Mercury and Moon He exospheres: Analysis and modeling

► We model the He exosphere at the Moon and Mercury with the same approach. ► Neither a partial energy accommodation nor a total energy accommodation can fit all the observations. ► These observations and their modeling suggest a diurnal variation of the energy distribution of the He ejected from the surface. ► Any of the present theories on the gas-surface interaction in surface-bounded exospheres can explain satisfactorily these observations.