Heating Efficiencies in the Thermosphere of Venus Reconsidered

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, f _v , 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 f _v 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.