Heating Efficiencies in the Thermosphere of Venus Reconsidered

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Heating efficiencies less than 10% are required by recent models of the low neutral temperatures in the daytime thermosphere of Venus. We consider here whether such values are justifiable from a molecular point of view. The primary uncertainty in a calculation of the heating efficiency is the fraction of energy, fv, that appears as vibrational excitation of product molecules in quenching, photodissociation and exothermic chemical reactions. The current state of knowledge of energy partitioning in chemical reactions is discussed and a range of likely values for fv is deduced. The calculated heating efficiencies fall in the range 16–25% over the altitude range from 115 to 200 km. It is suggested that the heating efficiency should not be taken as a free parameter and that the cold Venus thermosphere has not yet been satisfactorily explained.



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