Isolation of major Venus thermospheric cooling mechanism and implications for earth and Mars

The additional cooling mechanism responsible for the unexpectedly cold dayside Venus thermosphere is isolated. Consideration is given not only to the condition that the calculated temperatures must be reduced to the observed dayside temperatures but also that the theoretical temperature variations must be in accord with the observed low amplitude of temperature variations associated with the 27-d variations of the sun. It is found that the low amplitude of the 27-d oscillations, combined with the cooling necessary for observed 300-K dayside temperatures, cannot be explained by eddy conduction cooling and can only be explained by very strong 15-micron cooling. Implications of the decrease in amplitude of the 11-yr Venus thermospheric variability due to the 15-micron thermostat effect are examined.