar system (which is what the Romans also intended to indicate by giving it its name). I argued (Worlds in Collision, 1950) that, under its massive envelope Venus, with a short but stormy history, must be very hot, even incandescent, owing to the presence of natal heat, and to the disturbance in motion with "a thermal effect caused by conversion of momentum into heat" (Worlds in Collision, p. 371). I made this statement when Venus was thought, due to the strong reflecting power of its clouds, to have a ground surface temperature of only a little above that of the Earth.
I offer the new proposition as another crucial test of my theory. Since "Venus gives off heat" (Worlds in Collision, p. 371), the drop in the temperature of the ground surface must be reflected in a smaller drop in the temperature of the cloud surface. In the 1920's E. Pettit and S.B. Nicholson measured the cloud surface temperature and obtained ca. -25°C for both sides, illuminated and shadowed, which seemed paradoxical. But almost 30 years later they recalculated their original finds and gave -38°C for the day side and -33°C for the night side, which is even more paradoxical. In the 19mm wavelength, Mariner II also found that the ground surface is warmer on the night side, but hottest on the terminator.
In 1956 Strong and Sinton made known their measurements (1953-54) of the cloud surface of Venus, indicating "approximately -40°C" on both sides of the planet, but found a gradient of 5° which they ascribed to the heating effect of the sun, on the assumption that Venus rotates directly, but since it rotates retrogradely, the phenomenon is again in conformity with the Pettit-Nicholson observations.
Bolometric measurements can be made to detect even a small fraction of a degree centigrade. Were it possible to take the Pettit-Nicholson and the Strong-Sinton figures as a basis for comparison, the drop of circa 1°C per eight years would already be attested. But in view of the implications of the test, also for the cosmology of the solar system, an exact series of measurements needs to be organized, possibly by more than one team of observers. If Venus has revolved on its orbit for billions of years, there should be no measurable drop in the temperature of the planet that could be detected from its cloud surface. But, if Venus' history is measured in thousands of years only, there will be found a detectable drop in the temperature from the top of the cloud envelope.
 E. Pettit, Astronomical Society of the Pacific, 67, (1955), 293ff.
 F. T. Bareth et al., "Microwave Radiometers in Mariner II Preliminary Report," Science, 139 (March 8, 1963).
 J. Strong, W. S. Sinton, Science, 123 (April 20, 1956), 676.
Thus I maintained that should subsequent measurements show a falling of the cloud surface temperature, if only in fractions of a degree per year, it would reflect a substantial loss of heat at the ground surface of the planet and thus document its youth. I repeated this claim in Yale Scientific Magazine, 41 (April, 1967).
In the July, 1968, issue of the Journal of Atmospheric Sciences, Gillett, Low and Stein published a short paper ("Absolute Spectrum of Venus from 2.8 to 14 Microns"). They obtained a definitely lower figure for the cloud surface temperature than did Sinton and Strong eight years earlier. Gillett and his co-workers wrote: "The reasons for the disagreement are not understood at present."
In the same paper Gillett et al, wrote on a different topic: "Two fundamental problems are now apparent: (1) What mechanism accounts for the strong absorption of sunlight in the 3-5 micron region? (2) What property of the clouds causes the low brightness temperature between 8 and 10 microns?" They only raised, but did not attempt to answer the question.
These are truly basic facts for the problem of the presence of organic material (hydrocarbons and carbohydrates) in the atmosphere and clouds of Venus. On this subject I intend to write for a subsequent issue of Pensée.
PENSEE Journal I