The etymology requires some explanation, as the modern French word for the planet Mars is Mars. I was guessing some Celtic/Gallic deity identified with Mars, but I haven't been able to turn up a good answer. (There's Lenus Mars...?)

The given explanation is taken from a USGS page, (which also confirms my suspicion that Carl Sagan was involved!) [[1]]

The "stub" etymology is repeated on this page: List of valles on Mars

--mkmori