Laboratory measurement of microwave absorption in models of the atmosphere of Venus

Abstract

Coefficients of induced absorption in model atmospheres contaming CO2, N2, A, and Ne, needed to calculate the properties of the lower atmosphere of Venus from the radio observations on the assumption that the atmosphere is dry and massive, have been measured in the temperature range 240–500°K to pressures as high as 130 atm. Since the microwave region lies on the low-frequency wing of both the translational and rotational spectrum, the microwave-induced absorption coefficient is proportional to the square of the frequency, and all measurements have been made at one frequency, 9260 Mc/s. Absorption due to small amounts of water vapor in N2 has also been studied at 9260 Mc/s, over a comparable pressure range, and over the temperature interval 393–473°K. The absorption coefficient in this case is found to be approximately twice that calculated on the basis of the Van Vleck-Weisskopf theory from all of the significant microwave and infrared transitions of the water molecule. An expression for the absorption coefficient for all the atmospheres studied that is faithful to the laboratory data to a few per cent over the range of pressures relevant to Venus is α = P2 2(273/T)5(15.7ƒ + 3.90ƒ ƒN2 + 2.64ƒ ƒA + 0.085ƒN22 + 1330·ƒH O) × 10−8 cm−1 where P is the pressure in atm, is the frequency in wave numbers, T is the Kelvin temperature, and ƒCO2, etc., are the various molar fractions. The term in ƒH2O strictly applies only when N2-H2O collisions are the major source of pressure broadening of the H2O lines. When this expression is used to calculate the brightness temperature as a function of frequency for Venus, it is concluded that (a) water vapor can account for the microwave spectrum only if water is several orders of magnitude more abundant than the infrared studies suggest and that (b) if induced absorption in a CO2-N2 atmosphere is responsible for the spectrum, if CO2 is a relatively minor constituent of the atmosphere, and if the lapse rate is close to the adiabatic, then the ground pressure must lie in the range 100–300 atm.