Abstract

Global mean sources and sinks of radiative energy are calculated for the upper atmosphere of Venus. We especially consider the region between 90 and 130 km, where the equilibrium temperature is largely controlled through infrared absorption and emission by vibrational-rotational bands of CO2. Source functions for bands deviating from thermodynamic equilibrium are determined as part of the calculation. Radiative transfer in the region of non-overlapping lines is calculated by summing the contribution of individual Voigt lines. Many isotopic and hot bands contribute amounts to 15μ cooling or near-infrared heating at some levels comparable to the contributions by strong bands. The emission of the fundamental 15μ state of C120216 is maintained at near local thermodynamic equilibrium values by the radiation field to pressures three orders of magnitude less than would be expected in considering only relative values of radiative and vibrational relaxation rates. Conversion of near-infrared solar photon energy into thermal energy occurs through collisional relaxation of the 15μ fundamental states. Mean near-infrared heating rates increase from 1K (earth day)−1 at 65 km to more than 300K (earth day) −1 at 115 km. The 15μ cooling is dominated at most levels by cooling to space. The time scale for radiative damping in the cooling-to-space approximation varies from 30 earth days at 65 km to 1/20 of an earth day at 120 km. The calculated equilibrium temperature profile decreases from 250K at an altitude of 66 km to a minimum of 158K at 88 km, increases to a peak value of 190K at 113 km, and again decreases to a mesopause minimum of 180K at 122 km (8×10μb). The calculated thickness between the 100-mb level and the level of the ionospheric peak differs by less than 1 km from that observed during the Mariner 5 radio occultation experiment.

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