Abstract: Energetic neutral atoms are created through solar wind charge exchange collisions with neutral gases. I have simulated these energetic neutral atoms as they transfer their energy to thermal atoms in different astrophysical environments, in particular the Martian atmosphere and the interstellar medium. I will discuss the simulation results and their implications, including Martian atomic escape fluxes as well as the possibility of a diffuse, extrasolar energetic neutral background in our solar system.