Why not Marsian?

The planet's name is from Roman god of war, Mars, the oblique form of which is Mart-, as in Martian, martial (arts), and round-aboutly March.

And the adjectival form of Venus is Venereal, though many baulk and write or say Venusian. What about a war between the Martians and the Venereans?

Ceci n'est pas un seing.