The Effect of Photoionization on the Loss of Water of the Planet

Hydrogen and oxygen can rapidly escape from the atmosphere of planets as a result of the photolysis of water by strong X-ray and ultraviolet (XUV) radiation of a star, which are fundamental to the origin of life and habitability. We developed an ion-atom mixed model to describe the escape of water from planets with water-dominated atmosphere. We showed that the oxygen ions easily escape from the atmosphere with the hydrogen owing to the strong interactions between oxygen ions and hydrogen. The atomic oxygen can escape from the atmosphere with hydrogen in the environment of high XUV irradiation. However, they decouple from hydrogen and oxygen ions with the decrease of XUV flux. We found that the critical XUV level is about 12-16 times or 30-40 times the present value for planets with the mass and separation of Venus and Earth, respectively. Below the critical XUV values, the oxygen can deposit in the atmosphere. Our results show that ions effectively transfer momentum between interacting species. The neglect of ions in the hydrodynamic model results in lower momentum exchanges between hydrogen and oxygen so that the critical XUV values increase by a factor of 2. Finally, our model predicted that the planets with a mass and separation of Venus (Earth) can lose the hydrogen of 21.5 (9.1) Earth oceans and oxygen of 2.6 (0.6) Earth oceans. Thus, the hydrodynamic escape could have dried up the water of their atmosphere in the period of early evolution.