What splits water molecules ($ ext{H}_2 ext{O}$) into hydrogen and oxygen high in the Martian atmosphere?

Answer

Intense ultraviolet radiation from the Sun.

Intense ultraviolet radiation from the Sun can strike water molecules high in the atmosphere, splitting them into their constituent parts: hydrogen and oxygen.

What splits water molecules ($	ext{H}_2	ext{O}$) into hydrogen and oxygen high in the Martian atmosphere?
MarsSpacewaterplanetevidence