The effects of the velocity distribution of impact generated water ice ejecta on exospheric escape from airless bodies

We explore the velocity distribution of impact generated water ice ejecta from planetary surface. We show that the ejecta velocity distribution should be modeled by a mixed Maxwellian distribution, rather than the commonly used single Maxwellian distribution, in order to better capture the outcome of the different emission processes and the dynamics of the clustered water molecules in the ejecta. We find that, comparing to the mixed Maxwellian distribution, the single Maxwellian distribution may lead to a significant underestimation of the impact -ejected water molecules that stay gravitationally bound for exosphere models. While the relative difference is within 10% for celestial bodies with a large escape velocity such as Mercury, it can be about 1000% for celestial bodies with a small escape velocity such as Enceladus.