Within the framework of ESA's Mars Express mission, flux and energy distributions of planetary energetic hydrogen atoms around Mars for low and maximum solar wind conditions are investigated. Recent work shows that the energetic neutral atom distribution in the Martian environment is characterized by particles who have their origin in the solar wind and by a second particle population which comes from the planetary atmosphere. This study is related to the study of the Martian water inventory, since energetic planetary hydrogen atoms have their source in the Martian surface and subsurface ice reservoirs. After the evaporation from the Martian surface the water vapor reaches the upper atmosphere where the solar UV radiation can break up the molecules into hydrogen and oxygen atoms, which can be ionized and swept away by the solar wind. A test particle model which involves the motion in the external electric and magnetic field is used.

It has been found that after a planetary neutral hydrogen atom is transformed into an ion via charge exchange with solar wind particles, solar UV or electron impact, it is accelerated to higher altitudes and energies by the interplanetary magnetic field and gradually guided by the solar wind plasma flow around the planetary obstacle. Furthermore some of these newly born planetary ions take part in a charge exchange reaction with particles of the upper Martian atmosphere and will thus be transformed into planetary energetic neutral atoms. Our study shows that planetary energetic atoms have their origin close to the planet and are negligible outside of about a half Martian radius. It is important to note that we can show that the Energetic Neutral Particle (ENA) detector on board of the Mars Express space craft can separate the planetary energetic hydrogen population from the solar wind population by a careful analysis of the particle energies. Such a separation is important for the study of the evolution of the Martian atmosphere and its water inventory.