Etymology
Burroughs frequently invented words of the languages spoken by the people in his novels, and used these extensively in the narrative. In Thuvia, Maid of Mars he included a glossary of Barsoomian words used in the first four novels. The word "Barsoom", the native Martian word for Mars, is composed of the Martian name for planet, "soom", and the Martian word for eight, "bar". This reflects counting Mars as the eighth body in the inner solar system, by counting not just planets, but the Sun and the satellites of Earth and of Mars.
wiki Barsoom