Every planet has an exosphere, the outermost layer of an atmosphere that fades away into space. But Mars' exosphere is weird: it undergoes dramatic changes in response to Mars' extreme seasons and changes in the solar wind. This is because Mars' exosphere is mostly hydrogen. The hydrogen come from broken-down water molecules which originate near the surface of the planet. Seasonal warming allows more water to rise up, get broken apart by solar ultraviolet light, and fill up the exosphere. Hydrogen is also the main ingredient of the solar wind, a low-density stream of particles that originate at the Sun and spread through the solar system. The most common particles are protons, the same thing as a hydrogen nucleus. Interesting things happen when fast protons run into all that hydrogen filling up the Mars exosphere. First, the protons actually 'steal' an electron and become normal hydrogen atoms. Then the fast atoms plow into the atmosphere, colliding with the gases there. Each collision causes the atom to give off ultraviolet light: 'proton aurora'! So seasonal warming on Mars combined with strong solar wind cause the atmosphere to glow in the ultraviolet, and the MAVEN spacecraft records images when it happens.