Radium is a light sensitive material found in the universe, it is often used as a power source. When exposed to light it will explode, making it valuable in martian firearms.
History[]
According to a forward by Edgar Rice Burroughs, the term "radium" as used in the books is artistic license on his part. Carter's writings use an untranslatable martian glyph to refer to the material but some of it's properties made him think of earthly radium.